Method for the disposal of a residue containing sodium sulfate

p0001The invention relates to a method for the elimination of industrial waste comprising sodium sulfate.

p0002Human activities produce large quantities of sulfur residues that are harmful to the ecology. Thus, the combustion of fossil fuels (coal, petroleum), especially in power plants for electricity production, leading to the formation of a large volume of sulfur oxides, their release into the atmosphere n ' is generally not admissible.

p0003A known and recommended technique for the purification of sulfur oxides in flue gas is to treat them with carbonate or sodium bicarbonate (Chemical Marketing Reporter, July 7, 1986, pages 5 and 27: "Sodium Chemicals Get Nod for Commercial FDG in '90's "); sulfur oxides in the treated gas are thereby converted into sodium sulfate, readily separable from the gases. This known technique provides efficient treatment of industrial flue gas sulfur oxide, but it leads to the formation of a residue comprising sodium sulphate, the elimination of which presents difficulties. The high solubility of sodium sulfate in water in fact prohibits its rejection as such on a heap or a heap. To solve these problems, consideration was given to waterproof the residue containing sodium sulphate, by mixing fly ash, clay and water (ibid). This waterproofing method however has the disadvantage of requiring several different raw materials and involves a complex and expensive apparatus. It also requires the centrifugal immobilization areas for storing waterproofed residues heap.

p0004According to EP-A-0,005,301, the gas to be purified is treated with sodium (bi) carbonate to form sulfite and sodium sulfate which is dissolved in water, and the resulting aqueous solution is treated with calcium chloride to precipitate calcium sulphate dihydrate which is collected. The extracted calcium sulfate dihydrate is used in the building industry or rejected.

p0005In DE-A-3411998 describes a process for removing a solid residue, wherein this residue is dispersed in water and the aqueous dispersion thus formed is sent in an underground cavity of a salt deposit, which is withdrawn from an aqueous solution of sodium chloride.

p0006The invention aims to provide a new process that eliminates industrial waste containing sodium sulfate, easily and economically, without requiring immobilization of a spreading area, which also leads to obtaining a recoverable industrial product.

p0007The invention consequently relates to a process for removing an industrial residue containing sodium sulphate, by treatment of the sodium sulphate with a calcium salt, the method being characterized in that, before treating the sodium sulphate with the salt of calcium, the residue is dispersed in water to dissolve the sodium sulfate is separated from the resulting aqueous dispersion an aqueous solution of sodium sulfate and redroidit the solution to crystallize sodium sulfate decahydrate that are treated with the calcium salt, and in that sends the calcium sulfate in rock salt which is withdrawn to deposit simultaneously an aqueous solution of sodium chloride.

p0008In the process according to the invention, the calcium salt should be selected from those which are capable of decomposing sodium sulphate forming calcium sulphate. Preference is given to calcium chloride to produce sodium chloride simultaneously with the production of calcium sulphate.

p0009In the process according to the invention, treatment of the sodium sulphate in the residue with the calcium salt to be operated at a controlled temperature to crystallize calcium sulphate. In practice it is preferred to select a temperature at which crystallized calcium sulfate dihydrate.

p0010The treatment of sodium sulphate with the calcium salt is generally made in aqueous medium. The rate of dilution of the sodium sulfate and the calcium salt in the aqueous medium is not critical, and its optimum value can readily be determined by routine laboratory work, depending on the other operating conditions.

p0011To cure the sodium sulphate in the residue with the calcium salt, it may for example disperse the residue comprising sodium sulphate in the solid state in an aqueous solution or an aqueous suspension of the calcium salt. One can also, alternatively, dispersing the residue in a sufficient amount of water to dissolve the sodium sulfate, and then mixing the calcium salt to the resulting aqueous medium. In this alternative method, may optionally separating insoluble material from the sodium sulfate solution, before treating it with the calcium salt.

p0012The calcium sulfate precipitate is then sent to a rock salt deposit, which simultaneously extract an aqueous solution of sodium chloride.

