Cutting plan for wire mesh

The Manufacture of a Cement Extender fromRice-Husks using a Basket-Burner

by M. L. AllenPrince of Songkla University,

Thailand.

Ingredients

The only ingredient is rice-husks.However, to make a strong mortar,some additional calcium oxide(quick-lime) or Portland cement willbe required.

Husk is obtained as a by-product ofthreshing padi. In fact, about 20% ofthe dry mass of harvested padi ishusk. Of this residue, somewherebetween 17% and 23% by weight isleft as ash when the husk is burned.The ash is about 95% pure silica,and, if properly prepared, it is in anactive form1 which behaves very likecement.

The main problem in producingactive silica from rice-husks is toburn them at a temperature belowabout 800°C to stop the silicaforming inactive crystals. But it isalso necessary to burn them for longenough so that all the cellulose burnsaway and leaves a white or grey-coloured ash.

The use of asimplebasket-burnerensures thattheseconditionswill occur.

Equipment

The basicequipmentcomprisestwo baskets -- one largeand one small.One alone will work but results are

more consistent if a small basket isplaced concentrically inside thelarger one. The reason is that thesmall basket allows for a moreeven combustion with sufficientaccess for air.

The larger basket should be about600 mm in diameter and about 900mm high. An inner basket of250 mm diam and 750 mmdepth should be positionedconcentrically as shown in thesketch.

Both baskets are convenientlymade from steel mesh having nofewer than four holes to thecentimetre. (25 mesh/ins isvery suitable).

The attached sketch showshow the mesh can be cut from a rollof mesh to minimise waste. There isno need for the mesh to begalvanised (coated with zinc)because this will burn off in the

operation of the process.However, using stainless-steel mesh is worthwhile ifregular use is planned. Thisis because it can stand thehigh temperature, rain andsome rough-handlingwithout corrosionoccurring. The basketshould be supported withsmall pieces of brick or flatstone about 40 mm abovethe ground to let air getunderneath it. Of course theground should be level andfree of plant growth for thesame reason.

Because sudden rain showerscan limit combustion and a wind

blowing on the basket can increasethe local temperature, the basket mayhave to be placed in a 200 litre drumand fitted with a cover.

The drum does not have to be new oreven clean so long as it keeps windand rain off the rice-husks in thebasket. To ensure that sufficient aircan get to the rice-husks in thebasket, at least 10 holes of about 10mm diameter should be drilled orpunched in the base of the drum. It isalso a good idea to put flat stones orbricks under the drum as well asunder the basket.

Quantities

The basket described takes about 20kg of rice-husks which occupy about0.1 m3. Typically, one burning takesabout 15 hours and produces 4 kg ofRice Husk Ash (RHA).

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Pakistani burner

The Process

The basket should be positionedaway from homes because itproduces smoke. However, it can beplaced under a roof provided thatthere is sufficient ventilation. If rainor winds are expected, the basket-burner should be under such a coveror inside a 200 litre drum asdescribed in the EquipmentSection above.

Dry rice-husks are placed in thelarge basket until a layer of 150mm isformed. Thesmaller basketcan then beput in place and spacebetween filled withhusks.Thereshould beno husksplaced inside the innerbasket.

Ignition can be achieved by droppinga piece of burning paper or hotcharcoal into the central basket.Alternatively, a piece of rag soakedin used engine oil and ignited can bedropped into the central basket.

YOU SHOULD NEVER USEPETROL TO IGNITE THEHUSKS !!

About 10 minutes after ignition youwill probably see some flames. Thenthe burner should produce a littlesmoke (which keeps away flyinginsects but makes your eyes water!)

After about an hour, the side surfacesshould be smouldering and turningblack. This smouldering spreadsslowly up the sides and the bedheight reduces. There should be noflames produced at this stagealthough, as the husks collapse, theremay be some sparks and a flame fora few moments.

If the process is started in themorning, the ash should be ready forremoval on the following morning.However, it may still be hot enoughto cause burns, melt plastic bags or

ignite paper bags SO BE VERYCAREFUL. The ash should bewhite or grey with only a little blackchar. If the ash is to be used as acement-substitute or extender, itshould be used as soon as possiblebecause it loses its activity in moistair. Alternatively it could be storedin air-tight cans or double plasticbags. The basket is immediately

available for making moreRHA and needs nopreparation. If your object is tomake more RHA, you might

consider using more baskets.However, forfullcommercialoperation, this

is expensive.There are twoalternatives:

1) In Indiaand Pakistan,

brick burnersare used. They

are built inmodules on a square baseapproximately 1 metre by 1 metre.The bricks are built into a wall butwith some bricks omitted to allow airto enter. However, to stop husksspilling out, the modules are linedwith mesh.

2) Two rolls of mesh can be unrolledand held upright with steel rodsdriven into the ground. The meshprovides the walls for a roughlycircular path about 800 mm wide ---rather like a race-course. Rice-husksare loaded into the gap between thetwo walls for a distance of about 3metres. The leading edge can then beignited and more husks added to thecool face ahead of the burning.Husks can be added right around thecircle to the opposite side. The nextday, RHA can be removed until itgets too hot to handle and morehusks added to the cool side toreplace the ash removed. In this way,the fire is encouraged to go round thepath continuously with husks addedahead of the fire and ash removedfrom behind it. Some sheets ofcorrugated iron can be used to builda shelter over the fire and rice-husks. This may be moved

around as needed or built into apermanent cover over the processsite.

