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Title: Method of producing low-carbon, white husk ash Document Type and Number: United States Patent 4049464 Link to this page: http://www.freepatentsonline.com/4049464.html Abstract: Low carbon, white husk ash suitable for use in manufacturing building materials, more particularly refractory building materials are prepared by first removing volatile constituents by heating the husks, normally rice husks but alternatively, for example, wheat, oats or barley husks, to a first relatively low temperature below the ignition point of the husks. Fixed carbon is then oxidized in the presence of a reagent by heating the husks to a second temperature above the separation temperature but below the crystallization temperature of the SiO.sub.2 in the husks, following by a heat treatment at a third temperature above the crystallization temperature of the SiO.sub.2, to produce a uniform SiO.sub.2 crystal structure. Inventors: Tutsek, Alexander (Gottingen, DT) Bartha, Peter (Bovenden, DT) Application Number: 720089 Filing Date: 09/02/1976 Publication Date: 09/20/1977 View Patent Images: Images are available in PDF form when logged in. To view PDFs, Login or Create Account (Free!) Referenced by: View patents that cite this patent Export Citation: Click for automatic bibliography generation Assignee: Refratechnik GmbH (Gottingen, DT) Primary Class: 106/406 Other Classes: 423/335 Field of Search: 106/288 B, 288 Q, 309 423/335 US Patent References: 1528371 Mar, 1925 Gambel 423/335.

3125043 Mar, 1964 Gravel 110/28. Foreign References: 2,070,383 Sep, 1971 FR

697,474 Sep, 1953 UK

Primary Examiner: Garvin, Patrick P. Assistant Examiner:

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Sheehan, John P. Parent Case Data: This is a continuation, of application Ser. No. 562,098, filed Mar. 26, 1975 now abandoned. Claims: We claim: 1. In a method of producing low-carbon, white rice husk ash suitable for the production of building materials, and more particularly refractory building materials, in which method, for purposes of removing volatile constituents and for purposes of transforming the fixed carbon into a gaseous compound, the rice husks are heated to a temperature below the crystallization temperature of the SiO.sub.2 of the rice husks, whereupon the resultant husk ash is heat-treated at a temperature above the crystallization temperature of the SiO.sub.2 for the purposes of obtaining a uniform SiO.sub.2 crystal structure, the improvement comprising: the first step of heating the rice husks to a first relatively low separation temperature within the range of from 200.degree. to 450.degree. C. in the absence of air, exclusively for separating the volatile constituents that are capable of after-burning; subsequently the second step of heating the rice husks free of volatile constituents to a gasification temperature in the range of from 450.degree. to 550.degree. C. while feeding an oxidizing agent thereto for transforming the fixed carbon into a gaseous phase prior to heat treatment; and the third step of subsequently heating the thus obtained rice husk ash to a heat treatment temperature in the range of from 700.degree. to 800.degree. C. 2. The method of claim 1, in which the husks are first brought to a separation temperature of from 200.degree. to 350.degree. C. at a heating rate of from 10.degree. to 40.degree. C./min, from a feed temperature of less than 100.degree. C., and are subsequently heated at the same rate and in the presence of the oxidizing agent to the gasification temperature in the range of from 450.degree. to 550.degree.C., after which the resulting husk ash is heated at the same rate to the treatment temperature between 700 to 800.degree. C. 3. The method of claim 2, in which the rate of heating is 25.degree. C./min. 4. The method of claim 1, in which the oxidizing agent supplied for oxidising the fixed carbon in the second process step is combustion-supporting air having an air coefficient of n = 4.0 to 6.0. 5. The method of claim 4, in which the oxidizing agent supplied is water-vapour. 6. The method of claim 1, in which HCl is added so as to inhibit the crystallization of SiO.sub.2 in the gasification step. 7. The method of claim 1, in which husks are comminuted in order to increase the bulk weight thereof before heat-treatment. Description: The invention relates to a method of producing low-carbon, white husks suitable for use in the manufacture of building materials, more particularly refractory building materials. Rice is a basic food in Asia, more particularly India and Japan, and substantial amounts are also cultivated in various countries in America and Europe. The present

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world rice production of about 400 million tons per year will probably increase in the future, owing to the great increase in population, paticularly in Asian countries. When rice grains are husked, the husks make up about 14 to 35%, depending on the variety of rice; since the husks have a low bulk weight of about 100 kg/m.sup.3, they take up 560 to 1400 million m.sup.3. Chemical analysis of rice husks shows the following typical composition (referred to substance free from loss on ignition):

______________________________________ Water 9% Protein 3.5% Fats 0.5% Cellulose 30 to 42% Pentosan 14 to 18% Mineral ash 14 to 30% ______________________________________ The composition of the mineral ash is normally within the following limits:

______________________________________ SiO.sub.2 92 to 97% Al.sub.2 O.sub.3 0.75 to 3% Fe.sub.2 O.sub.3 0.17 to 2% CaO 0.36 to 3% MgO 0.32 to 1.5% ______________________________________ In addition, up to 30% carbon is found, depending on the degree and nature of combustion of the organic constituents before analysis. In view of the composition of rice husks, a number of suggestions have been made for large-scale use thereof in agriculture or industry. These suggestions relate to direct use, with or without comminution; chemical decomposition of rice husks on an industrial scale to obtain organic chemical basic materials; combustion for obtaining heat, and use of the mineral ash residues. The possible applications include; the use of rice husks as fodder, for loosening up the earth, for pressing fruit or the like, for producing building materials such as slabs or the like, or use as fillers, packing materials or oil-absorbing substances. In industrial technology, rice husks are a basic material for obtaining furfurol for hydrolysis of wood, and for obtaining acetic acid and other organic basis materials. Rice husks are also used as abrasives, heat-transfer media for steam and electricity generation, and for producing ferrosilicon, silicon carbide, silicon nitride, lignin and nitrogen-lignin compounds, sodium silicate, silicon tetrachloride and the like. More particularly, rice husk ash is used in the glass industry, the ceramics industry, the cement industry and especially in the refractory-materials industry, since it has the properties of a porous silicic-acid raw material. As already mentioned, about 92 to 97% of rice husk ash consists of SiO.sub.2 and, therefore, has a melting-point above 1600.degree. C. It is, therefore, particularly suitable, as a silicic-acid carrier, for use as a raw material for manufacturing heat-insulating, refractory and heat-

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resistant building materials such as refractory, chemically bonded or burnt bricks and materials, and for manufacturing light building materials bonded with hydrated calcium silicate (tobermorite synthesis). The aforementioned materials have to be produced from ashes which have a defined carbon content and also have a given structure. In the case, for example, of rice husk ash suitable for tobermorite synthesis, the silicic acid content must be in an "amorphous", vitreous, and therefore very reactive, form. For refractory purposes, on the other hand, the silicic acid should occur in crystalline form, for example as quartz, cristobalite or tridymite, so as to have the required dimensional stability at high temperatures. It has been found that refractory building materials or the like can be prepared only from white rice-husk ash, that is ash which is substantially free from carbon or contains only a small amount of carbon, since black rice-husk ash (that is ash containing a high proportion of carbon) is water-repellent owing to the carbon residue, so that the ash particles are insufficiently wetted with aqueous binder solutions and it is more difficult to bond the bricks and obtain a mechanically strong structure. The presence of carbon likewise interferes with tobermorite synthesis, since it prevents, of at least adversely affects, the formation of tobermite. When ceramic products are produced, an undefined carbon content may result in undesirable reduction phenomena in the pots. A controlled carbon content and a definite structure are also required in other applications of rice husk ash, for example when used as fillers, filter materials or the like, since the colour of the ash is then uniform. It has been found that white, low-carbon rice husk ash is obtained when the husks are charred or "roasted" in open piles. In countries where rice is grown, this process is usually performed in the open air. The husks are ignited at a certain point and then heat up, and smoulder from the centre outwards. This process, which produces hardly any flames, lasts about 3 to 6 months. Aromatic vapours and gases are produced, and give off a very unpleasant smell over a wide area. Furthermore, particulars of ash having a high silicic-acid content are blown round the neighbourhood and are liable to produce silicosis in men and animals. The burnt-out heap of rice husks, consisting of ash, does not have any uniform colour, chemical composition, crystal structure or degree of crystallisation, since "roasting" occurs under uncontrolled conditions, so that the colour of a pile of ash varies between pink, white, grey and black. It is difficult to separate the individual constituents, which differ in chemical, mineralogical and physical respects, consequently ash of the aforementioned kind does not satisfy the conditions which are particularly necessary for use as a constituent of refractory building materials. Another point is that the specific weight of "amorphic" and crystalline ash is non-uniform -- for example 2.12 for amorphous rice husk ash and 2.28 for crystalline ash containing cristobalite. The quality of the finished building material, however, depends on a uniform crystal structure, on both the crystal modification and the degree of crystallinity of the SiO.sub.2 in the ash, since these factors have an effect particularly on the coefficient of thermal expansion. Owing to the high calorific value of rice husks (3430 to 3995 kcal/kg), the "roasting" of rice husks in open piles occurs automatically, but the process is inefficient and also, as already mentioned, takes about 3 to 6 months and can, therefore, not be used on a large industrial scale for producing white rice husk ash (more particularly because of the aforementioned pollution problem). It is particularly noteworthy that the aforementioned process cannot yield ash having a uniform crystal structure and a uniform low carbon content.

