03-07-2012, 03:54 PM
Ballarpur Industries ltd. Yamunanagar on Soda Recovery
Soda Recovery.pptx (Size: 667.32 KB / Downloads: 58)
ABSTRACT
History-> The word paper derives from the Greek term for the ancient Egyptian writing material called papyrus, which was formed from beaten strips of papyrus plants. The soda process for making pulp from wood was developed by Watt & Burgess in 1851. NSSC pulping yields highest fibers up to 65%-80% which results in reduced stream pollution problems. But quality and bleachability is low.
Types-> Chemical pulp is of three types
1) Kraft or Sulfate Pulp (Alkaline)
2) Sulfite Pulp (Acid)
3) Neutral Sulfite Semi-Chemical (NSSC) Pulp
Density->The density of paper ranges from 250 kgm-3 (16 lbft-3) for tissue paper to 1500 kgm-3 (94 lbft-3) for some specialty paper. Printing paper is about 800 kgm-3 (50 lbft-3).
In BILT, Unit: Kraft Process is employed for pulping. Veneer, Veneer chips, Bamboo, Poplar wood and Poplar rulla, Eucalyptus wood & rulla etc. are the raw materials used in that unit due to their easily availability in nearby areas.
SODA RECOVERY
INTRODUCTION :
Recovery of chemicals has always been considered as the backbone of pulp and paper industry in the phase of increase of pulping chemicals. Strict laws of pollution control and power crisis arising from mushrooming of industries the recovery of pulping chemicals has become more important.
THE KRAFT PROCESS
INTRODUCTION:
Kraft process is one of the main chemical pulping .
Kraft or sulfate pulping is an alkaline process .
It is cooked with a strong (12%) solution of NaOH and Na2CO3. The material added to the cooking liquor for the Kraft process is Na2SO4, hence the common name of sulfate process.
The cooking however is done with a solution containing Na2S, NaOH and Na2CO3 formed from the sulfate during preparation and recovery of the cooking liquor.
All sorts of wood can be cooked by the Kraft process and the fibers produced are bleachable and strong.
It is very important that the chemicals used can be recovered and recycled, reducing or even eliminating stream pollution.
RECOVERY OF THE BLACK LIQUOR
The B/L removed from the pulp washer contains 95 to 98% of the total chemicals charged to the digester. Organic sulfur compounds are presented in combination with sodium sulfide. Na2CO3 is present as are also small amounts of Na2SO4, salt, silica and traces of lime, Fe2O3, alumina and potash. Total solids usually average about 20%. Then black liquor is concentrated, burned and limed.
In the smelting furnace any remaining organic compounds are broken down, the carbon burned away and the inorganic chemicals melted. At the same time the following reaction takes place:
Na2SO4 + 2C Na2S + 2CO2
The carbon comes from the organics in the wood.
Evaporator Section :
In this section black liquor is concentrated to a required concentration and sent to the recovery boilers for heat recovery & formation of green liquor. The black liquor is concentrated with the help of evaporators. In this section there are mainly two types of evaporators, one is free flow falling film evaporator (FF) and other is short tube vertical effect evaporators (STVE).
ROLE OF RECOVERY BOILERS
Feed - concentrated B/L coming from the evaporators. At both of these boilers B/L is fired to recover the heat content of the spent cooking liquor in form of production of high pressure steam. The lignin in the liquor is burnt away and what remain are the chemicals (in the form of smelt).
This smelt is then dissolved in weak white liquor coming from Causticizing Plant, which gives a characteristic green color (hence called green liquor; G/L).
This G/L contains some solid impurities which are separated out by means of a clarifier. The main aim of these boilers is to recover all unused chemicals from the B/L stream in form of G/L. Chemical composition of G/L is as follows:
Na2CO3 (70%); NaOH (15%); Na2S(15%)