Burning lime

For interest – you’re probably not going to do this, but…

If you do intend to do it, you must buy Michael Wingate’s book, Small-scale Lime Burning; it covers all aspects of setting up a small lime-burning operation, and discusses quarrying, site operations, fuel, raw materials and kiln design.

How widespread is lime?

  • Not sure if there are any countries that don’t have limestone. Deposits are usually very thick; around 10% of all the world’s sedimetary rocks are limestones.
  • Limestone is calcium carbonate, but so is coral and seashells – and these can be burnt to produce quicklime too. Not a good idea though – coral reefs are precious and endangered.
  • Wood is the best fuel for burning lime (see above), and trees are found pretty much everywhere too (have to be re-planted for this to be an environmentally-friendly option though – Wingate estimates around 14 hectares of tree plantation are needed for each daily tonnage of quicklime produced by the kiln).
  • A relatively small outcrop of limestone can produce quite a lot of lime. a deposit of limestone one metre thick (and they are usually thicker) covering just one acre would yield around 30,000 tonnes of limestone, which would allow a small kiln, after quarrying losses, to produce 3 tonnes of quicklime per day for 10 years, bearing in mind that a tonne of limestone will yield slightly less than half a tonne of quicklime (Wingate, Small-scale Lime Burning).
  • Best to burn a sample of the local limestone as a trial to see what kind of quicklime it produces, and then to slake it to see what kind of lime putty it produces.

Kilns

  • Wingate’s book covers different types of kiln, from heaps of wood and limestone (i.e. no kiln at all), to pits, domes, basic kilns and very expensive and sophisticated modern kilns.
  • Simple kilns can be built of brick, but with lime mortar (firebrick and fireclay mortar around the burn zone), and definitely not cement, as it is inflexible and will crack as it expands with the high temperatures; kilns can be made of stone too, but (of course) make sure it’s not limestone, as your kiln will turn to quicklime!

Technique

  • Using wood to burn limestone means relatively low temperatures (around 900°C) which produces good, reactive quicklime. The process is relatively slow using wood at low temps. Another reason that wood is a good fuel for lime burning is that it produces a long flame, without any concentrations of areas of high temperature. This is important in burning the lime evenly, resulting in a uniform batch of quicklime with no ‘underburnt’ or ‘overburnt’ parts.
  • In most small-scale, middle-of-the-road kilns (i.e. not basic ones or very expensive ones) the fuel and limestone are added from the top (i.e. in a ‘vertical shaft’ kiln), and lumps of quicklime fall and are removed from the bottom. it is a continuous process – the fire in the kiln doesn’t go out unless the kiln needs to be inspected or repaired.
  • These kind of kilns can be home-made, but there are off-the-shelf models available too.
  • Layers of wood and lumps of limestone are built up in the kiln – about 5 of each, with kindling at the bottom. The limestone is in lump form so that there will be gaps between the limestone and fuel through which the flames, heat and air can rise.
  • The skill of lime burning is in keeping the fire about half way up the kiln, with new material being pre-heated above, and quicklime cooling below, waiting to be removed.
  • For every tonne of limestone burnt, around 250kg of wood is needed.
  • Coal, coke or gas can be used as a fuel, but why use a fossil fuel when you can use a renewable fuel, especially when the renewable fuel produces better results? Charcoal can also be used, but again, wood produces better results, so why not just cut out the charcoal-making process and use wood in the first place.
  • Modern kilns often don’t use wood, and achieve temps of around 1200°C to speed up the reaction. Over this temperature, the pores of the lime can be closed and the quicklime will be unreactive.
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