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On the other hand, when the rice husks are burnt in industrial plants for obtaining heat (for example to produce steam), the ash occurs in the form of black residues having a high carbon content, usually between 10 and 25%. For the reasons given, however, ash having a carbon content as high as this is unsuitable for many applications, more particularly for the manufacture of refractory materials. In the present case, the slowness of carbon to react is probably due to the fact that, after the silicic acid has crystallised, the carbon is not sufficiently accessible for combination with oxygen. No success has to date been obtained in attempts to control the combustion of rice husks so as to produce a low-carbon or carbon-free ash. On the other hand, it has been shown that subsequent roasting of black-rice husk ash is uneconomic, since the carbon is in very inert form. Table 4 hereinafter shows that no white ash is obtained even at high temperatures such as 1000.degree. C. and even after 11/2 hours treatment. There is no known process, therefore, which operates under conditions at which the organic constituents are converted into energy or separable decomposition products and in which a low-carbon white ash is obtained for the manufacture of building materials. An object of the invention, therefore, is to provide a large-scale industrial method of producing white, low-carbon rice husk ash, the process being efficient and economic and adapted to produce homogeneous, white rice husk ash having a uniform low carbon content and a well-defined SiO.sub.2 crystal modification and a homogeneous degree of SiO.sub.2 crystallisation. To this end, in the method of the present invention, volatile constituents are first removed by heating the husks, in the absence of air, to a first, relatively low temperature below the ignition point of the husks, after which the fixed carbon in the husks is oxidised in the presence of a reagent by heating the husks to a second temperature above the separation temperature but below the crystallisation temperature of the SiO.sub.2 in the husks, after which the resulting husk ash is heat-treated at a third temperature above the crystallisation temperature of the SiO.sub.2, to produce a uniform SiO.sub.2 crystal structure. The husks are preferably rice husks. In a preferred embodiment of the method, the volatile constituents are separated at a temperature in the range of from 200 to 450.degree. C., and the fixed carbon is oxidised at a temperature between 450 and 700.degree. C. According to another preferred feature, the rice husks are first brought to a separation temperature in the range of from 200.degree. to 250.degree. C. at a heating rate of from 10.degree. to 40.degree. C./min, starting at a feed temperature of less than 100.degree. C., and are subsequently heated at the same rate and in the presence of the reagent to an oxidation temperature in the range of from 450.degree. to 550.degree. C., after which the resulting rice husk ash is heated at the same rate to a treatment temperature between 700 and 800.degree. C. Preferably, the rate of heating up is 25.degree. C./min. According to another embodiment, the reagent supplied for oxidising the fixed carbon in the second process step is combustion-supporting air having an excess air coefficient of n = 4.0 to 6.0 that is an amount four to six times that required for a complete combustion or

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reaction. Alternatively, the reagent supplied is water-vapour. In other words, the inventive concept of the method of the invention is as follows: The rice husks are ashed in a three-step process; in a first step, the husks are separated, in the absence of air, from volatile constituents, after which the fixed carbon is oxidised at a higher temperature, which is followed by heat-treatment at a still higher temperature in order to adjust the degree of crystallisation and the SiO.sub.2 crystal modification. The combustible gases produced during the separation of volatile constituents and the gasification of the fixed carbon are subsequently burnt; the resulting energy is sufficient for the entire process and under particularly efficient conditions, can provide additional energy. The method according to the invention can be used, under industrial conditions, to manufacture white rice husk ash having a low carbon content and a defined composition and structure, the separated organic constituents being volatilised and used for heating. The heat obtained from the organic constituents of the husks can be used for the ashing process itself and/or (as already mentioned) for other purposes. Alternatively, of course, the organic thermal-decomposition products of the husks can be further processed in known manner and synthesised to furfurol, wood vinegar, methanol or the like; alternatively, of course, rice husks can be ashed according to the invention after being treated in a known process step for obtaining furfurol. The method according to the invention, therefore, is adapted for efficient, large-scale manufacture of white rice husk ash having a definite composition and structure, of the kind which cannot be obtained by subsequent roasting of black ash from conventional combustion processes, since such processes are insufficiently economic owing to the additional energy and time required. The invention is based on the surprising discovery that an optimum low carbon content is not obtained by burning the rice husks at a high temperature; instead, a number of tests at between 100.degree. to 1000.degree. C. have yielded the unexpected result that optimum white ash is obtained at temperatures between 450.degree. and 550.degree. C., whereas the ash obtained at lower or higher oxidation temperatures is normally black or grey and unserviceable. This finding applies both to continuous heating and to temperature shock treatment in a given temperature range. According to another feature of the present invention, the husks are normally comminuted before heat treatment, to increase the bulk weight. The resulting increase in bulk weight (to about 0.5 t/m.sup.3) considerably increases the energy yield and the throughput of the process according to the invention. Other features and advantages of the invention will be clear from the claims and the following description, in which the invention is illustrated by an example and by experimental results. EXAMPLE Rice husks, untreated or pretreated to obtain furfurol, have the composition and properties given in Table 1.

Table 1 ______________________________________ Rice husks from

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Rice husks Italy used in the Designation from Italy furfurol process ______________________________________ Ash 14.4% 20.7% Combustible constituents 85.6% 79.3% Volatile constituents 69.7% 53.9% Lower calorific value 3430 kcal/kg 3995 kcal/kg Ignition point (air) .about.250.degree. C .about.250.degree. C C 39.6% 43.0% H 5.4% 4.5% S traces -- N 0.32% 0.44% O 40.28% 31.36% ______________________________________ Differential thermoanalysis (DTA) and differential thermogravimetry (DTG) in a nitrogen atmosphere and in the presence of air showed that the decomposition reactions proceed as shown in Table 2.

Table 2 ______________________________________ Rice husks Rice husks (.degree. C) N.sub.2 atmosphere no exclusion of air ______________________________________ 100 100.degree. C endothermic reaction: decomposition of pentosan and formation of volatile constituents and tar 200 230-500.degree. C exothermic reaction: Decomposition of cellulose and oxidation of the free carbon 300 270.degree. exothermic reaction: decomposition of cellulose and formation of volatile constituents (CO) 400 360.degree. C exothermic reaction decomposition of lignin and formation of volatile constituents (CO,CH.sub.4) 500 600 700 700-800.degree. C exothermic 700-800.degree. C exothermic

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reaction: reaction: crystallisation of SiO.sub.2 crystallisation of SiO.sub.2 and fixed C residue 800 Black rice husk ash White rice husk ash ______________________________________ The preceding Table 2 shows that the thermal separation of the volatile constituents reaches a maximum at approximately 360.degree. C. and is substantially complete at approximately 450.degree. C. In a nitrogen atmosphere, the fixed carbon content remains in the ash, whereas in the presence of air it can be removed by oxidation under optimum conditions at 500.degree. to 550.degree. C., that is precisely by the method according to the invention. Structural research on the silicic-acid content of the husks has shown that the acid is mainly in the form of very imperfect cristobalite, apparently with mainly oxidative bonding in the "amorphous" state. Depending on the temperature used, the cristobalite either remains amorphous in X-rays or crystallises out. If mineralising agents are not added, crystallisation begins at 700.degree. C., with marked vibration of the ash. If mineralising agents are added, crystallisation begins at 300.degree. C. Experiments have shown that alkali-metal halides (such as NaF, NaCl and KCl) are particularly effective mineralising agents. It is sufficient, for example, to wet the husks with 0.2 N aqueous NaCl solution. It has also been found that crystallisation of the silicic acid can be delayed if the husks are wetted with HCl before thermal decomposition. Very reactive white rice husk ash can be obtained in this manner at 700.degree. C. It has also been found that the removal of fixed carbon from the ash can be accelerated by adding mineralising agents such as NaF, NaCl or KCl. Systemmatic ashing tests on a semi-industrial scale in the presence of stagnant air have shown that a white ash is obtained in the temperature range between approximately 450.degree. and 550.degree. C., as shown in the following Table 3.

