Processing of pulp to the production of paper

The pulp and paper industry has extended production beyond fulfilling the demand for paper, to include heavier grade products; packaging such as plain and corrugated liner board for shipping containers.

In keeping up with these demands in recent times, chemical pulping has been more prevalent to improve the efficiency of wood processing to explore the better release of cellulose from timber. Beyond the pulp and paper mill, subsidiary operations can include plywood, particleboard, chipboard manufacture, and production of alpha-cellulose for cellulose acetate.

Water as a raw material in the pulp and paper manufacturing process

The modern pulp and paper mills are located at early mills sites and use hydraulic or hydroelectric power in the grinding operations. The use of water and its treatment requirements largely depends on individual manufacturing processes, which include pulping, bleaching, and wastewater discharge. The industry consumes a large quantity of water due to the washing of the pulp at several points in the manufacturing process and the transport of pulp fibers through various refining operations.

Chemical Pulping: The Kraft Process

The Kraft Process is one of many examples of chemical pulping used in the industry and contributes to 75% of the total pulp output in the United States. In the digestor, a mixture of wood chips and white liquor (sodium sulfite and sodium hydroxide) are measured and brought up to a temperature of about 177 °C. The operational temperature is achieved through direct steam injection or by recirculation of liquor through a steam-driven heat exchanger.

With the wood chips and white liquor being a great contributor of calcium after kraft pulping conditions, it increases the chances of calcium carbonate precipitation. The use of CrestoGuard 556A effectively targets calcite scale formation and alleviates the problem of deposition occurring in the digesters, reducing any unwanted disruptions assigned to cleaning and scale removal. 

After the cooking process, the digester contents are directed to a blow tank for recovery and treatment of valuable by-products and noncondensible gases respectively. The pulp is sent to washers to remove the spent pulping liquor to prevent the reduction in pulp quality. Countercurrent water flowing through the pulp collects the pulping chemical to produce a 'weak black liquor' containing about 10-12% total solids.

The 'weak black liquor is concentrated to over 55% of total solids in an evaporator to form a 'strong black liquor'. In a mix tank, sodium sulfate salt cakes are mixed with the black liquor to make up for chemical losses in the system to regenerate sulfur-bearing liquor. The liquor pumps into a recovery furnace and is dehydrated when sprayed onto the walls. Secondary combustion takes place in the furnace, with the intense heat of fusing the inorganic elements of the 'black liquor' to form a molten chemical known as smelt.

The smelt runs into a dissolving tank where it is mixed with water to produce a 'green liquor'. The 'green liquor' is then clarified to remove any dregs or suspended solid particles. The clarified liquor is then reacted with lime to initiate the slaking and causticizing reactions. The causticizing reaction between calcium hydroxide and sodium carbonate produces calcite and pirssonite scales, which can clog the pipelines to and fro the dissolving tanks. CrestoGuard 455 effectively inhibits the mixture of scale formation to allow the pipelines carrying the 'green liquor' to be free of deposition. Reduced deposition decreases the occurrence of unscheduled downtime needed for cleaning and ensures productivity with continuous operations.

With the completion of causticization, 'white liquor' is regenerated from the 'green liquor' and reused in the cooking process in the digester.