As the name suggests the recovery section was installed to recover the cooking chemicals from the Black liquor received from the Brown Stock Washing section. Although, this section is much cost-intensive it needs to be installed to avoid the use of 100% fresh cooking chemicals. As the losses in the Brown Stock Washing is 8-12%, the requirement of the fresh chemicals will be only 8-12%.

The main subheads in the Recovery plant are,

• Multiple Effect Evaporators
• Low-Temperature Incinerator
• Recaustisicing Plant

Multiple Effect Evaporator Section

The Black Liquor received from Brown Stock Washing Section is having a concentration of 0.8- 1.5% depending upon the use of Raw Material. The Concentration of Black Liquor is increased to 25-30% in the evaporator section. There are different types of evaporators being used in the paper industry depending upon the quality of Black Liquor. These are Long Tube Vertical evaporators, Falling Film Evaporators, Natural Circulation Evaporators, etc.

The most common evaporator being used in the paper industry are Falling Film Evaporators which are having advantages over Long Tube Vertical Evaporators, which are as follows.

• In Falling Film Evaporator, the black Liquor is pumped from the Vapour Head to the Distribution Plates located above the Tubes allowing the liquor to fall down within the tubes.
• The Falling Film Evaporator working at a low Temp. Difference and which allows many effects to be installed in series. It also has a better steam economy than other types of evaporators.

Low-Temperature Incinerator

The basic concept of this system is the use of fluidized bed technique for the treatment of black liquor received from Multiple Effect Evaporator to recover the Pulping chemicals as well as to reduce the pollution load on the environment. The Low-Temperature Incinerator is used in the paper mill, where the Black Liquor solids are less than 300 tons per day.

IN LTI, The solids in the liquid waste containing heat energy are amenable. The advantage of LTI is,

• Elimination of Air as well as Water Pollution.
• Removal of BOD & color, etc. from residual organic products.
• Removal of all particulate matter & gases before discharged into the atmosphere.

The system consists of mainly

• Low-Temperature Incinerator Proper With Accessories
• Venturi Scrubber System with Cyclone Separator
• Attrition Mill and other Accessories
• Air Pre-heater with HSD Burner

The Semi Concentrated Black Liquor from the evaporator is fed to Venturi Scrubber, which works on the Direct Contact Evaporator. The Concentration of Black Liquor is increased to 45%. This Black Liquor is fed to LTI- where the Temp. Is around 600-700 Deg. C is maintained. The water in the Black Liquor is evaporated & the solids are converted to Soda Ash.

The Soda Ash produced is withdrawn from the side of the reactor at a controlled rate by a variable speed Rotary Valve. The Soda Ash is conveyed through Water Cooled Screw Conveyor before being stored in a Silo.

Recaustisizing Plant

The Soda Ash (NA2CO3) produced in the LTI- is further treated with milk of lime (Ca(OH)2) to get Caustic Soda (NaOH) which is used in the Digester for the Cooking of the Raw Material & the Calcium Carbonate (CaCO3) produced is either used as Landfills or sold to Cement Industry.

The Major Equipment in Recaustisicing Section is as below.

• Lime Slurry Preparation
  • Table Feeder: A table Feeder is installed to feed the lime to slaker at a controlled rate.
  • Rotary Drum Slaker: In Drum Slaker, the water is added along with the lime to make the slurry. The slaker rotates at a fixed speed & required time is given to complete the reaction. The grits in the lime are removed from the slaker.
  • Classifier: The green liquor from LTI Section is mixed with Lime Slurry in Rake Classifier. Any grits in the system are removed in the Classifier.
  • Causticizer: The Slurry from Rake Classifier is pumped to Causticizer, which is installed in series to provide the required retention time to complete the reaction. The Causticizer has an agitator & the size depends on the size capacity.
  • White Liquor Clarifier: The slurry from Causticizer is pumped to White Liquor Clarifier, which is provided with a rake mechanism having blades to ring the sludge in the center. The Overflow from the clarifier is taken to the Storage Tank, from where it is pumped to the Digester House to be used as Cooking chemicals.
  • Recausticizers: The underflow from White Liquor Clarifier, is taken to Recausticizers- where it is mixed with Washed Liquor from Ist stage Lime Mud Washer.
  • Lime Mud Washer: Lime Mud Washers are similar to White liquor Clarifier, which are provided with Rake Mechanism. The Lime Mud Washers are provided in Series to recover the chemicals from the Lime Sludge & the wash water is used in the Counter-Current system.
  • Lime Mud Filter: The Sludge from the final Lime Mud Washer is pumped to Lime Mud Filter, which works on the vacuum. The inlet consistency of lime sludge is around 1-1.5% and outlet consistency is around 60%. The lime sludge from the lime mud filter can be used either in landfills can be sold. Lime Mud Filter is provided with a filtrate tank.