Table 3 ______________________________________ Processing temperature Weight loss Colour of ash residue ______________________________________ 300.degree. C 29.8% Black 400.degree. C 66.8% Grey 500.degree. C 77.1% White 600.degree. C 74.7% Grey 700.degree. C 69.4% Black 800.degree. C 73.5% Black 1000.degree. C

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75.8% Black ______________________________________ Table 3: Loss of weight during thermal decomposition of rice husks, and colour of ash during spontaneous heating to the temperature indicated. Duration: 1 hour. Stagnant air. Table 3 also shows that the organic constituents of the husks are not decomposed to an adequate extent if the temperature is below 450.degree. to 550.degree. C. Above 550.degree. C., heating-up is spontaneous and the ash is unsuitable (black, carbon-containing). If the temperature is increased far above 1000.degree. C. (this was also tried experimentally), there is no improvement in the rate of decomposition and the purity of the inorganic end product, compared with decomposition in the aforementioned region of 450.degree. to 550.degree. C. Table 3 shows that the greatest loss of weight occurs during spontaneous heating at approximately 500.degree. C. The end product is white, whereas the quality of the ash progressively decreases above and below 500.degree. C. With regard to the rate at which husks must be brought to the temperature of 450 to 550.degree. C. the optimum rate was found to be 25.degree. C./min, as shown from Table 4 hereinafter:

Table 4 ______________________________________ Processing temperature Weight loss Colour of ash residue ______________________________________ 20 - 300.degree. C 37.1% Black 20 - 400.degree. C 72.1% Grey 20 - 500.degree. C 77.5% White 20 - 600.degree. C 76.9% White 20 - 700.degree. C 78.0% White 20 - 800.degree. C 76.1% White 20 - 1000.degree. C 78.6% White ______________________________________ Table 4: Loss of weight during continuous heating of rice husks to the temperature indicated. 25.degree. C/Min. stagnant air. Table 4 shows that white ash is obtained at 500.degree. C. onwards, after gradual heating at 25.degree. C./min from room temperature. It has been shown that 10.degree. C./min and 40.degree. C./min can normally be regarded as the upper and lower limit for the rate of heating at which ash having adequate "whiteness" (C. <2%) can be obtained.

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Table 5 hereinafter shows the progressive loss of weight at the optimum temperature of 500.degree. C.:

Table 5 ______________________________________ Time Weight loss Colour ______________________________________ 0 min 0% gold-yellow 1 min 41.7% black 3 min 45.6% black 5 min 49.2% black 10 min 56.0% black 30 min 76.1% grey 60 min 78.8% white ______________________________________ Table 5: Weight loss during thermal decomposition of rice husks after spontaneous heating to 500.degree. C, in dependence on time. Stagnant air As Table 5 shows, 41.7% of combustible constituents have been removed from the husks after 1 minute. After 10 to 30 minutes, all the volatile constituents have been driven off. At the same time, the fixed carbon constituent begins to vaporise and is substantially removed within 60 minutes. Table 6 hereinafter shows the weight loss during subsequent oxidation of black rice husk ash obtained by high-temperature combustion:

Table 6 ______________________________________ Weight loss/min Temperature 25 60 120 180 Colour ______________________________________ 300.degree. C 2.4% 2.61% 2.6% 2.9% Black 400.degree. C 8.93% 8.98% 9.05% 9.06% Black 500.degree. C 24.26% 24.47% 24.68% 24.90% Grey 600.degree. C 24.60% 24.87% 24.95% 24.96% Grey 700.degree. C 24.00% 24.01% 24.05% 24.07% Grey 800.degree. C 24.41% 24.50% 24.55% 24.54% Grey 1000.degree. C 24.45% 24.83% 24.81% 24.90% Grey ______________________________________ Table 6: Weight loss during subsequent oxidation of black rice husk ash obtained by high-temperature combustion. Table 6 shows that the carbon in black ash can likewise be removed at 500.degree.

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C. by oxidation. No advantage is gained by increasing the temperature or the time. Our discussion of the results illustrated in Tables 3 to 6 shows that white rice husk ash having the desired low carbon content (C. <2% or, at least, <3%) and a given structure can be obtained if the volatile constituents are separated according to the invention at temperatures preferably below 450.degree. C., whereas the fixed carbon is vaporised, preferably between 450.degree. and 550.degree. C., by oxidation, reaction with water gas or the like. If required, additives such as NaCl should be added so as to accelerate the removal of fixed carbon in gaseous form during the second process step. The distillation products obtained by the method according to the invention can be synthesised in known manner to obtain heat or organic raw materials. In order to obtain a white ash containing silicic acid in a reactive state (tobermorite synthesis), the maximum process temperature must remain below the crystallisation temperature of the SiO.sub.2 in the rice husks, that is below 700.degree. C. In this case, crystallisation-inhibiting admixtures such as HCl can be used if required. A white ash in which the silicic acid has crystallised, as is particularly advantageous in the use of rice husk ash for manufacturing refractory building materials, can be obtained according to the invention by heat-treating the ash in the range preferably from 700.degree. to 800.degree. C. after the organic constituents have been separated. Crystallisation occurs at temperatures between 300.degree. and 800.degree. C., depending on whether mineralising agents such as NaCl, NaF or KCl are added or absent. The method according to the invention can take place in one or more steps in known devices such as rotary furnaces, blast furnaces, retort furnaces and sintering plants. The inventive concept which relates more particularly to the manufacture of rice husk ash suitable for use in refractory materials, can of course be modified by the skilled addessee from the form in which it has been described and is defined in the claims. Of course, the process according to the invention can also be applied to the treatment of other suitable husks, for example, wheat, oats or barley. The features of the invention disclosed in the preceding description and the following claims can be used either singly or in any combinations for putting into practice the various embodiments of the invention. Title: Method of producing active rice husk ash Document Type and Number: United States Patent 5329867 Link to this page: http://www.freepatentsonline.com/5329867.html Abstract: Active rice husk ash is produced by a method which includes placing a hollow platform having many holes of a size too small for rice husk to enter on an enclosed floor slab, erecting a chimney on the hollow platform in communication with the interior of the hollow platform, forming a cone of rice husk around the chimney to completely cover the hollow platform, igniting the rice husk at the small holes for

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smolderingly incinerating the rice husk into carbonized rice husk, and allowing the carbonized rice husk to self-burn into ash.

Inventors: Sugita, Shuichi Application Number: 111569 Filing Date: 08/25/1993 Publication Date: 07/19/1994 View Patent Images: Images are available in PDF form when logged in. To view PDFs, Login or Create Account (Free!) Referenced by: View patents that cite this patent Export Citation: Click for automatic bibliography generation Primary Class: 110/346 Other Classes: 106/406, 110/246, 110/347 International Classes: F23G 005/00 Field of Search: 110/346, 347, 246, 226 106/406 US Patent References: 3959007 May, 1976 Pitt 106/406.

4049464 Sep, 1977 Tutsek et al. 106/406.

4829107 May, 1989 Kindt et al. 106/406.

5010831 Apr, 1991 Halfhide 110/347. Other References: Proceedings Fourth International Conference, Istanbul, Turkey, May 1992: Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete S. Sugita, et al. pp. 495-512 "Evaluation of Pozzolanic Activity of Rice Husk Ash". Primary Examiner: Favors, Edward G. Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt Claims: What is claimed is: 1. A method of producing active rice husk ash comprising the steps of placing a hollow platform having many holes of a size too small for rice husk to enter on an enclosed floor slab, erecting a chimney on the hollow platform in communication with the interior of the hollow platform, forming a cone of rice husk around the chimney to completely cover the hollow platform, igniting the rice husk at the small holes for smolderingly incinerating the rice husk into carbonized rice husk, and allowing the carbonized rice husk to self-burn into ash. 2. A method of producing active rice husk ash comprising the steps of connecting downstream and upstream rotary kilns in tandem, heating the upstream rotary kiln to a controlled temperature for carbonizing rice husk, heating the downstream rotary

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kiln to a controlled temperature for burning rice husk into ash, supplying rice husk to the upstream rotary kiln to be smolderingly incinerated into carbonized rice husk therein, supplying the carbonized rice husk to the downstream rotary kiln to be burned into ash therein, and discharging the resulting active rice husk ash from the downstream rotary kiln. 3. A method of producing active rice husk ash according to claim 2, wherein the upstream rotary kiln is heated to a temperature of 300.degree.-400.degree. C. and the downstream rotary kiln is heated to a temperature of about 600.degree. C.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of producing active rice husk ash exhibiting a high degree of amorphousness and suitable for use as a concrete aggregate in place of fly ash, silica fume and the like. 2. Description of the Prior Art In technology relating to rice husk and rice husk ash, the fundamental thinking has conventionally been first to consider rice husk as a heat source and second to try to find practical uses for the resulting rice husk ash. When rice husk is used a heat source, it is necessary to blow air into the rice husk after it is lit so as to achieve the highest combustion temperature possible. However, almost all of the rice husk ash obtained by such high-temperature burning is crystalline, very low in chemical activity and black in color. It therefore has few practical uses. When air is blown into to the rice husk for burning it at high temperature, the resulting rice husk ash progressively crystallizes, a fact that can be readily confirmed from the X-ray diffraction pattern of the rice husk ash. The electrical conductivity of a saturated calcium hydroxide solution of the rice husk ash is very low, generally measuring around 0.3 mS/cm. Such rice husk ash cannot be expected to produce a significant effect when used as, for example, a pozzolan for concrete. To be of high practical utility, moreover, a rice husk ash should preferably exhibit high chemical activity. It should also not be black. Since no method is available for low-cost industrial production of rice husk ash exhibiting high activity, there is a need for developing one. At the Fourth CANMET-ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete held in Istanbul, Turkey, between May 3 and May 8, 1992, the inventor on May 8 announced a method of using an ordinary muffle furnace as an electric furnace, charging 800 g of rice husk directly into the furnace, raising the temperature of the furnace from normal room temperature to 280.degree. C. and maintaining it at this temperature for 1.5 hr, raising the temperature of the furnace to 350.degree. C. and maintaining it at this temperature for 1.5 hr, raising the temperature of the furnace to between 400.degree. C. and 800.degree. C. and maintaining it at this temperature for 2 hr to conduct incineration, and removing rice husk ash from the furnace 5 hr later. Differently from rice husk ash obtained by directly igniting rice husk and burning it in a short time, the rice husk ash obtained by this method exhibited excellent chemical activity and

