Page 1 of 16

 

Effect of Black Liquor Firing Conditions on Salt Cake Properties

Inga-Lill Samuelsson, ALSTOM Sweden, Växjö Fredrik Johansson, ALSTOM Sweden, Växjö Denise Levesque, ALSTOM Canada, Ottawa Vic van Hattem, ALSTOM Canada, Richmond

ABSTRACT During the last 10-15 years, important changes have been made to the design of soda recovery boilers. Air systems with more than a single level of air above the black liquor nozzles appear to have an effect on the process conditions in the flue gas and the performance of the precipitators. For several years, operating experience as well as salt cake data has been gathered from many recovery boilers and precipitators around the world to better understand the causes behind the changed precipitator performance. To further investigate how the salt cake properties have changed, a test campaign was conducted during the fall of 2012 at three Swedish pulp mills, focusing on combustion conditions in upgraded kraft recovery boilers and the precipitator performance. Dust measurements were performed at different boiler operating conditions and dust samples were collected. Various analyses of the dust samples were made to determine the composition and the structure of the dust particles. Despite similar liquor compositions, significant differences were observed in precipitator performance between the plants. This paper discusses the outcome of the investigations and the impact of modern boiler operation on dust characteristics, and ultimately, on the precipitator performance. INTRODUCTION During the last 10-15 years, substantial changes in soda recovery boiler design have been implemented. New boilers and rebuilt boilers have been equipped with new air systems having more than one interlaced air level above the liquor guns instead of the earlier single level of air. The design changes affect the combustion conditions and hence the properties of the flue gas and the dust, which, in turn, affect the operation and dust collection efficiency of the precipitator (Fig. 1). Some precipitators show an improved performance after a boiler upgrade, while others demonstrate deterioration in performance (Fig. 2), which so far has been difficult to explain.

Fig. 1: Upgraded boiler operations affect the dust emission from the precipitator

Changed boiler design and combustion philosophy

  

Page 2 of 16

 

Fig. 2: Current reduction in precipitator (SE3) after boiler upgrade with new air system For several years, operating experience as well as data and dust samples have been gained and collected from different plants around the world during inspections, performance measurements and other investigations. The collected data has been evaluated and compared with previous findings with the intent to understand which process properties have changed, the reasons behind the changes and how the new dust properties affect the precipitator operation. Furthermore, Alstom conducted a test campaign during the fall of 2012 focusing on precipitators on old, relatively small, heavily loaded and upgraded kraft recovery boilers. The results from these measurements were then compared with data collected from other plants. Some of the findings as well as the most important conclusions are presented in this paper. TEST CAMPAIGN To further deepen the understanding of the relationship between the boiler operation and the precipitator operation, three pulp and paper mills in Sweden were selected as suitable candidates for a major test campaign. The chosen plants were:

BillerudKorsnäs, Korsnäs Boiler 4 Metsä Group, Husum Boiler 8 Smurfit Kappa, Piteå

All three recovery boilers are more than 35 years old but were recently refurbished and upgraded to modern design including new levels of combustion air. The precipitators have been modernized with rigid discharge electrodes and high frequency transformers. From now on these three plants will be called SE1, SE2 and SE3 in this paper and are not in the same order as listed above. The measurements were conducted in the fall of 2012, a few weeks after the maintenance period at each plant. The timing was chosen to minimize the risk of mechanical disturbances on the precipitator performance. To carry out the measurements in the most efficient manner possible, an external, certified testing company was hired to perform all the dust and gas measurements. The recovery boilers were operated at 2-3 different loads and measurements were made during each load case. Dust samples from the flue gas and from the precipitator dust transport system were collected at all load cases. Several analyses were done of the dust samples to determine particle size, chemical composition and mechanical properties of the dust.

