Chemical Recovery in Pulp And Paper Industry

With increasingly high energy and chemical costs and stringent environmental regulations nowdays, the need for improved recovery of chemicals from the pulp and paper making process has been a critical economic factor in the industry. It is essential that paper mills maximise steam and power production capacity, reduce recirculating chemical dead loads, and minimise chemical losses. Paper is an essential outcome from the forest industry, used in various forms and shapes. These different forms of paper are made by pulping of the wood, bleaching the pulped wood, spreading these bleached pulp into sheets ultimately converting it to paper. With different stages of designing the paper, various chemicals are used to give the paper its present properties, such as the bleaching chemicals that make paper white (and which also enable it to subsequently be colored). Sulfate or Kraft pulping is the main pulping process was invented in Germany in 1884 and remains the dominating technology today. Usually known as “kraft pulping” which relies on a combination of heat, chemicals and mechanical pulping to convert the wood into a smooth, soft pulp suitable for use in paper making.

The Kraft pulping process has three main functions:

• minimizing the environmental impact of waste material (black liquor) from the pulping process.
• recycling pulping chemicals, NaOH and Na2S;
• co-generating steam and power.

For every single ton of pulp to be produced, the kraft pulping process generates about 10 times more tons of weak black liquor or dry solids that are further processed through the chemical recovery process. This generation of black liquor in such a large quantity makes black liquor the fifth most important fuel in the world, along with coal, oil, natural gas, and gasoline. As black liquor is produced from wood, it is one of the most important renewable bio-fuel.

Advantages for Chemical Recovery Process:

• It can be used with virtually all wood species.
• It can easily handle the extractives in most coniferous wood
• The pulp has very good strength.
• The recovery process for the chemicals is well established.
• More effective at removing impurities like resins.

This traditional Kraft Pulping has undergone many changes since 1884, and its till have many opportunities in future. Some the recent trends are:

Gasification

Developments in the chemical recovery process, has introduced black liquor gasification which proves to be a potential replacement for recovery boilers. Black liquor gasifiers with this capability are still in the development stage and full-scale implementation is still a futuristic approach.

Borate Autocausticizing

The process involves adding sodium borate into the liquor system so that it forms tri-sodium borate (Na3BO3) in the recovery boiler smelt. This technology has the potential for completely eliminating the causticizing plant and the lime kiln, making the kraft process much simpler.
 
The chemical recovery process dictates the quality and quantity of the white liquor, which in turn, limits pulp production and the profitability of the kraft pulp mill. There are numerous ways in which the economics, energy efficiency, and environmental protection associated with the recovery process can be improved.
 
With today’s increasingly high energy and chemical costs, and stringent environment regulations that limit particulate and gaseous emissions, solid waste disposal and mill effluent discharge, the need for improved recovery of energy and chemicals from the black liquor has become a critical economic factor in kraft pulp mill operation. It is essential for mills to maximize the steam and power production capacity, reduce recirculating chemical dead loads, and minimize chemical losses. The reliability and efficiency of recovery boilers, evaporators, causticizing plants and lime kilns have a direct impact on the quantity and quality of white liquor, and ultimately the quantity and quality of pulp produced by kraft mills.
 
The greatest opportunity for increasing energy and chemical recovery efficiency and for improving overall operating performance at existing kraft pulp mills is through advanced energy integration and mill-wide control.