p0013Any suitable means may be used to introduce the precipitate of calcium sulfate in rock salt deposit and to extract the aqueous solution of sodium chloride. A commonly used way is to introduce the calcium sulfate in the reservoir in the form of an aqueous suspension in water or a saturated brine or dilute sodium chloride. To this end, it is advantageously make use of the technique of push two pipes in the rock salt deposit, adjacent to each other (usually a tubular duct and an axial annular duct), to inject the suspension aqueous calcium sulphate under pressure in one of the pipes and to extract the aqueous solution of sodium chloride by the other conduit (Dale W. Kaufmann, "sodium chloride", American Chemical Society Monograph Series, 1960, Reinhold Publishing Corporation Chapman & Hall, Ltd., London, pages 142-185).

p0014In a particular embodiment of the process according to the invention the calcium sulfate precipitate is sent to the rock salt deposit with a sufficient quantity of water to dissolve therein a volume of rock salt at least equal to the volume sulfate calcium. In this embodiment of the method according to the invention, the amount of water used is now set to form in the reservoir, by dissolving rock salt, a sufficient volume of cavity to accommodate any calcium sulfate therein is sent and, if appropriate, other substances insoluble residue.

p0015Sometimes the industrial residue to be removed contain soluble or insoluble substances it is not recommended to reject the rock salt deposit or recover in the aqueous solution of sodium chloride. To this end, in another particular embodiment of the method according to the invention is dispersed industrial residue in a sufficient amount of water to dissolve the sodium sulfate, the resulting aqueous solution is recovered after separated into namely, if necessary, insoluble matters, cooled under controlled conditions to crystallize sodium sulfate decahydrate, the precipitate formed and treated with the calcium salt as described above. In this embodiment the method of the invention, the decahydrate calcium sulfate precipitate has a very high purity (Kirk-Othmer - Encyclopedia of Chemical Technology - 3rd Edition -. Vol 21 - John Wiley & Sons - 1983 - 247 pages 249 (see page 249)). It is optionally combined with sodium carbonate decahydrate, if the subject process industrial residue containing sodium carbonate or sodium bicarbonate, as is generally the case of residues obtained after the desulfurization of a flue gas with carbonate or sodium bicarbonate. This embodiment of the method according to the invention is especially recommended in the case-industrial residues contaminated with heavy metals.

p0016In a variant of this embodiment of the method according to the invention, only a fraction of the precipitate of sodium sulfate decahydrate (optionally combined with sodium carbonate decahydrate) was treated with the calcium salt to its sending in the rock salt deposit. The remaining fraction is used to treat a sodium chloride brine contaminated with calcium cations to precipitate thereof to calcium sulfate state.

p0017In a preferred embodiment of the method of the invention, the calcium salt is calcium chloride from the treatment with calcium hydroxide, to the mother liquor of sodium bicarbonate in the manufacturing process of the soda ash. In this embodiment of the method according to the invention, the treatment of the sodium sulphate with the calcium chloride may be performed by mixing the sodium sulfate, a waste liquor to a distillation column from the mother liquor from the manufacture sodium bicarbonate by the ammonia process. Examples of the constitution of these residues in the state of dilute aqueous suspensions, are provided in the Treaty of TE-PANG HOU "Manufacture of Soda," second edition, Hafner Publishing Company, 1969, page 237 and in the publication "the rehabilitation of the Solvay process waste Beds" Frederick W. Boecker, BSEC, Syracuse University, 1968, page 20. This embodiment of the method of the invention has the advantage of not implement that residues of industry, as all are eliminated in the rock salt deposit. It involves implementing an amount of water capable of dissolving, in the deposit, a volume of rock salt at least equal to the total volume of the calcium sulphate and insoluble materials of the waste liquor of the soda plant. Alternatively, it may submit the waste liquor to filtration or decantation to remove beforehand insolubles contained therein.

p0018In another embodiment of the method according to the invention is implemented residual calcium chloride in the manufacture of sodium bicarbonate by the technical amines, as described in GB-A-1082436 ( KAISER ALUMINUM & CHEMICAL CORPORATION) and in patents and patent applications BE-A-899 490 and EP-A-148,524 (Solvay & Cie).

