Smelt kraft

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. 

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. 

Chemical reactions were shown to play a minor role in a smelt-water explosion, although gas samples from kraft smell-water incidents showed that hydrogen evolution could be correlated with Na2S content. There was general agreement that the explosion mechanism was physical in nature. 

The smelt-water explosion problem is of primary interest in kraft recovery furnaces where, from operational error or an equipment failure. 

Whereas Sallack was concerned with soda smelts. Nelson and Kennedy (1956a,b) were interested in kraft smelts in which sodium sulfide was present in significant amounts. In Table XI, typical analyses are shown for both types of smelts. 

The experimental procedure followed by Nelson and Kennedy was very similar to that used by Sallack, and their results agreed with Sal-lack s conclusions in regions of overlap. With kraft smelts an explosion was easier to achieve and the presence of Na2S was cited as a possible... 

Combustion Engineering, Inc. (1966). Summary Report on the Nature of and Remedies for Explosive Reaction(s) Between Smelt and Water in Kraft Chemical Recovery Furnaces. Inst. Pap. Chem., Appleton, Wisconsin. 

Krause, H. H., Simon, R., and Levy, A. (1973). Smelt-Water Explosions, Final reports to Fourdrinier Kraft Board Institute, Inc. Biittelle Laboratories, Columbus, Ohio. Lemmon, A. W. (1980). Explosions of molten aluminum and water. In Light Metals 1980 (E. McMinn, ed.), p. 817. (Proceedings of Technical Sessions Sponsored by TMS Light Metals Committee at 190tb AIME Armuat Meeting.)... 

Lougher, E. H., Blue, G., Goddard, S., Gurev, H. S., Miller, J. F., Putnam, A. A., and Simon, R. (1968). Feasibility Study on Smelt-Water Explosions, Summary report to Fourdrinier Kraft Board Institute, Inc. Battelle Memorial Institute, Columbus,... 

Waste-to-energy boilers Coal-fired Boilers Pulp and paper digesters Kraft recovery boilers Municipal waste containing chloride, sulfur, alkali metals, zinc and lead Sulfidation attack boiler tube wastage (50-60 mpy) Thiosulfate and polysulfides Sulfate, thiosulfate Chloride 59 (alloy 625 overlay weld metal used) 8 (alloy 625 and 309 SS) 21 (overlaid with 309 SS) 11 (309 L on lower furnace sidewalls 625 on floor tubes and smelt opening)... 

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. 

Kraft pulp mills Digesters batch and continuous Multiple-effect evaporators Recovery furnace Weak and strong black-liquor oxidation Smelt tanks Lime kiln Mercaptans, methanol (odors) H2S, other odors H2S, mercaptans, organic sulfides, and disulfides h2s Particulates (mist or dust) Particulates (dust), H2S Condensers and use of lime kiln, boiler, or furnaces as afterburners Caustic scrubbing and thermal oxidation of noncondensables Proper combustion controls for fluctuating load and unrestricted primary and secondary air flow to furnace and dry-bottom electrostatic precipitator noncontact evaporator Packed tower and cyclone Demisters, venturi, packed tower, or impingement-type scrubbers Venturi scrubbers... 

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. 

These bricks find relatively frequent use in Kraft mills because of the amount of alkaline media encountered. They are used in the liquor regeneration systems, the smelt tank, the lime slaker, the causticizer, the liquor storage tanks, and the caustic storage tanks. They, of course, can be used in any industry having corrosive alkaline conditions. As with any corrosion resistant material of construction, proper care must be exercised to insure the proper application of Portland cement brick. 

Both neutral sulphite and bisulphite pulping liquor can be reeovered by the Tampella process (Rimpi, 1983). The residual eooking liquor is burnt in a kraft type recovery furnace and the smelt of sodium earbonate and sodium sulphide obtained as described in the seetion on kraft pulping. The dissolved smelt is carbonated with flue gas to form sodium hydrosulphide and sodium bicarbonate and the partially carbonated liquor stripped with steam in order to liberate hydrogen sulphide gas. The gas formed plus make up sulphur is then burnt to sulphur dioxide for the preparation of sulphite eooking liquor. 

The spent sodium-base sulfite liquor is burned under reducing conditions, as in the kraft process, yielding a smelt of sodium sulfide. 

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. 

GREEN LIQUOR - The liquor resulting from dissolved molten smelt from Kraft recovery furnace in water. 

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