Sulfate black liquor

Case A is a standard sextuple effect evaporator compared against mechanical compression for evaporating sulfate black liquor at a rate of 300,000 Ib/hr (1978). 

Black Liquor Soap Acidulation. Only two-thirds of a typical black Hquor soap consists of the sodium salts of fatty acids and resin acids (rosin). These acids are layered in a Hquid crystal fashion. In between these layers is black Hquor at the concentration of the soap skimmer, with various impurities, such as sodium carbonate, sodium sulfide, sodium sulfate, sodium hydroxide, sodium Hgnate, and calcium salts. This makes up the remaining one-third of the soap. Cmde tall oil is generated by acidifying the black Hquor soap with 30% sulfuric acid to a pH of 3. This is usually done in a vessel at 95°C with 20—30 minutes of vigorous agitation. Caution should be taken to scmb the hydrogen sulfide from the exhaust gas. 

Recovery of inorganic chemicals is crucial to the cost effectiveness of the Kraft process. The black liquor which is obtained from delignification is rich in solubilised lignin and carbohydrate degradation products and, after concentration, is combusted in a recovery furnace. The Carbon dioxide which is produced during combustion converts unused sodium hydroxide into sodium carbonate. In addition, the sodium sulfate is converted, under the reducing atmosphere of the furnace, to sodium sulfide. 

Table 4 shows the typical ranges of black liquor constituents and characteristics of Kraft evaporator condensates. The composition of liquors may vary significantly, depending upon the type of raw material used. Inorganic constiments in black liquor are sodium hydroxide, sodium sulfate, sodium thiosulfate, sodium sulfide, sodium carbonate, and sodium chloride [11]. 

A number of studies have evaluated the toxicity of pulping hquors, in particular the black liquors generated from Kraft mills. Table 8 shows a partial representation of toxicity data compiled by the NCASI (National Council of the Paper Industry for Air and Stream Improvement) and McKee and Wolf for Kraft mill pulping wastewaters [15,16]. The table indicates that hydrogen sulfide, methyl mercaptan, crude sulfate soap, and salts of fatty and resin acids are particularly... 

Two other important side products of the kraft process are sulfate turpentine and tall oil. The turpentine is obtained from the gases formed in the digestion process. From 2-10 gal of turpentine can be obtained per ton of pulp. Tall oil soap is a black viscous liquid of rosin and fatty acids that can be separated from the black liquor by centrifuging. Acidification gives tall oil. These side products will be discussed later. 

Black liquor (a heated mixture of sodium carbonate, sodium hydroxide, sodium thiosulfate, and sodium sulfate) [34]... 

The production of wood pulp from lignocellulosic materials by treatment with various chemical liquors, particularly the neutral sulfite semichemical process and the kraft or sulfate process, gives residual black liquors. These contain salts that carry acetic acid and formic acid equivalent to 5% or more of the dry weight of the wood. 

Liquors from Neutral Semichemical Pulping and from Kraft Pulping. In making neutral sulfite semichemical pulp, the black liquors may have about 10 parts of water to 1 part of total solids, of which about one-third is sodium acetate and sodium formate. After evaporation to about 1 part solids to 1 part water, sulfuric acid is added to the concentrate to free the acetic and formic acids. When the concentrate is extracted with acetone, the mixed acids are obtained, the acetone is separated for recycle, and the acids are concentrated and refined. The raffinate is stripped and is passed to the usual furnace to be burned for recovery of the inorganic salt values. This process gives a smelt of sodium sulfate, which may be used in the kraft process as chemical makeup. The loss of the fuel value of the acetic and formic acids is practically negligible. 

Hence, a part of the black liquors (10-50%) of a kraft plant may be processed to free, and subsequently to recover, the acetic acid as described by the addition of sulfuric acid and the formation of sodium sulfate. This new sodium sulfate from the sulfuric acid is present in the raffinate from the extractor it is thus the makeup for the balance of the liquors (50-90% ) that can be processed conventionally without recovery of the acetic and formic acids. The liquors from the raffinate are added to the other liquors before they go to the drier and the furnace. This part of the liquors (10-50%) recovers that much of the volatile acids that would otherwise be lost in the burner as salts. 

This process also transfers heat from the flue gases to the black liquor so that it leaves the last stage of evaporation hot. At this point, it is referred to as strong black liquor. Addition of make-up chemicals, either sodium sulfate ( salt cake ) or sulfur as necessary is carried out at this stage to replace any losses of sodium and sulfur occurring elsewhere in the process. In fact, sulfate process is synonymous with kraft process because the primary make-up chemical requirement of the kraft process is sodium sulfate, although sodium sulfate is not an active constituent of kraft pulping liquor. 

Sometimes known as the sulfate process, since the sulfur is added as sodium sulfate. It is then reduced to S ° by burning with the evaporated black liquor (lignin and carbohydrate fragments from a previous cook) in an insufficient supply of air. Water extraction yields green liquor , a solution of (largely) sodium sulfide and sodium carbonate and which is treated with slaked lime [Ca(OH)2j. Insoluble calcium carbonate precipitates and the resulting solution of sodium hydroxide and sulfide ( white liquor ) is used for a new cook. 

Fontes, Costa, and Azevedo [79] used 300 pm diameter Type R thermocouple in an alumina sheath with an external diameter of 4 mm and placed inside a 3 m or 5 m long water-cooled stainless steel probe to measure gas temperatures in a black liquor recovery boiler. Calculations showed the "true" temperature in the regions of highest temperature did not exceed measurements by more than 8%. Measurements were made at multiple elevations, multiple locations at each elevation, and at multiple insertion depths into the boiler. A traversing mechanism was used to insert the probe, which was checked frequently for deposition of black liquor particles and sulfate deposits. The temperatures ranged from approximately 700°C to 1000°C. 

The principle use of lime in the paper and pulp industry is in the sulfate (or Kraft) process. Kraft black liquor is dehydrated and burnt in a furnace to produce a smelt which consists primarily of sodium carbonate and sodium sulfide [32.28]. The smelt is causticised with slaked lime (see section 31.20). The products are insoluble calcium carbonate, which is removed by filtration, and sodium hydroxide liquor, which is recycled. Some 90 % of the calcium carbonate, so produced, is calcined in a suitable kiln (see section 16.4.11), slaked and re-used. About 250 kg of quicklime per tonne of pulp are required for causticisation. 

BLACK LIQUOR - The liquid material remaining from puipwood cooking in the soda or sulfate papermaking process. 

In the making of paper pulp in the sulfate process, wood chips are digested or cooked and spent black liquor is obtained after washing the pulp. This solution contains primarily sodium carbonate and organic sulfide compounds. This solution is concentrated by evaporation in a six-effect system (Kl, SI). 

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