p0019The method of the invention has the advantage of providing an easy, economical and reliable for removing an industrial residue, without requiring immobilization of a storage area. It has the additional advantage of producing an aqueous solution of sodium chloride, which can be exploited in an industrial process.

p0020The method of the invention applies to any solid or liquid industrial waste containing sodium sulfate. It finds an advantageous application for the disposal of solid residues FGD treatment containing sulfur oxides by means of sodium carbonate or bicarbonate. It advantageously solves the problem of disposal of such residues from the treatment of fumes from industrial centers functioning to sulfur fossil fuels, especially in power plants for electricity production.

p0021Features and details of the invention will be apparent from the following description of some embodiments, with reference to the accompanying drawings.<ul><li>Figure 1 is a diagram of an installation implementing a first embodiment of the method according to the invention;</li><li>Figure 2 is the diagram of an installation coupling an embodiment of the method according to the invention and a sodium bicarbonate manufacturing process;</li><li>Figure 3 is a diagram of a second embodiment of the method according to the invention.</li><li>Figure 4 is a diagram of an installation implementing a third embodiment of the method according to the invention.</li></ul>

p0022In these figures, the same reference numbers denote identical elements.

p0023In installations schematically in Figures 1 to 3, the method according to the invention is applied to the removal of a solid residue obtained after desulfurization of a flue gas from the combustion of coal or petroleum derivative. To this end, the flue gas 1, contaminated with sulfur oxides, is introduced into a reactor 2 which also is supplied with sodium bicarbonate 3. In the reactor 2, the sulfur oxides react smoke with sodium bicarbonate to form sodium sulfate. The gaseous reaction mixture 4 that is extracted from the reactor 2 is sent to a succession of filters (advantageously including electrostatic filters) 5 wherein separating a gas substantially free of dust and 6 a solid residue powder 7. This comprises sodium sulfate, sodium carbonate and usually dust present in the flue gas (fly ash, heavy metals).

p0024The residue 7 is treated according to the inventive method.

p0025In the embodiment of the method according to the invention, implemented in Figure 1, the solid residue 7 is introduced into a dissolving chamber 8, where water is added 9 are sufficient to dissolve all of the sodium sulfate and sodium carbonate residue 7. the resulting aqueous mixture 10 is fed to a crystallization chamber 11 that also is supplied with an aqueous solution of calcium chloride 12 in an amount sufficient to decompose the sulfate and sodium carbonate and form calcium sulfate and calcium carbonate precipitate, and sodium chloride that passes into the aqueous solution. Conditions are adjusted pressure and temperature in the chamber 11 so that calcium sulphate is crystallized in the form of dihydrate or gypsum, preferably acicular structure. Is collected from the crystallization chamber 11 an aqueous slurry 13 which, after the addition of additional water 14 is introduced into an underground deposit of rock salt 15 which is withdrawn from an aqueous solution of sodium chloride 16.

p0026Water not saturated aqueous slurry 13 dissolves the salt deposit 15 by providing it a cavity 17 wherein the insoluble materials of the slurry (including calcium sulphate) settle to form a solid deposit 18. The additional water 14 must therefore be adjusted so that the volume of salt gem dissolved and extracted from the deposit with the solution 16 is at least equal to the volume of solids of the slurry 13.

p0027In an alternative embodiment of the method which has just been described with reference to Figure 1, the aqueous mixture 10 is subjected filtration, decantation or centrifugation to extract the undissolved substances before sending it in the crystallization chamber 11. This alternative embodiment of the process reduces the volume of insoluble materials introduced into the pool; consequently, it requires less water intake 14 and reduces the dissolution rate of the deposit and the flow of the sodium chloride solution 16 withdrawn from the deposit 15.

p0028The plant shown in Figure 2 includes the process of the invention in a soda ash plant with ammonia 19. The soda ash plant with ammonia is well known technique (HOU TE-PANG "Manufacture of Soda, Second Edition, Hafner Publishing Company, 1969). in it, reacting an aqueous solution of sodium chloride 20, ammonia 21 and carbon dioxide 22 into a set 23 of reactors known per se, which is collected an aqueous slurry 24 of sodium bicarbonate. by treatment of the slurry 24 of a filter 25, there is obtained sodium bicarbonate solid 26 that is valued and a mother liquor 27 which is treated with calcium hydroxide in aqueous suspension 28 a reactor 29 comprising a distillation column supplied with water vapor. separately was collected from the reactor 29, on the one hand, ammonia which is recycled 21, and the other, a waste liquor 30 . the latter is an aqueous slurry consisting, essentially, of a saturated aqueous solution of calcium chloride and sodium chloride containing various suspended substances (including calcium sulphate, calcium carbonate, the magnesium hydroxide, iron oxides and silica).

