29 January 2003

Development of a Low Cost, Heat Resistant, Kiln Brick, for Production of Purifiers

Village potters in southerly, 'less developed countries' tend to fire their wares at low temperature, around 700 degrees centigrade.  Their kilns are quite adequate for their products, water containers, flower pots, etc., but the lower temperatures do not lead to the high strength that is preferable for a purifier that will have a long life time.  What is needed is a kiln that will fire to a slightly higher temperature, 900 to 1000C.  The village potters may well play an integral part in purifier production, but the guiding role may be more appropriately played by the entrepreneur/ developer, who enters the picture with an improved, appropriate kiln, starting with a less expensive brick 

It is important to note that it is easily possible to improve the capability of a kiln to reach higher temperatures, simply by enclosing it, sealing it against a lot of heat loss.  Next we need to consider that the biggest cost of kilns is the high tech bricks so widely believed to be necessary.  But the reality is that in brick production the simple addition of common 'creek sand'  to a common, red firing earthenware clay, plus sawdust, can be easily undertaken by those who are in the business of producing building brick.  This will give a heat resistant brick for kiln construction, at little additional cost to that of ordinary building brick. 

Talk to a ceramist about this kind of low cost alternative and you may find that the idea of an inexpensive 'refractory brick' flies in the face of convention.  The prevailing wisdom is that if you want a good brick you spend the money, usually about US$1.00 apiece.  So you end up with a kiln that costs $1000.00 or $2000.00.  But this high cost will only drive up the price of the purifiers, something that is unacceptable.

A Triaxial Diagram for Composition of Appropriate Refractory Brick

The real price of an appropriate brick may be about $0.10, that is 10 cents.  The brick should consist of common clay, common sand and wood sawdust as combustible.  The problem is that common clay, red brick, all by itself will not have the heat resistance needed for repeated firing of a kiln that will reach the 1000C, or so, that temperature best suited for production of durable purifiers.  So 'creek sand' as added, a material that contributes the desirable, additional heat resistance.  And sawdust, at about 60 mesh, can be burned out, leaving voids in the brick.  Sawdust can contribute greatly to an insulating brick, cutting fuel costs and as an aid to environmental responsibility.

The ranges of boundaries for the content per material, by weight, as indicated in the shaded areas of the triaxial, are: 1. Clay:  45 to 85%,  2.  Sand:  15 to 35%,  3.  Sawdust:  0 to 40%    Certain qualities of the materials prevent going outside these limits.  As to sawdust, as much as 40% would only be indicated in the case of very hard wood.  Softer wood sawdust would suggest closer to 20 to 30%, but a rough gauge of the amount to use would be 'that percentage weight' which give no more than 50% 'by volume.'

As to material attributes and limitations, note that if clay is less than about 45% it may not have sufficient capacity for binding the materials together, both before and after firing.  For sand, if this is less than 15% it will not be sufficient for the more heat resistant brick that is desirable. If sand is greater than 35% a kind of 'secondary shrinkage' will occur,  an increasing number of micro-cracks appearing over time, with each additional firing.  This can lead to the bricks breaking apart.  If sawdust is near the lower limit, zero content, the brick will be hard and not insulating.  This hard brick may be best suited to the outer wall of the kiln, the insulating brick for the inner wall.

In practice, in making the brick it may be a good idea to use 75% clay to 25% creek sand, then whatever amount of sawdust, whether hard brick or insulating.  A few bricks only could be made in the ratios 65/ 35 and 85/ 15, clay to sand, and these bricks could be set within the kiln, monitored over time for performance.  This may be necessary because every clay and every sand will perform a little differently, but the exercise may be valuable as an indication of the amount of materials that will be suitable for future bricks.

Hopefully many, if not most countries would have a local expert, capable of designing and building a kiln using such inexpensive bricks.  And the kiln will be high maintenance.  This is to say that after each firing it may be necessary to inspect the brick work, adding a little refractory mortar here or there and occasionally replacing bricks.  But there is a clear advantage in savings, an appropriate kiln costing perhaps 10 to 30% of what would be expected with 'standard refractory brick.'

So what would be the upshot of using purifiers fired at lower temperatures, say 700C rather than 1000C?  For the most part the life time would be somewhat shorter, perhaps six months rather than several years. 

Shown is a kiln at the compound of Ceramiques d'Afrique, located in Guinea, West Africa.  Firing at 700C the output of this kiln has been high quality, fuel efficient cook stoves.  This kiln should also be quite suitable for firing purifiers of life time that is shorter than optimal, perhaps six months.