Boiler upgrade

Current in A-field

Current in B-field

  

Page 3 of 16

 

DISCUSSION AND RESULTS Results of Measurements at the Swedish Pulp Mills Despite quite similar black liquor compositions at all three plants (Table 1), the measurements show a wide variation in precipitator performance (Table 2). The three precipitators are operating at different loads, meaning they are not expected to achieve the same dust collection efficiency. “Performance” therefore refers to actual precipitator efficiency compared to predicted efficiency at similar process conditions. One significant difference between the three test plants is that two of them have an economizer located downstream of the precipitator. These precipitators therefore have a higher flue gas temperature than the plant with the entire economizer positioned directly upstream of the precipitator. Despite the significant differences in flue gas temperature, no clear connection between precipitator performance and operating temperature was found. The most notable difference in measured process data between the three precipitators is the dust concentration. The dust concentration to the precipitator that showed the best performance during the test campaign was extremely high, > 35 g/Nm3 d.b. at present day normal load, whereas the dust concentration to the other two precipitators was quite moderate, 15-20 g/Nm3 d.b. at normal load. The reason for the large difference was not clear. Gas and dust concentration measurements gave no obvious clues for the large difference in performance between the plants. It appeared that the precipitator operation was affected by factors that are not measured during typical performance measurements.

Analyzed substance in

black liquor 

SE1Content of substance 

[% by weight] 

SE2Content of substance 

[% by weight] 

SE3 Content of substance 

[% by weight] 

Sodium [Na] 20,8 18,9 20,2 Potassium [K] 1,42 2,34 2,84 Sulfur [S] 5,6 5,7 6,1 Chlorine [Cl] 0,37 0,13 0,28 Carbon [C] 32,6 31,7 33,0 Oxygen [O] 35,6 37,6 34,1 Nitrogen [N] 0,10 0,07 0,08 Hydrogen [H] 3,5 3,5 3,4 Dry solids in liquor 76 72 74 

Table 1: Black liquor dry solids composition

  

Page 4 of 16

 

Plant data  SE1 SE2 SE3 Flue gas temperature at precipitator inlet [oC] 

 230

 178

 267 

Recovery Boiler bottom area load Dry solids flow/boiler bottom area [kg/(s x m2)] 

 0,23 

 0,25 

 0,19 

Furnace volume load DS flow/furnace volume (up to bullnose) [kg/(s x m3)] 

 0,0085 

 0,0110 

 0,0088 

Flue gas velocity in precipitator [m/s] 

 1,25

 1,29

 1,03 

Dust concentration at precipitator inlet [g/Nm3 d.b.] 

 >35!

 19

 16 

Actual vs predicted Precipitator performance ++ + -- 

Table 2: Load data and precipitator performance at present day normal load Dust Properties Particle size. Dust samples from the dust transport system of the precipitators were taken during the measurements. Dust samples were also taken directly from the flue gas flow in the duct upstream of the ESP with a specially designed sample probe. The probe is designed so that a small removable steel plate with carbon tape attached to it could be introduced into the flue gas flow. The plate was exposed to the flue gas for 1-2 seconds, enabling the particles to stick to the tape. This method gives an idea of the size and shape of the dust particles in the flue gas upstream of the precipitator before they are collected in the precipitator. Dust samples collected from the dust transport system and stored in containers agglomerate and get an altered structure. Therefore, such samples cannot be used to analyze the size of primary particles or agglomerates in the flue gas. The dust samples on the carbon tapes were analyzed with the Scanning Electron Microscopy (SEM) method. The results show that the dust samples contain not only a substantial quantity of very large agglomerates, but also some finer particles (Fig. 3, 4). It is very clear that the size of the agglomerates differs between the three analyzed plants. Plant SE3 showed much less large agglomerates of dust than the other plants. SE3 is also the plant which showed the worst ESP performance during the measurements (Table 1). SE1, which had the best performance and highest dust concentration, appears to have the largest amount of large agglomerates in the flue gas. A comparison of recent dust samples to samples taken several decades ago shows that large agglomerates are much more common today than before (Fig. 5a, b). Note that both photos are shown with the same magnification.