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was white in color. Since the method comprises two steps, the first for carbonization and the second for incineration, it will be referred to as the two-stage incineration method. The object of the present invention is to provide a method for efficient production of white rice husk ash exhibiting chemical activity which is based on this two-stage incineration method. SUMMARY OF THE INVENTION In the batch method for producing active rice husk ash according to one aspect of this invention, a hollow platform having many holes of a size too small for rice husk to enter is placed on an enclosed floor slab, a chimney is erected on the hollow platform in communication with the interior of the hollow platform, a cone of rice husk is formed around the chimney to completely cover the hollow platform, the rice husk at the small holes is ignited for smolderingly incinerating the rice husk into carbonized rice husk, and the carbonized rice husk is allowed to self-burn into ash. In the continuous method for producing active rice husk ash according to another aspect of this invention, downstream and upstream rotary kilns are connected in tandem, heating of the upstream rotary kiln is controlled to a temperature for carbonizing rice husk, heating of the downstream rotary kiln is controlled to a temperature for burning rice husk into ash, rice husk is supplied to the upstream rotary kiln to be smolderingly incinerated into carbonized rice husk therein, the carbonized rice husk is supplied to the downstream rotary kiln to be burned into ash therein, and the resulting active rice husk ash is discharged from the downstream rotary kiln. As will be understood from the foregoing, since the present invention first carbonizes rice husk by burning it in an incineration furnace without flaming and then burns the carbonized rice husk into ash, it enables easy production of white rice husk ash exhibiting excellent chemical activity. The above and other objects and features of the invention will be better understood from the following description made with respect to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is sectional view of an embodiment of an apparatus for conducting the batch method of producing active rice husk according to the present invention. FIG. 2 is a perspective view of the apparatus of FIG. 1. FIG. 3 is an explanatory view of an embodiment of an apparatus for conducting the continuous method of producing active rice husk according to the present invention. FIGS. 4(a) and 4(b) are graphs showing how the compression strength of concrete incorporating rice husk ash produced by the continuous production method of the present invention varies with the rice husk ash (RHA) content. FIG. 5 is a graph showing how the compression strength of concrete incorporating rice husk ash produced by the batch production method of the present invention varies with the RHA content.

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DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 and 2 are explanatory views of a batch production apparatus for carrying out the method of producing active rice husk according to the present invention and FIG. 3 is an explanatory view of an apparatus for continuously carrying out the method. In FIG. 1, reference numeral 10 denotes a concrete floor slab on which rice husk is stacked for burning. The floor slab 10 is enclosed on three sides by a three-walled enclosure 11 and the remaining side is provided with a vertically sliding shutter 12. The floor slab 10 is supported on a four-sided lower enclosure 13 that is provided on the side of the shutter 12 with an entrance 14 for the apparatus operator. The lower enclosure 13 is for defining a space 13' for setting fire to the rice husk. The space 13' is about 1 meter high. An approximately 50-cm opening 15 is formed through the floor slab 10 to reach the side of the space 13'. A hollow platform 16 measuring 20-30 cm in height and slightly larger than the opening 15 in diameter is placed on the perimeter of the opening 15 and made unmovable. The surface of the hollow platform 16 is formed with many holes 17 measuring a few millimeters in diameter, a size too small for rice husk to enter. A hole measuring about 12-15 cm in diameter is formed in the upper wall of the hollow platform 16 and a chimney 18 of a diameter slightly larger than the diameter of the hole is erected on the perimeter of the hole. The apparatus is installed at a step in the ground so that the ground surface GL.sub.1 at the upper level of the step can be used by a dump truck or the like for dumping rice husk onto the floor slab 10 and the ground surface GL.sub.2 at the lower level of the step can be used by a tank truck or the like for loading the product rice husk ash after the shutter 12 has been opened. When rice husk is to be burned, rice husk is dumped onto the floor slab 10 enclosed by the enclosure 11 and the shutter 12 in such manner as to completely cover the hollow platform 16 and form a conical pile 19 around the chimney 18. When the size of the batch of rice husk to be burned is 500 kg, the angle of repose of the pile 19 is about 35.degree. and the height of the pile is about 1.5 m, so that the floor slab 10 can measure about 3.5 m square and the enclosure 11 and the shutter 12 can be about 1.0 m high. When the pile 19 is to be ignited, the operator enters the space 13' through the entrance 14 and sets fire to the rice husk at the holes 17 in the hollow platform 16 by inserting burning paper, cloth or the like into the opening 15 at the center of the floor slab. Once the rice husk has been ignited, smoke and heat of combustion pass through the holes 17 in the hollow platform 16 into the interior of the hollow platform 16 and then rise to the exterior through the chimney 18, while air passes from the exterior through the pile 19 and into the interior of the hollow platform 16, as indicated by the arrows, whereafter it rises through the chimney 18 and is dispersed in the atmosphere together with the smoke. Flaming of the rice husk during the combustion is prevented by the smoke passing through the chimney. More specifically, since the rice husk is smolderingly incinerated at a temperature of about 370.degree.-380.degree. C., namely at a temperature lower than its fire point of

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410.degree. C., it is carbonized, i.e., becomes carbonized rice husk. When left as it is, the carbonized rice husk then self-burns at a temperature of about 700.degree.-750.degree. C. to become an ash, namely the white active rice husk ash which is the product aimed at by the method of this invention. When necessary, the rice husk ash can be comminuted with a ball mill or the like. A 500 kg batch of rice husk requires about 24 hr to carbonize and about 48 hours to burn into ash following carbonization, and yields about 100 kg of active rice husk ash. These processing times can be regulated to some degree by varying the diameter and height of the chimney. The aforesaid production method can be conducted without need for any complicated equipment and does not require the supply of electricity, gas or any other source of combustion heat other than that for igniting the rice husk at the start. The combustion of the rice husk proceeds solely by flameless self-burning and carbonization is achieved through the process of smoldering incineration. The method thus enjoys the advantages of not requiring any heat source other than that for ignition and of enabling high volume production, notwithstanding that the burning may be somewhat uneven. Moreover, the method enables production of highly amorphous active rice husk ash exhibiting a difference in electrical conductivity of 2-3 mS/cm and a SiO.sub.2 content of 90-95%. FIG. 3 shows an apparatus for continuously producing active rice husk ash which comprises an upstream rotary kiln 20 and a downstream rotary kiln 30 connected in tandem via a material supply hopper 31. A burner, electric heater or other heat source (not shown) is controlled for heating rice husk in the upstream rotary kiln 20 to 300.degree.-400.degree. C. and for heating rice husk in the downstream rotary kiln 30 to about 600.degree. C. The length of each rotary kiln is, for example, 1.5 m. The rice husk conveyance speed in the rotary kilns is adjusted so that rice husk supplied to a material supply hopper 22 of the upstream rotary kiln 20 is discharged as rice husk ash from a product discharge port 32 of the downstream rotary kiln 30 in about 1 hr. The waste heat from the downstream rotary kiln 30 is preferably used to preheat the rice husk in the material supply hopper 22 of the upstream rotary kiln 20. If convenient, the material supply hopper 31 can be removed from the product discharge port 21 of the upstream rotary kiln 20 and the two kilns be connected directly to each other. When the illustrated arrangement is employed, i.e., when the upstream product discharge port 21 is connected with the downstream material supply hopper 31, it is preferable to insulate the product discharge port 21 and the hopper 31. During operation of the apparatus, rice husk supplied into the upstream rotary kiln 20 from the material supply hopper 22 is converted into carbonized rice husk as it is tumbled and conveyed through the upstream rotary kiln 20. The carbonized rice husk discharged from the upstream rotary kiln 20 through the product discharge port 21 and into the material supply hopper 31 is then supplied to the downstream rotary kiln 30. It is thereafter incinerated as it is tumbled and conveyed through the downstream rotary kiln 30 to be discharged from the product discharge port 32 of the downstream rotary kiln 30 as highly active rice husk ash of very high whiteness, a SiO.sub.2 content of 92-95% and a difference in electrical conductivity of 6-6.5 mS/cm. When necessary, the rice husk ash can be comminuted with a ball mill or the like. For every 100 weight units of rice husk supplied to the upstream rotary kiln 20, 45 weight units of carbonized rice husk is obtained at the product discharge port 21.