The pulp after washing contains uncooked shives which are required to be removed before the pulp is used for kraft paper making or to be used in the bleaching section to make white grade papers.

Screening is carried out in 3 stage screening process with a centrifugal screen in the primary stage. The accept from the primary screen is further cleaned in the centricleaning system & the accept of centricleaning system is thickened in the Vacuum Thickener to be stored in the High-Density Tower at the consistency of around 10%.

The rejects from the Primary screen are further cleaned in the secondary screen to recover the pulp from the rejects.

The aim to install Brown Stock Washing System is to wash the pulp after the cooking process to recover the chemicals & other organic compounds. The washing system runs on the Counter-Current washing system, where the hot water is used on the final washer and the black liquor is sent to the Recovery section from the First washer. The washers are designed to work on Barometric Leg Vaccum System.

The inlet consistency of the washer is generally 1.5-2.5% and the discharge consistency is around 10%. Shredder Repulper is installed at the inlet of the next washer to make a homogeneous mixture of 1.5- 2% consistency by using the filtrate of the previous washer.

The Filtrate from each washer is collected into the Filtrate Tank & is used in the previous washer for the Dilution purpose. As the liquor is very dilute in nature, there is a tendency for Foam Generation, which is controlled by installing Foam Breaker on a Foam Tank.

Chips from the storage silo are loaded into the digester, where it is cooked with cooking liquor (Caustic Soda) at a Temperature of 170-175 Deg. C & Pressure of around 9 kg/cm2. The cooking is carried out by an Indirect heating process to remove lignin & other organic compounds. After attaining a desired pressure & temperature, It is kept at the same pressure & temp. for 2 hours. Then, the whole mass is blown to the Blow tank under pressure. The Vapors coming out of the blow tank are utilized to heat the water in the blowing heat recovery system.

Vertical Stationary Diester With Preheater:

Digester has got cylindrical portion in the center & a conical shape at the bottom & top end. It has got screen plates at the top, middle & bottom. Material of Construction of digester is SA 516 Gr. 70 & the thickness is approx. 36 mm. The designed pressure is 10.5 kg/cm2 & test pressure is 16.5 kg/cm2. The screen plates MOC are AISI 304. Digester body has got connection for putting liquor in the center & discharge at the top & bottom. Blow line to the digester is connected at the bottom, from where the complete mass is blown after the cooking cycle completes.

As the cooking process is carried out by Indirect heating method, Digesters are connected with Liquor Preheater- where the steam is injected at the outer body of the preheater & the liquor is circulated in the preheater through liquor circulation pumps in the tubes of the preheaters.

Blow Heat Recovery System:

Blow Heat recovery system is installed to condense the contaminated vapors originating from the blow tank during blowing of pulp from the Digesters. The water from the Accumulator tank is circulated to condense these vapors & raise the water temperature. This hot water is used to heat the fresh water from ambient Temperature to 70 Deg. C in plate type or Tube type heat exchanger. The heated fresh water is used in washing section.

Wood Logs having Diameters from 50- 250 mm and lengths of 6 meters are received in the factory. Debarking of these logs is carried out either at the source or in the factory as the bark is a Non- Fibrous material & not useful for the pulping process. Debarked Logs are fed to the chippers to cut into approx. size of 25 mm x 25 mm x 3 mm thick. So that, during cooking the liquor is impregnated into the chips to facilitate the cooking process. The Chips from the chipper are fed to the screens, where accepted chips are, and oversized materials & dust are separated. The accepted chips are stored in the Silo, from where it is fed to the Digester. The Oversize material is taken to rechipper & the chips are again fed to the screen. 

Chipper:

The Chipper consists of a fabricated drum with 4 No. chipping knives & 1 no. dead knife. 2 Nos. Feed rolls are provided in front of the chipper to pull the logs. The chipper is normally provided with a startup motor with VFD to avoid the Jerk load during the Startup.

Generally, the Hydraulic system is provided for the hood & Dead knife changing. Limit switches & Rotation sensors are provided to avoid accidents.

Chip Screen:

Chip screen is the triple-deck- two separate accepted chips, oversized chips & dust. The screen is a rope hanging with an oscillation arrangement. The accepted chips- mesh size is 40 x 40 mm and Dia. 6 mm perforated screen for the dust. The screen has got movable eccentric assembly with bearing housing & base frame. One side is coupled with Vat & another side with a counterweight for balancing.

Rechipper:

Rechipper is identical to the main chipper with lesser capacity to feed the oversized material received from the chip screen. It has got three no. of fly knives on the drum & one no. of the deadly knife.