  

Page 5 of 16

 

Fig. 3: SEM photo of dust particles in the flue gas at the three Swedish mills in the test campaign

Fig. 4: Typical particle agglomerate with sintered primary particles

SE1 SE2 SE3

2 mm

100 µm 

SE1

  

Page 6 of 16

 

Fig. 5a: Dust sample, North America (NA2), 2008 Fig. 5b: Dust sample, Sweden (SE4), ca 1990

Chemical composition of the dust. The chemical composition of a particle affects its properties in different ways. A compound whose effect on precipitator operation has been discussed for many years is sodium carbonate. A common view is that high concentration of sodium carbonate results in tougher precipitator operating conditions and results in a reduction of the dust collection efficiency. The chemical composition of dust samples taken at the three Swedish plants at current normal load were determined by Energy-Dispersive X-ray Spectroscopy (EDX) and supplemented by separate carbonate analysis and pH determinations (Table 3). To improve the accuracy of the EDX method, which is semi-quantitative, the values in Table 3 are average values from several analysis points on the same carbon tape. A dust analysis from a North American plant (NA1) has also been added. An indication of precipitator performance has been included in the table. Analyzed substance in salt cake  

SE1 Content of substance 

[% by weight]

SE2Content of substance 

[% by weight]

SE3Content of substance 

[% by weight] 

NA1Content of substance 

[% by weight]Sodium [Na] 31  27 28 29,4Potassium [K] 2  5 5 6,5Sulfur [S] 15  12 18 15,2Chlorine [Cl] 1  - 2 1,2Carbon [C] 3  4 1 3,0Oxygen [O] 48  52 46 44Carbonate [CO3

2-] 12,9  14,7 5,2 14,4pH 11,4  11,4 11,3 11,5Relative performance ++  + -- -

Table 3: Composition of dust samples collected at present day normal boiler load The carbonate content is high to very high in all samples, resulting in aqueous solutions with pH values >11.3. Although all three boilers at the Swedish plants run at relatively high load, the measured carbonate content is unusually high, especially considering that the sulfur content of the black liquor ranges between 5.6 and 6.1% by weight. A comparison between the carbonate content and the precipitator performance shows no correlation. Therefore, one cannot conclude that high carbonate concentration negatively affects precipitator performance. All dust samples have relatively low content of chlorides. This is an advantage from an evaluation standpoint, since high levels of alkali chloride have a strong negative impact on the precipitator operation [1-3].

10 µm 10 µm

  

Page 7 of 16

 

Sediment. The various salts normally found in the salt cake are readily soluble in water and give clear and colorless solutions. Sometimes unburned black particles of soot and charred remains of liquor droplets can be found. In recent years the appearance of the aqueous solution has changed. Dust samples collected from precipitators downstream of small to medium sized, heavily loaded recovery boilers often give cloudy, colored solutions. The color ranges between beige-yellow-red-brown-black (Fig. 6a). After a few hours, and up to 24 hours, the color disappears and the solution becomes clear with brownish sediment formed at the bottom of the vessel (Fig. 6b).

Fig. 6a: Cloudy solution of dust, North America (NA1) Fig. 6b: Sediment with flakes of rust (NA1) To determine the origin of the sediment and its characteristics, several analyses were done. Appearance and composition were determined with SEM and EDX analyses, while a pyrolysis-gas chromatography method was used for a determination of the organic matter. A dust sample from plant NA1 was also analyzed with IR-spectrometry and various microscopy techniques. The appearance of the aqueous solutions and the slow sedimentation velocity of the non-soluble matter suggest that the particulate is in suspension, which means that the non-soluble particles are very small or have low density. Depending on the composition, some sediment will, after drying, form polymeric networks with no distinct particle structure [4-6]. SEM analyses show that such polymeric networks are formed by the sediments from all three Swedish plants (Fig. 7a, b).

Fig. 7a: SEM photo of sediment, plant SE1 Fig. 7b: SEM photo of sediment, plant SE3

40 µm200 µm 

SE1  SE3

NA1  NA1

  

Page 8 of 16

 

The appearance of the sediment from plant NA1 is different from the Swedish sediments (Fig. 8a, b). It does not form a distinct network structure but instead consists of separate particles. The difference may not only be due to the composition of the sediment, but also due to sample processing. Further investigations of the sample using the light microscopy technique, Differential Interference Contrast (DIC), showed that it contains two completely different fractions. One fraction consists of spherical, hollow, rather colorless particles and the other has a strong reddish color and much smaller particle size (Fig 9).