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This continuous production method employing two rotary kilns enables the temperature control for production of carbonized rice husk in the upstream rotary kiln 20 and the temperature control for burning the carbonized rice husk into ash in the downstream rotary kiln 30 to be separately controlled for optimum effect. This makes it possible to continuously produce active rice husk ash free of uneven burning and having a high degree of amorphousness, in a very short time. Although the method requires a heat source, it has the advantages of enabling recovery and effective utilization of the heat of combustion, being capable of continuous 24-hour operation, being highly efficient, and enabling large volume production of high quality rice ash. As will be understood from the foregoing, in either of its aspects the method according to the present invention enables ready production of highly active rice husk ash exhibiting a high degree of amorphousness. Examples of using rice husk ash produced by the method of this invention as an aggregate in concrete will now be given. The graphs of FIGS. 4(a) and 4(b) relate to concretes incorporating rice husk ash produced using rotary kilns and show how the ratio of the compression strength of concrete containing rice husk ash to the compression strength of plain concrete (not containing rice husk ash) increases with increasing rice husk ash content. FIG. 4(a) shows the compressive strengths of concretes aged 7 days (about 1 week), curve a being for concretes having a water binder ratio of 45%, curve b for concretes having a water binder ratio of 55%, and curve c for concretes having a water binder ratio of 65%. FIG. 4(b) shows the compressive strengths of concretes aged 91 days (just over three months), curve d being for concretes having a water binder ratio of 45%, curve e for concretes having a water binder ratio of 55%, and curve f for concretes having a water binder ratio of 65%. In each of these graphs, the vertical axis represents compressive strength increase rate (i.e., 0% on the vertical axis corresponds to the strength of plain concrete) and the horizontal axis represents rice husk ash content (weight ratio of rice husk ash to cement). FIG. 4(a) shows that the rate of increase in compressive strength with increasing rice husk ash content was approximately the same for all of the 7-day-old concretes regardless of water binder ratio. Particularly noteworthy is that the concretes having a 15% rice husk ash content exhibited about a 40% increase in compressive strength. FIG. 4(b) shows that while 91-day-old concretes exhibited a pronounced increase in compressive strength with increasing rice husk ash content, the increase for the concretes having a water binder ratio of 45% was smaller than that for the concretes with water binder ratios of 55% and 65%. This tendency is observed not only for rice husk ash but also for other pozzolan materials. The concretes with water binder ratios of 55% and 65% exhibited an approximately 30% increase in strength at a rice husk content of 15%, indicating that the rice husk ash exhibited very high chemical activity. The compressive strength continues to rise when the rice husk ash content is increased even further. The graph of FIG. 5 shows the compressive strengths of 28-day-old concretes

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containing rice husk ash produced by the batch method and having a water binder ratio of 55%. Concretes with rice husk ash contents ranging between 0% and 40% were tested. The compressive strength of the concretes increased up to a rice husk ash content of 20%, at which it was 50% higher than that of plain concrete, and then stayed substantially the same between 20% and 30%. The concrete with a 40% rice husk ash content achieved only a 25% increase in compressive strength. The concretes with the highest compressive strength were those with a rice husk ash content in the vicinity of 25%. They exhibited more than a 55% increase in compressive strength. It is clear from the foregoing that when rice husk ash produced according to the method of the present invention is used as an aggregate in concrete it imparts the concrete with a pronounced increase in compressive strength. Title: Rice hull ash concrete admixture Document Type and Number: United States Patent 4829107 Link to this page: http://www.freepatentsonline.com/4829107.html Abstract: A cement admixture and cement composition containing said admixture comprising finely ground rice hull ash formed by slurry grinding of said ash to a volume medium particle diameter or up to 4 micrometers.

Inventors: Kindt, Lawrence J. (Woodbine, MD, US) Gartner, Ellis M. (Silver Spring, MD, US) Application Number: 159978 Filing Date: 02/24/1988 Publication Date: 05/09/1989 View Patent Images: Images are available in PDF form when logged in. To view PDFs, Login or Create Account (Free!) Referenced by: View patents that cite this patent Export Citation: Click for automatic bibliography generation Assignee: W. R. Grace & Co.-Conn. (New York, NY) Primary Class: 524/3 Other Classes: 106/123.11, 106/406, 106/709, 524/15, 524/492 Field of Search: 524/15, 3, 492 106/98, 288 B, 123.1 US Patent References: 4105459 Aug, 1978 Mehta 106/98.

4623682 Nov, 1986 Nicholson et al. 524/3. Other References:

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Rice: Chemistry and Technology, B. O. Juliano. Properties of Blended Cements Made from Rice Husk Ash, by P. K. Mehta, J. Am. Concr. Inst. 74(9):440-442, (1977). Cement Replacement Materials, Rice Husk Ash, Chapter 6, by D. J. Cook, Surrey Press (1986). Use of Rice Husk Ash in Concrete, by M. N. Al-Khalaf and Hana A. Yousif, Int. J. Cement Comp. and Lightweight Concrete, 6 241-248 (1984). Primary Examiner: Jacobs, Lewis T. Attorney, Agent or Firm: Troffkin; Howard J. Claims: What is claimed: 1. A product comprising rice hull ash having ultra-high fineness particle size of a volume-median particle diameter measured by laser-light scattering of up to about 4 micrometers, a BET specific surface area of at least 20 m.sup.1 /g, a silica content of at least 80% by weight, a carbon content of less than 10% by weight and being substantially amorphous as shown by x-ray diffractometry, said product being in combination with from about 0.05 to about 10 weight percent of a cement water reducing agent. 2. The product of claim 1 wherein the rice hull ash is RHA-PF product. 3. The product of claim 1 wherein the rice hull ash is RHA-LT product. 4. The product of claim 1 wherein the rice hull ash product has a silica content of at least 85% by weight. 5. The product of claim 2 wherein the particle size is up to about 3 micrometers. 6. The product of claim 3 wherein the particle size is less than 3 micrometers. 7. An aqueous suspension wherein the water reducing agent of the product of claim 1, 2, 3, 4, 5 or 6 is present in from about 0.01 to 10 percent by weight of the suspension and is selected from at least one of naphthalene-sulfonate formaldehyde condensates, melamine sulfonate formaldehyde condensates, lignin sulfonates, alkali or alkaline earth metal polyacrylates or copolymers thereof and said rice hull ash is present in from about 20 to 80 percent by weight of the suspension. 8. The aqueous suspension of claim 7 wherein the water reducing agent is present in from 0.1 to 5 percent by weight based on the total weight of the suspension. 9. The aqueous suspension of claim 7 wherein the rice hull ash product is present in from 40 to 60 percent by weight based on the total weight of the suspension. 10. An aqueous suspension comprising from about 0.01 to about 10 percent by weight of a water-reducing agent selected from at least one of naphthalene-sulfonate formaldehyde condensates, melamine sulfonate formaldehyde condensates, lignin sulfonates, alkali or alkaline earth metal polyacrylates or copolymers thereof; from about 40 to 60 weight percent of rice hull ash product composed of RHA-PF having a volume-median particle size of up to about 4 micrometers measured by laser-light scattering, a BET specific surface area of at least 20 m.sup.2 /g, a silica content of at

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least 80% by weight, a carbon content of less than 10% by weight and being substantially amorphous as shown by x-ray diffractometry; and up to about 15 wt. percent of calcium nitrite. 11. The suspension of claim 9 wherein the rice hull ash product has a particle size of up to about 3 micrometers. 12. The suspension of claim 9 wherein the water-reducing agent is a mixture of naphthalene sulfonate formaldehyde and a polyacrylate. 13. An hydraulic cement admixture comprising an aqueous suspension of claim 7. 14. An hydraulic cement admixture comprising an aqueous suspension of claim 9. 15. An hydraulic cement admixture comprising an aqueous suspension of claim 10. 16. An hydraulic cement admixture comprising an aqueous suspension of claim 11. 17. An hydraulic cement composition comprising a portland cement, sand, aggregate and water in proportions to provide a settable product, the improvement comprising the addition of from about 0.1 to 30 percent by weight solids of the admixture of claim 13 based on the dry weight of the portland cement. 18. An hydraulic cement composition comprising a portland cement, sand, aggregate and water in proportions to provide a settable product, the improvement comprising the addition of from about 0.1 to 30 percent by weight solids of the admixture of claim 14 based on the dry weight of the portland cement. 19. An hydraulic cement composition comprising a portland cement, sand, aggregate and water in proportions to provide a settable product, the improvement comprising the addition of from about 0.1 to 30 percent by weight of the admixture of claim 15 based on the dry weight of the portland cement. 20. An hydraulic cement composition comprising a portland cement, sand, aggregate and water in proportions to provide a settable product, the improvement comprising the addition of from about 0.1 to 30 percent by weight of the admixture of claim 16 based on the dry weight of the portland cement. Description: BACKGROUND OF THE INVENTION The present invention relates to a novel rice hull ash of ultra high fineness and other particular properties, as described fully hereinbelow, aqueous slurries of said ash and to their use as an admixture for hydraulic cement to enhance the properties of hydraulic cement compositions and the resultant set products made therewith. Rice hull materials, such as the shell, hull or husk of the rice grain, are generally a discarded waste product from rice production. The rice hull materials are viewed as having little, if any, commercial value in their raw material form. It is known, however, that rice hull materials typically contain a substantial amount, typically about 16 to 20 percent, of silica (SiO.sub.2) and when burned yield an ash which is rich in amorphous silica. This ash, referred to herein and in the appended claims, as "rice hull ash" or "RHA", has been used as a pozzolanic additive in hydraulic cement compositions. The RHA is viewed as a filler material capable of replacing or extending