Making Cement

Rice-Husk Ash can either substitutefor some of the cement in a mortar orit can be mixed with quicklime tomake a hydraulic, cement-freemortar.

If you are interested in making sucha material commercially or testingthe properties of various mixes, youshould consider grinding the rice-husk ash with cement (or calciumoxide) in a rod mill.

The accompanying sketch shows onemade at the Asian Institute ofTechnology in Bangkok using a 200litre drum mounted on a pair of carback-axles and driven by an electricmotor. (Alternatively, the drum canbe supported on rollers). The “rods”are from left-over pieces ofreinforcing steel and should beslightly shorter than the drum so thatthey are free to move inside it.

You are also advised to obtain abook by Cook and Suwanvitaya2

setting out the properties of variousmixtures.

--- CEMENT FROM RICE-HUSKS 3

AIT Rod-mill

They have shown that mortarsmade from lime (CaO) and rice-husk ash in the proportion of1:1.5 develop a strength of about 8MPa after 28 days.And this is quitestrong enough forsome simple, non-load bearingstructures. Cook andSuwanvitaya alsoshowed that ifincreased strength isrequired, some of thelime may be substitutedby cement.

Typically, a 1:1.5 cement:RHAmortar develops a 28 day strength ofabout 20 MPa and mixes containingcement:RHA greater than 1:0.67actually show approximately thesame 28 day strength as straightcement (i.e about 50 MPa)2. ThusRHA may be used as a substitute forexpensive cement to provide mortarspossessing a range of strengths. Thevarious possibilities are presented inTable 1 taken from the publicationby Cook and Suwanvitaya2. Youshould note that the highest strengthsare only achieved if the RHA andcement are ground together for abouttwo hours.

The incorporation of rice-husk ashinto a mortar is reported to alsoconfer extra resistance to acids3.Thus mortars made from RHA couldbe used to construct houses, rodent-proof rice stores, drains inacidic soils, bio-gas generatorsand milk-resistant floors inmilking sheds.

Some other possibilities

This Project Outline will be ofuse to village co-operativesand rice-mills where excesshusks are available duringmilling operations. This Outline may also be ofuse to schools as part of abuilding or fund-raisingexercise. The processdescribed also has someeducational value in scienceclasses.

It could usefully form the basis foreither undergraduate or post-graduate research at a Universitybecause the amorphous silica formedby the basket burner can bedissolved in sodium hydroxide toproduce sodium silicate. Indeed, thisprocess may be of value to anentrepreneur because sodium silicateis a high "value-added" chemicalused in the slip-casting of ceramicsand also as a multi-tonne inorganicadhesive for such products ascorrugated card-board.

Rice-husk ash has also been used asa low-cost sterile filler in plastics andsoap. It has been shown to bebiologically inactive and to make anexcellent hydroponic matrix in whichdisease-free plants can be raised andseeds germinated. A 12:1 rice huskash:cement mix has also been shownto form excellent castable insulating

bricks for use in making ceramickilns and domestic cooking stoves.

References

1) Rice-Husk Ash Cements:United Nations IndustrialDevelopment Organization(with Govt. of Australia),Vienna (1984)

2) Cook. D.J. and Suwanvitaya,P. :Properties and behaviourof lime-rice husk ash. UNCIVReport R-208, University ofNew South Wales, Australia,1983.

3) Mehta, P.K. :The chemistryand technology of cementsmade from rice-husk ash.Proc. UNIDO/ESCAP/RCTTWorkshop on Rice-Husk AshCement, Peshawar, Pakistan,113-122, 1979

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Compressive strength of cement-lime-ash mortar mixes

cement:lime:RHA water:cement Cube strength (MPa) Fineness(cm²/g)7

days28

days90

days365days

100:0:0 0.49 29.3 43.0 47.2 44.0 3190

80:0:20 0.61 23.4 49.9 51.3 54.4 4640

60:20:20 0.61 22.3 36.2 38.9 41.3 7480

60:0:40 0.62 19.1 41.6 47.1 52.3 5860

40:40:20 0.67 11.2 18.3 20.8 21.8 11890

40:20:40 0.69 14.9 24.1 27.1 28.3 8320

40:0:60 0.74 13.2 17.8 19.4 19.9 10830

20:60:20 0.79 4.8 7.8 8.5 8.4 16360

20:40:40 0.74 7.9 11.3 12.7 14.6 14100

20:20:60 0.75 8.4 13.6 14.5 14.8 11560

20:0:80 0.82 4.8 6.2 7.1 7.6 10690

0:40:60 0.86 6.3 8.5 8.5 8.5 14180

0:20:80 0.89 6.4 6.7 6.9 6.8 12190

Source: Cook & Suwanvitaya2

Table 1

This document was first published by the UNESCO-sponsoredTechnology Networks of Southeast Asia. It is free from anycopyright constraints and may be copied and distributed freely.However, because all eventual uses and users of this technologycannot be known, the author, the Prince of Songkla University,JourneyToForever.org and UNESCO cannot be held liable for anyaccidents, problems or losses which may arise from its use: Like allgood developmental technology, the responsibility for its use mustultimately depend upon the user!