The purpose of cooking the agro raw material is to chemically dissolve the lignin & other organic compounds in order to obtain unbleached pulp. The cooking of agro based raw material like bagasse is carried out in horizontal continuous Tube Digester at the temperature of 160 Deg. C & pressure of 6.5-7 kg/cm2. The Main components of the digester system are.

• Screw feeder
• Inlet Chamber with Stop Valve.
• Digester tubes.
• Cold Blow Discharger
• Blow House.

The washed raw material from Aqua Separator is conveyed by a Belt Conveyor to the metering screw conveyor in the digester house. Bagasse from the Metering Screw conveyor is fed to the pin feeder & the excess raw material is returned back to the wet washing plant.

In the Pin feeder, Raw material is fed to the screw feeder to make it a compact type mass before it enters to the inlet chamber.

The Cooking chemical & steam is given at the inlet chamber, where it is mixed with the raw material before entering the digester tubes. A pneumatically operated Blow back valve is provided in the Inlet chamber to avoid blow back of the raw material in case of improper feeding before the digester tubes.

Digester is having two horizontal tubes equipped with horizontal screw conveyors- which conveys the material from first tube to the second tube. The steam is injected in the first tube to control the temperature & pressure. The Speed of the screw in the digester tube is maintained to obtain the desired Kappa Number.

The cooked Raw material- that is pulp, is blown to the Cold Blow Discharger, where the cold black liquor is injected. From Cold Blow Discharger- it is blown to Blow Tank, which acts at a storage for the cooked pulp.

Wet Cleaning System comprises of pulper, aqua-separator, DSM screens & Clarifier etc. It is used to wash agro-based raw materials to remove impurities like silica dirt and remaining pith. After washing of bagasse water will go to the clarifier along with impurities like sand & silica. Impurities will settle down there & it will store in a tank by underflow pump. Clarifier overflow water will be used for washing of bagasse. Efficient washing of raw material and removal of pith with maximum recovery of water are the main functions of the wet cleaning system.

Pulper:

The Dry depithed bagasse is fed to the pulper where it is diluted with water to a consistency of 2.5- 3%. With the high agitation of the pulper rotor, the dirt & pith is loosened, so that- they can be removed in the aqua separator. The Pulper is designed depending on the capacity of the plant.

Aqua Separator:

The Aqua separators separate the loosed and dissolved organic and inorganic soluble and fines and also simultaneously drain the suspension by gravity. The Raw material is lifted by an inclined conveying screw of special design over the dewatering section of the screw trough, the screw and the trough are of welded steel sheet construction. The bottom of the trough is provided with a perforated sheet plate for drainage. The Aqua Separator consists of an inclined conveying screw of special design with screw flight in SS 304. The outlet consistency from the aqua separator is around 15-20%.

B-2 Thickener:

The water along with some fiber & pith, coming out from the aqua separator- is fed to B-2 Thickener where the fiber is separated from the water suspension. B-2 Thickener has fine wire mesh & it is driven by CI Rollers, Girth Gear & Pinion. The Fiber & Pith from B-2 Thickener is fed to the Boiler along with the Dry pith. The water from the B-2 Thickener is further treated in the clarifier.

Clarifier:

The filtrate water received from B-2 Thickener is further treated in the Clarifier to remove the suspended solids. The Dust & Silica coming along with the water are removed in the clarifier. The underflow from the clarifier is used in landfills & the clarified water is reused in the pulper with dry bagasse.

Bagasse received from Sugar mills contains around 30- 35% pith @ total moisture contains around 50%. As pith has got no fiber value- it has to be removed to make bagasse fiber suitable for the pulp mill. The first step is to remove the pith by hammering action in the Depither.

Pin Feeder:

Bagasse is conveyed by belt conveyor to the Pin Feeder. The Pin Feeder is provided to regulate the flow of raw material into Depither. It consists of two rollers rotating against each other. The rollers are equipped with Pins which distribute the raw material in a controlled flow to the Depither. The Pin Feeder is driven by a variable speed motor via a reduction Gear Box. After passing through the pin feeder, loosened Bagasse falls into the Depither.

Dry Depither:

The Depither is designed to separate the dust from Bagasse. The Bagasse is fed through the inlet spout or chute at a specific feed rate to the top of the Rotor Assembly in which the Hammers are rotating clockwise (as one looks from the drive end i.e., from the top) at a pre-determined speed.

The Bagasse falling down between the Rotor Hammers and Screen will be battered (hammered) successively by each group of Hammers. The pith and dust is then forced through Screen by Centrifugal force and discharged through dust chute to a conveyor, which takes the dust either to boiler for burning or for disposal. The fiber is discharged through the fiber chute. The fiber is then taken to a belt conveyor to washing streets.