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the more expensive ingredients of the composition, such as portland cement and the like. (See U.S. Pat. No. 4,105,459 to P. K. Mehta.) The RHA material has also been proposed as a filler or extender in other applications, such as in elastomeric and plastimeric compositions (see U.S. Pat. No. 3,951,907 to P. K. Mehta). Its use in these general manners have value in countries where rice is a major crop and where the other components which are replaced or extended are expensive and/or not readily available. The rice hull material is commonly converted into RHA by uncontrolled combustion methods in which the material is used as the fuel source. More recently, U.S. Pat. No. 3,959,007 disclosed a process in which a higher amount of energy is obtained when burning rice hull material under controlled combustion while still obtaining a RHA useful as a cement pozzolan or as an extender in other applications. The ash obtained from conventional modes of combustion may be further processed by dry grinding the ash to a powder form. The resultant ground material is a fluffy powder of low bulk density having a BET surface area of about 10 sq. m./g. or greater and, typically, a Blaine specific surface area of lower than 1 sq.m./gm. (The higher the Blaine value the smaller the particle.) Although the BET specific surface area of RHA preparations increases as the combustion temperature is reduced, and can be in excess of 200 sq.m./g., the Blaine specific surface areas of RHA's are usually much lower than this, typically less than 1 sq.m./g. This difference is because most of the specific surface measured by the BET technique is internal to the particle while the internal surfaces of a particle are not measured by the Blaine technique. The Blaine specific surface area measurements are greatly affected by particle size changes and are more indicative of particle size. Thus, the RHA material presently obtained and used is a somewhat coarse particle, having a low bulk density which makes the RHA hard to handle and deliver in desired amounts into a mix. In the case where RHA is viewed predominantly as an extender-filler, the particle size of the RHA is not deemed to be critical as long as it is not so large as to disrupt the matrix to which it is added. In the case of its use as a pozzolan in cement compositions, it is known that small increases in the strength of the resultant hardened cement composition can be achieved by the use of smaller particle size RHA. However, this relationship tends to plateau as the particle size reaches a Blaine surface area of 1 to 1.5 sq. m/g. Therefore, there has been no incentive to attempt to develop a means to further reduce the RHA particle size and achieve a resultant product. In summary, rice hull ash (RHA) is generally of high silica content (at least about 85% SiO.sub.2), but its usefulness as an additive in hydraulic cement mixes has heretofore been limited by the difficulty in obtaining it and handling it in a controlled, convenient and efficient manner. It has been demonstrated that the pozzolanic reactivity of RHA may be enhanced by burning the rice hulls at relatively low temperatures in specially-designed furnaces (U.S. Pat. No. 3,959,007). The strength of hardened hydraulic cements can be enhanced to small extents by using smaller particle RHA but this relationship plateaus. Because of this as well as the mechanical restrictions in dry grinding and the required need of specialized combustion techniques, there has been no desire to produce a RHA of very high Blaine value. Additional barriers to the need to form and use a high-Blaine RHA are the difficulty believed associated with handling the ultrafine dry powder, its presumed poor flow properties and with its dust hazards. SUMMARY

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The present invention is directed to RHA of ultra high fineness as more specifically defined hereinbelow. The present invention is further directed to cement admixture compositions containing said RHA of this invention which provides for a means of easily handling and metering the ultra high fineness RHA and to improved cement compositions containing said admixture. The present invention has been found to be an effective means of causing the resultant cement structures to exhibit substantial inhibition to permeation of materials which adversely affect the durability of the resultant structure, such as chloride ions and the like and thus form a structure of high strength and low corrosion potential. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a rice hull ash product of ultra fine particle size, to stable and easily handle aqueous suspensions containing said rice hull ash and to hydraulic cement compositions to which the present rice hull ash has been added. In order to provide a clear description of the present invention, the following terms are used in this description and in the appended claims with the meaning given below. The term "hydraulic cement" refers to a dry powder which sets and hardens to a solid mass when mixed with water. Among the cements included under this definition are plaster of paris, high alumina cements, lime-pozzolan cements, blastfurnace slag cements, portland cements, and blended cements based principally on portland cement. The term "hydraulic cement composition" is taken to refer to a mixture of an hydraulic cement with water, and also, if desired, with aggregate and admixtures. The term "aggregate" refers to an essentially chemically inert filler, such as a sand or gravel or crushed rock, whereas the term "admixture" refers to materials which when added to a cement composition in small amounts imparts a large influence on the physical and/or chemical properties of the uncured composition and/or upon the cured composition. Hydraulic cement compositions include a cement paste when there is no aggregate, a mortar when the aggregate is a sand, and a concrete when the aggregate also contains coarse particles such as gravel or crushed rock. A "pozzolan" is an inorganic material which consists principally of chemically reactive compounds of silicon and aluminum in their oxide forms, and which is capable of reacting with lime (calcium hydroxide, Ca(OH).sub.2), to form a hardened mass of calcium silicate hydrates and calcium aluminate hydrates. A common application is to use pozzolans as additives to enhance the economy or modify the properties of mixed based primarily on portland cements. In such cases, the pozzolans react with the lime evolved by the normal reaction between cement and water. In some cases the pozzolan is interground or interblended with the dry solids during the manufacture of the portland cement, while in other cases, pozzolans are added as an admixture during the preparation of portland cement compositions, such as concretes. The terms "rice hull ash" and "RHA" refer to the ash obtained from the combustion of rice hull materials, that is from the shell, hull and/or husk of the rice grain. Rice hull material is generally viewed as a waste product of little or no commercial value ("Rice: Chemistry and Technology" 2nd Ed. Chapter 19, by B. O. Juliano, 1985).

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The terms "rice hull ash product," "RHA product," "RHA-LT product" and "RHA-PF product" refer to the product of the present invention, as fully described below, which is formed from conventional RHA. The present invention is directed to a new rice hull ash product, aqueous suspensions of this product and to its use as a hydraulic cement admixture. The rice hull ash used to form the RHA product of the present invention must be an amorphous silica formed by the combustion of rice hull. The combustion can be accomplished by various modes, such as by the controlled slow combustion of rice hulls at low to moderate temperatures, preferably below 800.degree. C. and more commonly between about 500.degree. and 600.degree. C. Such a combustion process is described in U.S. Pat. No. 3,959,007, the teaching of which is incorporated herein by reference, or by other known low temperature processes. The rice hull ash produced by such low temperature modes of combustion is herein referred to as "RHA-LT." When the combustion period is maintained for an extended period the RHA-LT may be light gray or off-white in color while short combustion periods tend to yield a darker gray to black material. Material formed under such low temperature combustion has high internal porosity as shown by its high BET specific surface area values which are typically in the range of about 20 to 200, more typically about 100 to 200 sq.m./g. and greater. Higher values are obtained by the lower combustion temperature. Alternately, the rice hull ash useful in forming the present RHA product can be formed by combustion of rice hull material in a conventional pulverized fuel burner. The rice hulls, chopped to an average particle size of from about 0.1 to about 1 mm, are blown into a flame along with the combustion air. The combustion, although reaching temperatures well in excess of 1000.degree. C., occurs over a very short time period. Materials formed in this manner have BET surface area values of about 20 to 50 sq.m./g. and are identified herein as "RHA-PF." The use of a pulverized fuel burner as the combustion means is a simple method of manufacturing the needed and useful RHA, because the technology for pulverized fuel combustion is well established and is also well suited to the use of the combustion heat for steam or electricity generation. The RHA found useful in forming the RHA-product of the present invention must have a silica (SiO.sub.2) content of at least 80% by weight and preferably at least 85% and most preferably at least 90% by weight. It should consist primarily of amorphous silica as determined by x-ray diffractometry. The formation of RHA under high temperature for extended periods results in crystallization of the silica dto cristobalite or quartz. Such RHA is not useful. The carbon content should be less than 10% and more preferably less than 6% by weight. Therefore, an amorphous silica RHA having at least 80% by wt. SiO.sub.2 and less than 10% by wt. carbon is suitable but the preferred RHA would contain at least 85% by wt. SiO.sub.2 and less than 6% by wt. carbon. The BET specific surface area should be at least 20 sq.m./g. and normally will range from 20 sq.m./g. to 200 sq.m./g. Higher values are useful. The particle size of RHA which is ground by conventional means of dry processes normally has a Blaine specific surface area of less than 1 sq.m/gm. with the value rising as high as about 1.5 sq.m./g. By excessive dry grinding a Blaine specific surface area value of about 2 sq.m./g. may be achievable. Values in excess of about 2 sq. m./g. are not achievable by conventional means of dry grinding. The Blaine specific surface areas of RHA's are usually much lower than the BET values for the same material because most of the specific surface measured by the BET technique is internal to the particle, in the form of fine pores, which are not measured by the Blaine technique. Thus, the BET specific surface area is not significantly increased by further grinding RHA preparations, whereas the Blaine specific surface area is very

