Pulp and Paper
Lime is an important commodity for the pulp and paper industry. Most pulp and paper facilities operate their own kilns, and regenerate lime after use. Thus, this industry is primarily a market for “make-up” lime from commercial manufacturers.
By far the largest application of lime in pulp manufacture is as a causticizing agent in sulfate (Kraft) plants. Here, the waste sodium carbonate solution is recovered and reacted with high calcium lime to generate caustic soda for reuse in the process. Most large sulfate plants recover about 90-98 percent of the lime by dewatering the waste calcium carbonate mud, then calcining it in rotary kilns.
Sulfite plants use quicklime to prepare calcium bisulfite, an acid cooking liquor used to digest or dissolve the non-cellulosic constituents of wood chips (paper and pulp stock). This process is on the wane due to waste disposal problems.
In pulp manufacturing processes, the interaction of lime and chlorine is used in the preparation of calcium hypochlorite bleach liquor. Calcium hypochlorite, the oldest known and lowest cost bleach, is used extensively to bleach the pulp to the desired degree of whiteness.
Lime is used in the treatment of pulp and paper mill liquid wastes as a coagulant in color removal, a filtration conditioner and to a lesser extent as a neutralizing agent. It is also used in the recovery of valuable by-products from pulp and paper mill wastes. It is used commercially to recover alcohol, calcium lignosulfonate, and yeast.