Fig. 8a: SEM photo of sediment, plant NA1 Fig. 8b: SEM photo of sediment, plant NA1

Fig. 9: DIC microscopy of sediment, plant NA1 EDX analyses were made of sediment samples from all four sites, including the North American sample (Table 4). The limited accuracy of the analytical method and differences in sample processing affects the results, but should still give a good indication of the sediment composition.

100 µm  5 µm

20 µm

NA1  NA1

NA1 

  

Page 9 of 16

 

Analyzed substance in

sediment 

SE1 Content of substance 

[% by weight]

SE2Content of substance 

[% by weight]

SE3Content of substance 

[% by weight] 

NA1Content of substance 

[% by weight]Oxygen [O] 51,0  36,7 34,6 39,1Sulfur [S] 2,6  4,5 0,5 0,4Chlorine [Cl] 1,6  - - -Sodium [Na] 13,8  - - 7,8Potassium [K] 1,1  1,0 0,4 2,2Calcium [Ca] 3,6  2,7 3,4 10,2Magnesium [Mg] 2,3  8,5 11,3 6,4Manganese [Mn] 1,1  9,8 12,7 1,5Silica [Si] 16,1  10,5 10,1 18,5Aluminum [Al] 1,7  1,0 1,4 1,0Iron [Fe] 0,5  2,5 3,1 7,6Zinc [Zn] 4,6  22,9 23,0 5,2

Table 4: EDX analyses of sediment The analysis shows that the composition of the sediments from SE2 and SE3 are relatively similar, while the sediment from SE1 resembles more the sediment from NA1. Whatever the reasons for the differences, the immediate conclusion is that the components are basically the same as in wood ash, although the proportions are different. According to a supplementary IR analysis of the sediment from NA1, silicon occurs in the form of silicates, which was expected given the appearance of the samples (Fig. 10). Silicates can have a greatly varied composition, and depending on the composition and treatment it can form polymeric networks as well as hollow spheres [5-8]. The presence of organic material was also analyzed in all four sediments. The results show that all samples have sediments with extremely low levels of organic matter (Fig. 11). The small peaks shown in the pyrogram of the sediment from NA1 represents palmitic and stearic acids. No other organic substances were detected in any of the samples.

Fig. 10: IR diagram of sediment from plant NA1

OH+H2O

H2O

Si-O-Si

Sediment ash from precipitator dust 

sample 105 °C 

Sediment ash from precipitator dust 

sample 575 °C 

  

Page 10 of 16

 

Fig. 11: Pyrolysis-chromatography diagram of sediment from plant NA1 A reasonable assumption is that the sediment consists of the water-insoluble and ash-forming substances in the liquor, for example wood ash, corrosion products, various additives, particles from the lime cycle and biological treatment plants, etc. It also suggests that both the combustion conditions in the boiler and the liquor composition affect the sediment properties. An increased level of water-insoluble particles (hereafter called sediment ash) appears to influence the ability of the dust to clump together; the higher the content of sediment ash in the dust, the greater the tendency for clump formation (Fig. 12, 13).

Fig. 12a: Salt cake in water, South America (SA) Fig. 12b: Salt Cake, South America (SA)

SA  SA

Fatty acids

Brownish sediment from precipitator ash

  

Page 11 of 16

 

Fig. 13a: Salt cake in water, bigger boiler, Sweden (SE5) Fig. 13b: Salt cake, bigger boiler, Sweden (SE5) Fibers. SEM analyses of the dust samples revealed the presence of a particle type that was unexpected. The samples from all three Swedish plants included in the test campaign and the North American mill (NA1) contained plenty of unburned fibers. The appearance and length of the fibers (Fig. 14a, b), as well as the concentration, show that it is most likely a case of unburned cellulose fibers. The organic composition of the fibers was not verified, however, as the EDX-method is not useful for analyzing organic material and the sample treatment before the pyrolysis-chromatography analysis removed fiber particulate. An EDX analysis, which was made to check the fiber surface composition of inorganic elements, revealed high concentrations of Si, Ca, Al, Mg, Na and Fe. Several of these elements form chemical compounds which act as efficient fire retardants. For example Al2(OH)3 and Mg(OH)2 (known as ATH and MDH, respectively) are two of the most commonly used fire retardants on the market [9-11].