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amenable to increase by grinding up to the limitations discussed above. The RHA-product of the present invention is required to have all of the physical characteristics described above for RHA, that is the characteristics of morphology, carbon and silica content, and BET surface area except that its particle size must be significantly smaller than previously taught. It has been unexpectedly found that by processing such RHA by the manner described below one can achieve a product of ultra high fineness, that is the particle diameter is ultra small. The ultra-small dimensions of the particles of the present RHA-product is of values that can not be accurately measured by conventional Blaine surface area measurements. An appropriate means of measuring the present ultra-fine particle size is by a laser-light scattering analyzer which gives a statistical analysis of the volume-median particle diameter (D.sub.50). The present RHA-product must have a volume-median particle diameter (D.sub.50) of up to 4 micrometers, preferably up to about 3 micrometers and most preferably up to about 2.5 micrometers. Because of the systematic differences among the various analytical techniques for determining size with the presently required ultra-high fineness product, the laser-light scattering analysis is the basis used here. Specifically, a Leeds and Northrup "Microtrac" laser-light scattering analyzer or its equivalent was used. The Blaine air permeability method of specific surface area determination, which is commonly used in the measurement of the fineness of cements and even of microsilicas, is not recommended herein because it is an imprecise means of analyzing materials of ultra-small particle size especially if the particles are also very non-spherical and internally porous, as is the case with RHA. For example, a sample of presently formed RHA, which was determined to have a D.sub.50 of 3.7 micrometers by a laser-light scattering analyzer, was also found to have a Blaine air permeability specific surface area of 4.5 sq.m./g. Using this Blaine value and assuming that all of the particles were uniformly-sized spheres, a calculated diameter of only about 0.6 micrometers would be obtained. Knowing that the laser technique directly measures particle diameter, one readily observes that the Blaine technique is inappropriate for the present ultra fine product. The process required to achieve the RHA-product of ultra-high fineness is to grind RHA by wet grinding technique. The liquid medium should be water. The water should contain or have introduced therein at least one low or high range water reducing agent used in cement formulations to achieve a resultant stable slurry. The wet grinding can be accomplished by various means, such as a ball mill, tube mill, sand mill, or any type of stirred or vibrated media mill, operated either in batch or continuous mode. Use of stirred media mills, such as an Attritor Mill has been found to be particularly effective for ultrafine grinding of RHA to produce slurries of the subject invention. The wet grinding has been found to achieve the desired ultra fine particle sizes and also provides low operating costs, reduced dust emissions and reduced noise levels. Dry grinding does not achieve the present RHA-product of ultra-high fineness. Although the resultant RHA-product may be dried and used in its powder form, it is preferable to maintain and use the RHA-product in aqueous suspension. The suspension shall contain the RHA-product in from about 20 to 80%, preferably from about 30 to 70% and most preferably from about 40 to 60% by weight. The RHA-product suspension must also contain small amounts of conventional water reducing agent cement admixture material, such as naphthalene sulfonate formaldehyde condensate, melamine sulfonate formaldehyde condensate, lignin sulfonates, polyacrylic acid and its alkali and alkaline earth metal salts as well as copolymers of

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the polyacrylate and the like. The term "polyacrylate" shall include polymers of acrylic acid, methacrylic acid as well as C.sub.1 -C.sub.3 alkyl esters thereof which are water soluble by having a sufficient amount of metal salt therein. The copolymer may be mixtures of the polyacrylate monomeric units indicated above or with other olefinic monomeric units including ethylene, hydroxyalkyl acrylates and the like. The water reducing agent must be present in at least 0.01% by weight of solids contained in the suspension, with normal range of from 0.01 to 10%, preferably from 0.1 to 5%. The most preferred suspension contains a mixture of a naphthalene sulfonate formaldehyde condensate and a polyacrylate. It has been unexpectedly found that the present RHA-product provides a stable, non-setting and non-settling suspension even when used in high concentrations, such as greater than 40% by weight. The solid RHA-product of the present invention must contain substantially uniformly mixed therewith a water reducing agent, as described above, in amounts given above for the solid, powder RHA-product composition. In addition, other conventional cement admixture materials may be combined in the RHA-product suspension without causing detrimental effects such as solidifying, gelling or settling of the suspension. Other types of admixtures which have been found to be desirable to be incorporated into the present RHA-product suspension includes viscosifiers, such as polyhydroxyalkyl celluloses, polyvinyl alcohol, polyethylene oxide and the like; wetting agents such as vinsol resins, sulfonated organic compounds, polyethoxylated alkyl phenols and the like; and set retarders such as sugar derivatives, polycarboxylic acids, hydroxycarboxylic acids, phosphates, phosphonic acids, borates and the like; accelerators, such as chlorides, sulfates, formates and nitrates of alkali and alkaline earth metals, especially of calcium, sodium or potassium. Another class of accelerator which is very suitable for this application is the hydroxyalkylated amines, such as triethanolamine, and corrosion inhibitors such as alkali and alkaline earth metal nitrites. The subject slurry can be used directly as a hydraulic cement admixture. The slurry of RHA-product is stable and storable and is a means of readily transporting and metering the desired amount of RHA-product without incurring the handling and health (breathing) hazards associated with dry powders of rice hull ash and other conventional pozzolans. In addition, because the present RHA-product is capable of being contained in the slurry in high concentrations, the water content of the slurry does not have an adverse effect on the water to cement ratio and on the physical properties of the resultant cement composition. Another embodiment of the present invention relates to a particularly useful and unexpected observation that stable and pourable suspensions can be formed of a combination of highly desired materials. Specifically, when the subject RHA-product suspension, as described above, is formed from RHA described above as RHA-PF, one can include into the suspension high concentrations, such as up to about 10% by weight, and even up to about 15% by weight of the total suspension, of calcium nitrite. Calcium nitrite is a known agent which effectively inhibits corrosion of metal pieces (such as rebars and the like) contained in cement compositions when used in concentrations ranging from at least 0.5 to 10 percent and preferably of at least about 2 percent based on the portland cement contained therein. The present suspension permits the addition of the desired RHA-product and of sufficient calcium nitrite by a single application. Thus it alleviates on-site multiple application of admixture materials, and provides for accurate dosage of the desired materials. The

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ability to combine the present microsilica product with high concentrations of calcium nitrite to provide a storage stable, pourable suspension is unexpected as suspensions formed with other microsilicas, such as condensed silica fume, with calcium nitrite harden after only a few hours to unworkable consistency which can not be poured or metered. Thus, a particularly novel and useful form of this invention is suspensions containing from about 20 to 80% (preferably from 40% to 60%) RHA-PF, from about 0.5 to 10% (preferably from 2 to 8%) Ca(NO.sub.2).sub.2, and 0.01 up to 5% (preferably, 0.1 to 5%) by dry weight of a water reducing agent, as described above, which is preferably selected from naphthalene sulfonate formaldehyde or melamine sulfonate formaldehyde condensates. This has exceptionally good properties as an economical durability enhancing admixture for use in reinforced portland cement mortars or concretes, since it acts both to reduce the permeability of the hardened concrete to aggressive species such as chloride or sulfate ions, and also to inhibit the onset of corrosion of reinforcing steel even if aggressive species, including carbon dioxide, penetrate through to the steel. The RHA-product admixture slurry of the present invention can be added to conventional hydraulic cement compositions in amounts ranging from about 0.1 to 30, preferably to 20 percent solids of the slurry based on the dry cement used in the cement composition (S/S). The following examples are set forth to further illustrate and describe the present invention, and are not meant to limit its scope in any way except as defined in the claims appended hereto. All parts and percentages are by weight unless otherwise indicated. EXAMPLE 1 This example illustrates the production of RHA slurries by the wet-milling process. Rice hull ash of the RHA-PF class was purchased from Agrilectric Power Partners, Inc., of Lake Charles, La. They were waste byproducts of an electricity generation process. The samples had bulk densities of between 17 and 22 lb/cu.ft., silica contents of 92 to 93 wt.%, carbon contents of 2.5 to 5.5 wt.% and a moisture content of less than 2 wt. %, the remainder of the mass being principally in the form of calcium, potassium and magnesium oxide compounds. The median particle diameter was 65 um, as measured by laser-light scattering technique using a Microtrac (TM). The RHA was milled in a 3.4 gallon ceramic jar mill, with a grinding medium consisting of 23.3 kg of steel balls (1/4 and 3/8 inch). The mill was of 12-inch internal diameter, and was rotated on rollers at 38 rpm. It was first charged with the steel balls plus 2250 g of an aqueous solution containing 110 g of sodium naphthalene sulfonate formaldehyde condensate (Daxad-19) sold by W. R. Grace & Co. Then 2750 g of RHA was added in three increments over a period of 3 hours, after which the mill was allowed to continue running for a further 16.5 hours. At the end of this period, the resulting RHA product slurry was withdrawn from the mill, and analyzed for particle size distribution using a Microtrac laser-light scattering analyzer. The mass median particle diameter was found to be 2.6 um. The slurry was a stable liquid with a total RHA solids content of 55% by weight. It has a viscosity of about 1400 centipoise as measured by a Brookfield viscometer at 60 rpm, and was pourable. EXAMPLE 2 This example illustrates the production of a slurry RHA-LT. The RHA-LT was produced

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by burning raw rice hulls in a current of air at a temperature well below 700.degree. C., using a 2-foot diameter by 24 foot long externally-heated steel-shelled rotary calciner. The resulting product was off-white in color, and had a BET specific surfact area of about 150 sq.m./g, and a carbon content of less that 0.5%. This RHA-LT sample was ground in a 5-gallon steel ball mill, with a charge of 23.3 kg of steel balls (1/4 and 3/8 inch), operating at 54 rpm. The mill was first charged with the steel balls plus 3200 grams of a 2% aqueous solution of sodium naphthalene sulfonate formaldehyde (Daxad-19), and then 3200 grams of the RHA-LT was added in small increments over a period of 1.3 hours. The mill was then operated for a further 29.5 hours, with samples being taken at intermediate times. These samples were subjected to particle size analysis using the laser-light scattering analyzer (Microtrac). Results are summarized in Table 1. The resulting slurries contained 50% RHA and 1% Daxad-19 by weight, and were all pourable liquids. The samples were stored and later observed as being a readily pourable liquid after brief agitation.

