Spent Liquor Recovery

Chemical recovery ia sodium-based sulfite pulpiag is more complicated, and a large number of processes have been proposed. The most common process iavolves liquor iaciaeration under reduciag conditions to give a smelt, which is dissolved to produce a kraft-type green liquor. Sulfide is stripped from the liquor as H2S after the pH is lowered by CO2. The H2S is oxidized to sulfur ia a separate stream by reaction with SO2, and the sulfur is subsequendy burned to reform SO2. Alternatively, ia a pyrolysis process such as SCA-Bidemd, the H2S gas is burned direcdy to SO2. A rather novel approach is the Sonoco process, ia which alumina is added to the spent liquors which are then burned ia a kiln to form sodium aluminate. In anther method, used particulady ia neutral sulfite semichemical processes, fluidized-bed combustion is employed to give a mixture of sodium carbonate and sodium sulfate, which can be sold to kraft mills as makeup chemical. 

Kraft pulping is a common process in the paper industry. Figure 8.4 shows a simplifled flowsheet of the process. In this process, wood chips are reacted (cooked) with white liquor in a digester. White liquor (which contains primarily NaOH, NaiS, Na2C03 and water) is employed to dissolve lignin from the wood chips. The cooked pulp and liquor are passed to a blow tank where the pulp is separated from the spent liquor weak black liquor which is fed to a recovery system for... 

California Steel Industries, Inc., located in Fontana, CA, reclaimed wastes to increase profits and address water use issues. The facility, a steel mill, is situated in an area that does not have a ready supply of process water. Also, the offsite recycling facility used to dispose of spent process pickle liquor was soon to become unavailable. As a result of these concerns, the company constructed an onsite recycling facility designed to recover ferrous chloride for resale and to reuse water and hydrogen chloride for use in steel processing operations. Environmental benefits include the recovery and resale of 20 to 25 t/d of ferrous chloride, 13,440 L/d of hydrogen chloride, and 49,200 L/d of water. In addition, corporate liability was minimized because spent liquor was no longer sent to a disposal facility. 

Reducing smelting furnaces that produce a high-sulfidity, kraft-like green liquor are now employed at sodium-based sulfite mills. U.S. EPA anticipates that it would be necessary to replace the existing recovery boilers at ammonia-based mills if chemical substitution to a sodium base were employed. Additionally, it is likely that, because the heat value of sodium spent liquor is lower than ammonia spent liquor, evaporator modification may he required if excess capacity does not already exist. 

The spent liquors may contain lint and residual size that can be removed by filtration. Weakly alkaline liquors represent a cost problem, however. Although limited amounts of less dilute liquor may be recycled and used in boiling-off or scouring, the major proportion becomes a rather troublesome component of the effluent load. Neutralisation simply increases the salt content of the effluent. Recovery of the alkali by vacuum evaporation is the usual procedure [282,283]. 

Azarniouch and Prahacs have patented an electrochemical recovery procedure for NaOH and other valuable substances (lignin, organic compounds, H2S04, H2, 02 or 02/Cl2 mixtures) from spent liquors and bleach plant effluents [282]. 

Azarniouch MK, Prahacs S (1991) Recovery of NaOH and other values from spent liquors and bleach plant effluents, US 5,061,343 Chem Abstr 116 23226f... 

The spent liquor contains about 3% of dissolved substances, comprising 90-96% of oxalic acid and 3-6% of mercury in the form of mercurous nitrate. The recovery of mercury in the form of mercurous chloride or mercury proper is usually profitable. This is achieved by adding nitric acid to the spent liquor (1 1. of concentrated hydrochloric acid to 50 1. of liquid). The sediment which is precipitated is separated by decantation and dissolved in concentrated hydrochloric acid, using 10 kg of hydrochloric acid for 10 kg of sediment. Next a solution of pieces of tin (1 kg) in hydrochloric acid (6 kg) is added and pure mercurous chloride is precipitated. 

McCoy. September/October 1988. "Bipolar Membranes Applied to Spent Pickle Liquor Recovery." The Hazardous Waste Consultant. McCoy and Associates, Inc. Lakewood, CO. Pp. 1-18 to 1-21. 