Fig. 14a: Fibers in dust from plant SE2 Fig. 14b: Fibers in dust from plant SE3 Explanation of the “New” Dust Properties Why do carbonate concentration, agglomerate size, sediment concentration, presence of fibers and the properties of the agglomerates in kraft recovery dust seem to have changed in recent years?

1 mm  800 µm

SE2  SE3

SE5  SE5

  

Page 12 of 16

 

It is well known that the carbonate content is dependent on the balance between vaporized sodium and sulfur coming from the smelt bed and the molten droplets. High gasification temperature provides an excess of sodium, which results in the formation of sodium carbonate. There is an additional reason for high carbonate content. When the liquor droplets are completely combusted in suspension, their entire content of sodium and sulfur is being released. The estimated carbonate content, based on the composition of the liquor from SE1, becomes about 30% by weight of the particles when the droplets are completely combusted in an oxidizing atmosphere (Fig. 15). The conclusion is that the greater the proportion of the droplets being fully combusted in suspension, the higher the carbonate content in the dust.

Fig. 15: Gas and particulate formed during complete combustion of black liquor in an oxidizing environment An additional effect of complete combustion of droplets is the release of sediment ash. Instead of ending up in the smelt bed, where most of it is removed from the process via the green liquor sludge, the released ash follows the flue gas. The sediment ash in the flue gas is collected in the precipitator together with fume particles and is recycled to the black liquor. The concentration of sediment in the strong black liquor will increase until equilibrium is reached (Fig. 16a, b). The equilibrium level (D) depends on the concentration of sediment ash in the weak black liquor from the digester and how much of the strong black liquor is being burned in suspension (Table 5).

Composition of condensed salt particles (fume) at complete combustion in an oxidizing environment of liquor from SE1:

Component % by weight

Carbonate (CO3) 30,7 Sulfate (SO4) 29,5 Chlorine (Cl) 0,7 Sodium (Na) 36,6 Potassium (K) 2,5

 

  

Page 13 of 16

 

Fig. 16a: Circulation of sediment ash, D, at low percentage of droplet combustion

Fig. 16b: Circulation of sediment ash, D, at high percentage of droplet combustion Droplets combusted in suspension [% by weight]

Sediment ash in carryover (D) at equilibrium when A=1,0

Amount of sediment ash in carryover relative to the amount of sediment ash in carryover at 5 % droplet combustion

5 0,053 1 25 0,333 6,3 50 1,0 19,0

Table 5: Effect of suspension combustion on the amount of circulated sediment ash (dust emission from precipitator disregarded) Clearly a high level of suspension combustion is a very important cause of a high concentration of sediment ash in the dust to the precipitator.

  

Page 14 of 16

 

