Lime is used in multiple applications in the pulp and paper manufacturing industry.
Lime with especially consistent reactivity, high-purity, and high-brightness is used in the production of PCC (precipitated calcium carbonate). PCC is manufactured in a series of controlled chemical reactions. First, quicklime, CaO, is slaked with water to form a milk of lime (slaked lime slurry), to which in a second step carbon dioxide gas is added.
The resulting reaction consumes the carbon dioxide and produces a very fine precipitated calcium carbonate (CaCO3). The shape and size of the PCC crystals are modified by precipitation conditions (temperature, CO2 and lime hydrate concentration) during the reaction. This PCC is used as filler and coating pigment material in paper production to enhance paper's brightness, opacity, bulk, runnability and printability.
In the Kraft pulp production process, lime is used to convert sodium carbonate, contained in so-called green liquor, into soluble sodium hydroxide (caustic soda, NaOH), resulting in so-called white liquor on the one hand and calcium carbonate as a precipitate on the other, here also called lime mud.
This conversion reaction, called causticization, regenerates NaOH - a key reagent – together with sodium sulphide. Both are required to digest, i.e. convert, wood into paper pulp. The precipitated calcium carbonate is filtered and calcined in an onsite kiln to produce quicklime (CaO) that is re-used in the causticization reaction.
The precipitated calcium carbonate sludge (lime mud) also contains some impurities, necessitating disposal of a portion of the lime mud to act as a bleed for impurities from the circuit. This loss of calcium reagent from the recovery circuit typically necessitates the intake of make-up lime ranging from 5-15% of the total quicklime consumption of the causticization process during times of normal operation, and 100% during periods when the lime kilns are not operational, e.g. due to maintenance.
Recycled paper and board are produced from recovered wastepaper. Wastepaper contains sizing agents, glues, printing ink and other impurities that pose challenges to the paper production process. The impurities contained in wastepaper contribute to microbial growth, causing biofouling and noxious odours. Lime is used as a pH control agent to optimize the use of biocides for the control of microbial growth.
Lime is also used in paper recycling when excessive hardness occurs in the pulp. Here lime acts to precipitate soluble carbonate thus removing it from the water (pulp dehardening) and preventing carbonate scaling in process equipment, as well as in downstream biological water treatment units of the paper plant.
Milk of lime and magnesia can also be used as an alkaline reagent in the oxidative bleaching of mechanical, chemical and recycle pulp. As lime and magnesia have a lower, buffered pH than caustic soda, for example, they provide more benign and consistent bleaching conditions, resulting in less oxidative damage to paper fibres and stronger paper.
Bleaching with lime and magnesia also results in more selective bleaching, targeting specific discolouring functional groups of the fresh or recycled fibres, and thus generating fewer unwanted by-products, less fibre loss and dissolved organics in wastewater. The pulp or paper producer can also achieve higher fibre yield and reductions in emissions.