As the result of many experiments, we concluded that a 55% concentrate of desugared, spent liquor was the best lignin starting material, based on cheapness and paste oil recovery. We also concluded that light oils (containing many hydrocarbons, etc. such as cyclohexane and benzene),... 

An essential factor in the kraft process has been the recovery of the spent liquor from the cooking process. The black liquor removed from the... 

Since the prices of petroleum products have soared, valorization of natural resources has intensively attracted the attention of researchers in different fields. To fully utilize wood, which is a renewable raw material, one must make better use of its 25-30% lignin content which forms the principal dissolved material in the spent liquor of chemical pulping processes. The annual production of sulfite pulp in Canada is approximately 2.6 million tons (data based on 1979), thus, at least one million tons of lignosulfonate is produced, representing a potential resource of raw material for other uses. However, among 35 sulfite mills in Canada, only two or three of them possess a recovery system for spent liquor, that means a very small portion of lignosulfonate has been recovered. The recovered lignin has been... 

Xu, T.W. and Yang, W.H. (2004) Tuning the diffusion dialysis performance by surface cross-linking of PPO anion exchange membranes - simultaneous recovery of sulfuric add and nickel from electrolysis spent liquor of relatively low acid concentration. J. Hazardous Mater., 109 (1-3), 157-164. 

A furnace similar to the Tomlinson kraft recovery furnace is used for the combustion of magnesium-based sulfite spent liquors. In this case, however, no smelt is obtained instead the base is completely recovered as magnesium oxide in dust collectors the sulfur escapes as sulfur dioxide and is absorbed from the combustion gases in scrubber towers. However, because magnesium hydroxide has a very low solubility in water, a complete recovery of sulfur dioxide meets difficulties. 

Venter, J.S.M Van der Klashorst, G. H. The Recovery of By-Products and Pulping Chemicals from Industrial Soda Bagasse Spent Liquors. Paper presented at TAPPI Environmental Conference, Portland, OR, April 1987. 

The content of the reduction autoclaves is dischaiged into cone-bottomed flash tanks where separation of the powder from the spent liquor takes place. The nickel metal in slurry form is washed, dried, and packaged as powder or pressed into briquettes, sintered, and packaged ready for shipment. The spent liquor containing about 1 gm/liter Co and 1 gm/liter Ni is precipitated with HjS, filtered, and sent to the mixed sulfides stage for cobalt recovery. 

A feature of the kraft process vital to continued success is its integral, well-tested chemical recovery system. The digestion liquor for each batch of chips to be pulped is mainly obtained from the chemicals recovered from the spent liquor of previous digestions, and has approximately the composition given in Table 15.8. For kraft pulping it is usual to specify all of the components of the digestion liquor on a NaiO equivalent basis. This puts all the active constituents on the same sodium ion content basis. Thus, the actual concentration of sodium hydroxide present for a 73 g/F, NaiO equivalent is given by Eq. 15.12. 

The effect of various operating conditions on the recovery of the pulping chemicals and heat values in the case of a neutral sulfite semichemical (NSSC) spent liquor has been described recently (I). The thermodynamic equilibria and kinetics of gasification which seem to apply to this same set of experiments were discussed elsewhere (9). Gas compositions and yields examined in the latter study showed that under some of the operating conditions investigated, commercially significant quantities of ammonia and methanol synthesis gases could be produced from NSSC-type spent liquors. 

When spent cooking liquor recovery is not practiced, the wastewaters may be acichc (pH in the range of 2 - 3) and have higji concentrations of dissolved organics and inorganics. The solubilized organics and the type of cooking liquor will determine the characteristics of the wastewater. 

Chemical wood pulping is an important industrial process, with a relatively low yield. However, this process is attractive, because of the low requirements of wood quality and wood species. Moreover, it includes very short cooking times due to the easy penetration of alkaline solutions into wood. A well established processing of the spent liquor, generation of process heat, recovery of the pulping chemicals and production of valuable by-products such as turpentine and tall oil are also advantages of the kraft cooking. Major drawbacks are however, the odour and the dark colour of the unbleached pulp. 

The primary cooking chemicals in the kraft process are sodium hydroxide and sodium sulfide. The sodium sulfate which is added to the spent liquor is reduced to sodium sulfide in the recovery furnace. Sodium carbonate is also present but does not participate in the pulping process it is converted to sodium hydroxide during the recausticizing process. 

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