The question is how does the sediment ash affect the properties of the dust? It seems likely that the particles of the sediment ash act as condensation nuclei in the formation of fume particles from gaseous substances. There is also a natural agglomeration due to strong holding forces between submicron particles, therefore particles that collide tend to stay together. However, it is doubtful whether this fully explains the large agglomerates found. Chemical compounds of polyvalent metal ions and certain silicon compounds are used as effective flocculants in water purification [12, 13]. Can these compounds also act as flocculants in the flue gas? Flocculation is generally caused by opposite electrical charge of the flocculants and the suspended particles. It is well known that particles formed in a combustion process are generally pre-charged [14, 15] and that the polarity depends on the chemical composition of the particles. Based on this, one could conclude that the circulation of non-combustible sediment ash is a plausible cause behind the very large particle agglomerates. Another possible reason behind the heavily agglomerated dust is the formation of particles high in the furnace, where a large amount of heat transfer surfaces is located. Gaseous components can condense on these surfaces, forming agglomerated, sintered matter which is released to the flue gas due to high gas velocity or sootblowing. The frequent occurrence of fibers found in the dust also appears to originate from droplets more or less completely combusted in suspension. No investigation was done during this test campaign to clarify why the fibers themselves are not completely combusted. One possible reason may be that the fibers are released from the droplets high in the furnace, resulting in a short residence time in the combustion zone. This would be a function of the combustion air distribution and black liquor droplet size. Another possible reason is that the cellulose fibers, which are known to easily adsorb various metals [16, 17], are impregnated with metal compounds in the black liquor. Many of these substances act as flame retardants. Higher rate of suspension combustion not only increases the number of fibers in the flue gas, but is also expected to make the fibers more difficult to combust. The reason is the higher concentration of fire retardants in the black liquor caused by the recirculation of sediment ash. It is likely that both sediment ash and fibers increase the ability of the dust to agglomerate and form clumps and large piles. A portion of the sediment ash is extremely fine-grained, allowing this portion to function as a binder between the larger particles. Furthermore, the sediment ash may have sticky properties. For example, sodium silicate is commonly used as glue or cement [4-6]. Fibers act as structural reinforcement of the dust cake, which increases the ability to form large piles. EFFECT OF ALTERED DUST PROPERTIES ON PRECIPITATOR PERFORMANCE The impact of modern soda recovery boiler design and combustion philosophy on precipitator operation becomes more evident for heavily loaded, small to medium sized boilers than for moderately loaded, bigger boilers. The changed combustion conditions, with a greater proportion of droplets combusted in suspension, affect precipitator operation in several ways. One important change is the larger size of particle agglomerates in the flue gas. The larger the agglomerate size, the easier it is to charge the particles. Large agglomerates are therefore advantageous from the perspective of charging the dust. At the same time, large particle agglomerates increase the risk of uneven dust distribution across the inlet of the precipitator, which counteracts the positive effect of easier charging. A high concentration of fibers will always have a negative effect on precipitator performance. Long, narrow particles produce corona discharges and reduce the spark-over voltage and hence the precipitator current level. However, is the concentration of fibers in the dust from kraft recovery boilers high enough to significantly influence the precipitator operation? Since the collecting process in a precipitator "sorts" the dust particles with different properties, similar particles will be more or less concentrated in the dust cake on the collection plates and will also end up in different parts of the precipitator. Therefore, it is not uncommon that even quite small concentrations of certain types of particles can have a major effect on the precipitator performance. The presence of sediment ash as well as fibers increases the risk of formation of problematic dust build-ups inside the precipitator. This, in turn, increases the risk of deterioration of gas distribution and power distribution inside the precipitator. Such disturbances have a major negative impact on the dust collection efficiency.

  

Page 15 of 16

 

