Producing paper, like other industrial processes, leaves behind huge amounts of solid waste. It is estimated that for every tonne of paper produced 650 kilograms of ash and 600 kilograms of lime sludge are generated. Considering that India produces more than 20 million tonnes of paper, disposal of this waste poses environmental problems.
This is the problem that Prabhat Vashistha, a young researcher in the CSIR-Central Building Research Institute, Roorkee is tackling. So first he analysed the problem in detail.
Lime sludge, a wet cake of calcium carbonate in a weak alkali solution, is generated in the chemical recovery process of the paper industry. It cannot be reused since the sludge contains silica which creates scales in lime kilns. It could be used for stabilising soils that expand and contract with increase and decrease in water content. So it is useful in building roads. However, most paper mills in India are not located in such areas.
Lime sludge could be used to replace cement in concrete but the strength of the concrete is diminished. However, if the calcium carbonate is transformed into dicalcium silicate, it can be used as belite clinker – a strengthening material in portland cement.
Silica is available in plenty as boiler and fly ash, other waste materials from the paper industry. So Prabhat, guided and mentored by S K Singh, CSIR-CBRI and supported by Dharm Dutt, IIT Roorkee and Vivek Kumar, IIT Delhi performed a series of experiments.
They procured boiler and fly ash and lime sludge from a paper mill. The boiler and fly ash were products of the combined combustion of coal and agricultural waste. They found that, though fly and boiler ash both had silica in comparable quantities, fly ash had more aluminium. So they used boiler ash to extract silica.
First they treated the boiler ash with sulphuric acid to remove any metallic impurities. The silica in the boiler ash was then converted to sodium silicate by treating it with sodium hydroxide. Then they treated it with sulphuric acid again to recover nanosilica – spherical particles with diameters ranging from 10 to 25 nanometres.
The nanosilica was mixed with lime sludge in a ratio of about 1:3 by weight and pressed into pellets and heated to about 1000 degrees to get belite clinker. Then the researchers crushed the pellets and mixed them with blast furnace slag, another form of industrial waste, ground to granule size. The result was a cementitious binder.
The material showed good workability and had shorter dormant time than portland cement.
After seven days of curing, the mortar had more compressive strength than required for use in masonry, but lesser than required for high strength concrete. There was no deformation or shrinkage even after 21 days of curing.
The belite clinker was synthesized by firing at a temperature of 1000°C, less than the temperature usually used in the cement industry. So the process is more energy efficient than producing portland cement, says Prabhat Vashistha, CSIR-CBRI.
The cost of production at laboratory scale is comparable to that of portland cement. But when scaled up, the cost of production will come down to make it commercially attractive, says S K Singh, CSIR-CBRI.
There are more than 600 paper producing industries in India that can economically benefit from converting the waste into raw material for the construction industry. And, in the process, reduce environmental damage.
Cleaner Production, 224: 557-565 (2019);
Khushbu K Birawat
Global Academy of Technology
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