TABLE 1 ______________________________________ WET GRINDING OF RHA-LT SLURRIES Sample Grinding Time, hrs. Vol. median diameter, um ______________________________________ E2-1 7.0 4.0 E2-2 8.0 3.8 E2-3 13.5 3.3 E2-4 29.5 3.3 ______________________________________ EXAMPLE 3 This example illustrates the formation of RHA product slurries using a stirred-media mill. Samples of RHA-PF and RHA-LT as described in Examples 1 and 2 above, respectively, were milled in a 30-gallon "Attritor" stirred media mill, as supplied by Union Process, Inc. The milling process was started by adding an aqueous solution of the indicated water reducing agent to the mill charge in the mill chamber, the mill charge consisting either of 1/4 inch ceramic balls or 1/8 inch stainless steel balls. The mill stirrer motor was then started, and unground, dry RHA was fed in slowly at the top of the mill until the desired quantity had been added. The mill was allowed to run until the median particle diameter of the RHA in samples taken from the mill had reached the desired value, as determined by laser-light scattering (Microtrac) analysis, after which the slurry was discharged by pumping it out of the bottom of the mill chamber. Results of six such experiments are shown in Table 2. It is clear that both types of RHA can be ground to the desired particle size range of 4 um or less by this technique, and that median diameters as low as 1.1 um are achievable.

TABLE 2 ______________________________________ RHA Slurry Production with a Stirred Media Mill Code Ingredients, (% by mass) Media Median No. RHA Type* Diam um ______________________________________ E3-1 RHA-PF (52%); DX-19 (1%)

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C 4.0 E3-2 RHA-PF (52%); DX-19 (1%) C 2.9 E3-3 RHA-LT (50%); DX-19 (0.3%); C 3.0 DX-36 (0.3%) E3-4 RHA-PF (48%); DX-19 (2%) C 2.4 E3-5 RHA-LT (46%); DX-36 (0.3%) S 2.1 E3-6 RHA-PF (49%); DX-19 (2%) S 1.6 E3-7 RHA-PF (49%); DX-19 (1%) S 1.1 ______________________________________ Notes: *C = 1/4 inch diameter ceramic balls S = 1/8 inch diameter stainless steel balls DX-19 = Daxad19, (calcium neutralized NSFC, W. R. Grace & Co.) DX-36 = Daxad36, (sodium neutralized polyacrylic acid, W. R. Grace & Co.) EXAMPLE 4 This example illustrates the manufacture of a hydraulic cement composition using RHA product prescribed by the present invention. A standard high-strength concrete mix was formulated, using a water/cement ratio of 0.35, and tested as-is or with a RHA product slurry, the actual slurry used being slurry E3-6, as described under Example 3. Full details of the cement compositions are given in Table 3. As indicated additional amount of water reducing agent was introduced one minute after mixing commenced to cause the slump of the samples to be approximately equal. D.C. resistivities were measured on 4 inch diameter by 8 inch long concrete cylinders, by applying 60 volts end to end, using a 3.0N sodium hydroxide solution as the contacting medium. The method is a modification of the Federal Highway Administration's Rapid Chloride Permeability Test (FHWA-Report No. RD-81/119 [1981]), and gives results which can be correlated with chloride impermeability, (i.e., a higher resistivity indicates less permeability to chloride ions). The formed hydraulic cement compositions having the subject RHA-product (E4-2 and E4-3) therein exhibited multifold resistance to allowing adverse chloride ions to permeate therein in comparison to the untreated comparative sample (E4-1).

TABLE 3 __________________________________________________________________________ HIGH STRENGTH CONCRETES MADE WITH MICROSILICA SLURRIES Fresh concrete Properties Compressive Strength D.C. Resistivity Fresh Concrete Mix Proportions. lb/cu.yd. Final after moist curing after moist curing Mix RHA Air

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Slump Set, Density, 1 day, 28 days, for 28 days, Code Cement Sand Gravel Water Prod. DX-19 % in. hr lb/cu.ft. kpsi kpsi ohm-m __________________________________________________________________________ E4-1 744 1421 1707 260 0 10.2 2.2 6.5 8.8 153 3.6 8.2 81 E4-2 730 1395 1675 256 55 8.2 2.6 5.0 5.7 152 4.7 10.5 280 E4-3 740 1289 1696 259 111 12.0 2.5 4.5 7.5 152 4.8 12.0 530 __________________________________________________________________________ EXAMPLE 5 This example illustrates the effect of RHA type and particle size on the resultant permeability properties of cement compositions made using the methods of the present invention. Mortar cement compositions were mixed in a Hobart mixer, using a Type 1 portland cement, a concrete sand at a sand/cement ratio of 2.5, and a water/cement ratio of 0.45, including water added with any admixtures. Rice hull ash product was added, in slurry form, as a direct volume for volume replacement for sand, at a total dosage rate of 10% by weight of cement. A total of 1% (solids basis, relative to cement,) of water reducing agent (Daxed 19) was included in each mix,

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some of it coming from the RHA product slurries and the rest added directly with the mix water. Mortar cubes and cylinders were cast and moist cured following ASTM C109 procedures, and tested for permeability by measuring D.C. resistivity of the cylinders after 28 days. Results are summarized in Table 4. All samples containing the RHA product slurries exhibited low permeability as measured by electrical resistivity, compared to the blank. Compressive strength (cubes) of the blank was 8.4 kpsi while those containing RHA-product were within the range of about 10-12 kpsi after 28 days of cure. For comparative purposes, a class F fly ash pozzolan was used in the same manner as above to produce a sample. The sample was formed to have about the same air and flow properties but the D.C. resistivity exhibited very little increase. Further, two samples were made in the same manner using RHA-PF material having vol. median diameters of 65 and 6 micrometers, respectively. These samples also showed very little benefit to the composition.

TABLE 4 __________________________________________________________________________ EFFECT OF MICROSILICAS AND OTHER ADDITIVES ON MORTAR PROPERTIES Vol. median Fresh Mortar 28-day Cured Mortar Additive type Diameter, um Air, % Flow, % Resistivity, ohm-m __________________________________________________________________________ None (Avg. 3) -- 7.3 92 36 RHA-PF 4.0 2.2 116 145 RHA-PF 2.7 4.0 120 188 RHA-PF 1.8 1.7 102 219 RHA-PF 1.1 1.3 89 272 RHA-LT 3.3 4.2 64 301 RHA-LT 2.1 3.1 60 368 Class F Fly Ash* 3.0 126 41 RHA-PF* 65 4.3 50 49 RHA-PF* 6 2.4 107 78 __________________________________________________________________________ *Comparative Examples. EXAMPLE 6

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This example illustrates the ability to form aqueous slurries of RHA-product and calcium nitrite which are useful as cement admixtures. Samples of the formed slurry E3-4 described hereinabove were mixed with calcium nitrite in the amounts indicated in Table 5 below. The particle size of the RHA-product used was not altered by the required additional mixing to provide a substantially homogeneous composition. The pH of the samples was measured with a glass electrode immersed directly into the suspension. The samples were shaken and their ability to flow was observed. This visual observation is indicated in the Table 5.

TABLE 5 ______________________________________ RHA-PRODUCT/CALCIUM NITRITE SLURRIES Ca(NO.sub.2).sub.2 Storage Condition Stability Sample % of Slurry Days .degree.C. pH Fluidity ______________________________________ 1 0 84 23 -- Stable; Fluid 2 5 0 23 8.1 Stable; Sl. Thicker than Sample 1 3 5 4 50 7.4 Stable; Fluid 4 5 23 23 7.4 Stable; Fluid 5 8 4 23 8.0 Stable; Thick liquid 6 8 7 50 7.7 Stable; Thick liquid 7 8 23 23 7.3 Stable; Fluid