CONCLUSION Load increases and air system upgrades in small to medium sized kraft recovery boilers can alter the dust properties and thus affect precipitator performance. The test campaign has shown that changes to the air distribution and black liquor firing conditions have resulted in changes in dust properties such as increased particle agglomeration, elevated unburned fiber concentration, increased silica and metal oxide (sediment ash) concentration, and elevated carbonate levels. The test campaign also showed that it is possible to adjust the electrostatic precipitator operation by modifying the boiler operating settings. The upgraded boiler operation results in a higher degree of combustion of liquor droplets in suspension, which, in turn, results in a circulation and concentration of sediment ash in the black liquor loop. It also seems to be the reason behind the presence of fibers in the salt cake. More pronounced combustion in suspension therefore appears to be an important reason behind the altered dust properties. The altered dust size distribution is a key factor in the observed changes in precipitator operation. Larger agglomerates are easier to charge in the precipitator, which is an advantage with respect to dust collection efficiency. At the same time larger agglomerates are more difficult to distribute evenly over the cross section of the precipitator than submicron particulate, thus counteracting the advantage of easier charging. The presence of sediment ash and fibers also increase the tendency for the dust to cause build-ups on the precipitator internals. Dust accumulations may disturb the charging of the particles as well as the gas distribution inside the precipitator. It is a challenge to predict the overall effect of a boiler upgrade on the precipitator operation, as it is very dependent on the black liquor firing conditions. It is even more difficult if the upgrade is done in conjunction with a load increase or other process changes. The effects of the upgrade are generally more prominent on smaller, heavily loaded boilers, and it is more difficult to achieve stable operation on these boilers than on large, moderately overloaded boilers. REFERENCES  [1] Inga-Lill Samuelsson, Denise Levesque, Christer Mauritzson, Vic van Hattem, Alstom Power: Improved Operation of Recovery Boiler Precipitators. TAPPI ICRC 2010 [2] Inga-Lill Samuelsson, Alstom Power: Effect of Salt Cake Properties on Precipitator Performance. PAPTAC EXFOR 2000 [3] Inga-Lill Samuelsson, Alstom Power: Förbättrad filterfunktion vid svåra driftkonditioner. Sodahuskonferensen 2003 [4] Stacy A. Johnson, Patricia J. Ollivier, Thomas E. Mallouk: Ordered Mesoporous Polymers of Tunable Pore Size from Colloidal Silica Templates. Science Vol 283, February 1999 [5] Ralph K. Iler: The Chemistry of Silica: Solubility, Polymerization Colloid and Surface Properties and Biochemistry of Silica. Wiley-Interscience June 6, 1979, ISBN-10:047102404X, ISBN-13:978-0471024040 [6] Horacio E. Bergna, William O. Roberts (Editors): Colloidal Silica: Fundamentals and Applications. CRC Press Taylor & Francis Group, LLC, 2006, ISBN-10:0-8247-0967-5, ISBN-13:978-0-8247-0967-9 [7] Weiwei Wu, Xinhua Yuan, Sunsheng Cao, Yi Ge, Songjun Li, Zhiyuan Zhao, Long Fang: One-Pot Pathway: Fabricating Ordered Hollow Silica Spheres Using Sodium Silicate as the Precursor. Australian Journal of Chemistry Volume 64 (12), 2011 [8] Werner Stöber, Arthur Fink: Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. Journal of Colloid and Interface Science 26, 62-69, 1968

  

Page 16 of 16

 

[9] P.R. Hornsby: The Application of Fire-Retardant Fillers for Use in Textile Barrier Materials. Springer, 2007, ISBN-978-3-540-71917-5 [10] Alexander B. Morgan, Charles A. Wilkie (Editors): Flame Retardant Polymer Nanocomposites. John Wiley & Sons, 2007, ISBN-0470109025, ISBN-9780470109021 [11] Ahmad Aroziki Abdul Aziz, Sakinah Mohd Alauddin, Ruzitah Mohd Salleh, Maziyar Sabet: Influence of Magnesium Hydroxide/Aluminum Tri-Hydroxide Particle Size on Polymer Flame Retardancy: An Overview. International Journal of Chemical Engineering and Applications Vol. 3, No. 6, December 2012 [12] John C. Crittenden, R. Rhodes Trussel, David W. Hand, Kerry J. Howe, George Tchobanoglous: MWH’s Water Treatment: Principles and Design. Wiley, 2012, ISBN-1118103777, ISBN-9781118103777 [13] K.J. Ives (Editor): The Scientific Basis of Flocculation. Springer, 1978, ISBN-9028607587, ISBN-9789028607583 [14] Heejung Jung and David B. Kittelson: Measurement of Electrical Charge on Diesel Particles. Aerosol Science and Technology, 39:1129-1135, 2005 [15] H. Burtscher, A Reis, A. Schmidt-Ott: Particle charge in combustion aerosols. Journal of Aerosol Science, 17:47-51, 1986 [16] Ebhodaghe F. Okieimen, Justus E. Ebhoaye: Adsorption behavior of heavy metal ions on cellulose graft polymers. Journal of Applied Polymer Science, Volume 32October 1986 [17] Hale Bahar Öztürk, Hai Vu-Manh, Thomas Bechtold: Interaction of cellulose with alkali metal ions and complexed heavy metals: Lenzinger Berichte 87, 2009