Chemical Recovery from Black Liquor and Production of Paper | Chemical Technology - Chemical Engineering PDF Download

27. 1 Introduction 

The black liquor (lignin-rich) produced from the Sulfate pulp production contains 95-98% of digested chemicals. To reduce air and water pollution and to balance economy of operation, these chemicals should be removed before disposal.
In chemical recovery process, black liquor is concentrated, burned and limed.27. 1 Introduction The black liquor (lignin-rich) produced from the Sulfate pulp production contains 95-98% of digested chemicals. To reduce air and water pollution and to balance economy of operation, these chemicals should be removed before disposal.

In chemical recovery process, black liquor is concentrated, burned and limed.

Figure 27.1 Flow sheet of chemical recovery from black liquor

27.2 Functional role of various units (Figure 27.1)

(a) Multi effect evaporator

• It is a series of single effect evaporators.
• Multi-effect evaporator and disc evaporator concentrate the black liquor from 15-18% solid to 60-65% solid.

(b) Mix tank

• It is designed with two agitators to mix the product.
• Make up chemicals (Na₂SO₄ + S) are added. 

(c) Boiler

• It produces molten slag.
• Here organic carbon present in black liquor is burned.
• High pressure 28 atm to 30atm steam is produced.
• The reaction that takes place inside furnace is 

Na₂SO₄ + 2C→ Na₂S + 2CO₂

(d) Dissolving tank

• The molten chemical smelt dissolve immediately with cold water and yield green liquor (aqueous Na₂CO₃).

 (e) Clarifier

• It is used for filtering.
• It separates calcium carbonate sludge and white liquor.
• Filtering medium is monel metal.

(f) Washing tank 

• Impurities are washed away in this tank after clarifying.

(g) Causticizing tank

• It precipitates lime mud.
• Carbonate is causticized by adding Ca(OH)₂ 

27.3  Production of Paper (Figure 27.2) 

Paper is a sheet material made of fibers which are held together by hydrogen bonds. In wet process, fiber suspension in water is made and it is then made into sheet and dried. Paper making process is generally done on Fourdrinier machine. 

Figure 27.2 Flow sheet of production of paper

27.4  Functional role of various units 

(a) Beater

• Beater mechanically disintegrates the pulp fibers to make paper stronger, uniform, dense, opaque etc.
• It consists of metal blades attached with rotating drum.
• Finely ground fillers (to increase brightness, flexibility, softness and weight) and coloringagents  are also added.

(b) Jordan

• It is a conical refiner or Jordan engine.
• Metal bars and stones are set inside.
• Here, pulp is deformed, defibered and dispersed.

(c) Web forming

• 99.5% water fiber slurry are made to run on an endless belt at a speed of 50m/min to 500m/min.
• Pulp fibers are arranged into web.
• Water is drained out by gravity.
• Shaking motion is provided for better interlocking of fibers on mat.
• White water is collected and it is reused to conserve water and additives and to avoid pollution.

(d) Pressing

• Free water is removed by pressing with pressure roll, water mark roll and suction roll.
• Water content is reduced to 60-65% water.

(e) Drying

• Additional water is removed by smoothing rolls and series of steamheated metal drying rolls.
• Water is reduced from 60-65% to 5-6%.

(f) Finishing

• Here paper is passed through a series of calendaring rolls for producing smooth paper.
• It is wounded on a large winding roll.

27.5  Technical Questions 

1 Why are make-up chemicals added to the mix tank before the smelter?

Ans: White liquor chemicals  (Na₂S+ NaOH) are lost in the pulp manufacturing process in various ways.  These include loss along with pulp, waste sludge, wood knots and undigested residue.  Though significant portion of the make-up chemicals are recovered from the recovery plant, the make-up chemicals are still required to add up for the loss. These chemicals are added to the mix tank for smelting reaction. This is because Na₂SO₄ is not desired in the digester and Na₂S is desired which is  generated from the smelting reactions.

2 Discuss energy economy in the paper and pulp manufacturing process

Ans:

• Chip dust is burnt in the paper mill boiler to reduce the consumption of purchased fuel.
• Half of total fuel and electricity used is self-generated from biomass (primarily from spent pulping liquors, wood residues, and bark) and thus reduces energy costs.
• By products of paper industries such as tall oil, rosin, turpentine, gums, resins are well utilized.
• Steam produced in theboiler is used in various processes (causticizing and heat exchangers associated to the digester for temperature control).
• Flue gas produced in the furnace is used in the disc evaporator.

3 The smelt produced by boiler is in solid, liquid or  gaseous phase?

Ans: Smelt is a molten salt mixture.

4 How areenvironmental issues taken care in pulp and paper production?

Ans:   

• Chip dust is burnt in the boiler to eliminate environmental problems associated with chip dust disposal.
• Bleaching of pulp with chlorine produce dioxins and other undesirable products.So chlorine is replaced by hydrogen peroxide.
• White water is collected and  is reused to conserve water & additives and to avoid massive pollution.
• In pulp production process, hot water which is added in second filter is reused in first filter to conserve water.
• Waste-water treatment involving incineration is usually associated with the paper and pulp manufacturing plant.  This solves both environmental and energy issues of the pulp and paper plant.

5What is white water?

Ans:White water consists of flour (minute fibers) that drains from the wire part of the paper manufacturing machines.  It consists of about 0.5 % of pulp.

6 Why is bleed stream taken out?

Ans: White water contributes to the slurry viscosity and blending issues of the pulp + water mixture.  Since beyond a desired viscosity the pulp + water mixture would not provide good quality paper for fixed choice of other operating parameters of the paper manufacturing process, white water needs to be bleed out.  Also, usually, white water from the machinery where web forming is carried out is usually recycled and white water emanating from the pressing machines is sent for wastewater treatment and appropriate disposal. 

7 Explain how dryer performance can be controlled to obtain good paper quality in quantity.

Ans:The dryer needs to operate in such a way that the temperatures of different rolls  and their speeds be optimized.  This is due to the fact that paper with about 60 % water enters the dryer and leaves with about 5 % moisture in the paper.  Therefore, the drying rolls need to be efficient to quickly remove the moisture using steam heated rolls.  A variation in the steam heated rolls temperature is also expected, as progressively variant temperatures along the rolls will provide maximum removal efficiency of the paper.  In summary, there exists trade-offs with respect to the roller speed and roller temperature. Subsequently, the roller temperature is controlled using steam flow rate. In other words, both steam flow rate and roller speed contribute towards the dryer efficiency and providing required quality control for the manufactured paper.

8. Why is web forming process   supported by the  endless belt ?

Ans:Aslurry of 99.5 % cannot withstand its own weight anddisperses away.  Therefore, the endless belt would act as a support system to gradually form the web as the solution progresses ahead.