p0029According to the invention, the spent liquor 30 is introduced as such into the crystallization chamber 11 which is also fed with the aqueous mixture 10 containing sodium sulphate from the solid residue to be removed.

p0030In the embodiment of Figure 2, the solid deposit 18 which is formed in the cavity 17 of the rock salt deposit 15 comprises the solids which were suspended in the black liquor 30. The aqueous solution of sodium chloride withdrawn 16 the deposit 15 is sent to the soda ash plant 19.

p0031In an alternative embodiment of the method shown diagrammatically in Figure 2, subjecting the waste liquor to 30 filtration or decantation, so as to send into the chamber 11 an aqueous solution of sodium chloride substantially free of solid.

p0032The installation shown in Figure 3 applies to the treatment of solid industrial residues that contain undesirable substances, which does not want to find in the solid deposit 18 of rock salt deposit, or in the aqueous solution of chloride sodium 16 extracted from the deposit. Examples of such undesirable substances are heavy metals present in varying amounts in certain fuels.

p0033In the installation of Figure 3, the solid residue 7 containing sodium sulphate is first dispersed in water 9 in a dissolver 8 to dissolve the sodium sulfate. The amount of water and its temperature are adjusted to obtain a solution of all of the sodium sulphate in the residue 7. The resultant aqueous suspension 10 is treated on a filter 31 to remove insolubles 32. The filtrate 33 is sent to a crystallization chamber 34 where it is cooled to a set temperature between -1 ° C and 32.4 ° C for critalliser sodium sulfate decahydrate and sodium carbonate decahydrate. Is collected from the chamber 34 an aqueous slurry 35 which is treated in a dewatering device 36 for separating the precipitate of sodium sulfate decahydrate and sodium carbonate decahydrate 37 and a mother liquor 38. The precipitate 37 is then subjected to a treatment similar to that of the aqueous medium 10 in the plant of Figure 1. for this purpose, it is processed in crystallization chamber 11, with an aqueous solution of calcium chloride 12 to convert the sodium sulfate in calcium sulfate and sodium carbonate in calcium carbonate. The resulting aqueous slurry 13 is then sent into the salt deposit 15, with an addition of water 14. Water mother 38 can alternatively be recycled into the dissolving chamber 8.

p0034In a variant of the process which has just been described with reference to Figure 3, the filtrate 33 is treated in two consecutive steps, in two successive crystallization chambers, where is first crystallized sodium sulfate decahydrate pure, which was collected, then a mixture of sodium sulfate decahydrate and sodium carbonate decahydrate.

p0035In the installation shown in Figure 4, the solid residue 7 containing sodium sulfate is treated as in the plant of Figure 3 to obtain a coprecipitate 37 of sodium sulphate decahydrate and sodium carbonate decahydrate. The precipitate 37 is split into two fractions 47 and 39. Fraction 47 is processed, as in the case of Figure 3, with an aqueous solution of calcium chloride 12 in the crystallization chamber 11 to precipitate a mixture of calcium sulphate and calcium carbonate; the resulting aqueous slurry 13 is sent, with an addition of water 14, the rock salt deposit 15, from which is extracted an aqueous solution of sodium chloride 16. This is usually contaminated with calcium cations and magnesium from the salt deposit. To purify these cations in the solution 16 is first treated in a reactor 40, with the fraction 39 of the precipitate 37 to precipitate calcium cations in the calcium sulfate state and calcium carbonate. is additionally introduced into the reactor 40, sodium hydroxide 41, to precipitate the magnesium cations in the form of magnesium hydroxide state. After separation of precipitates 42, the aqueous solution of sodium chloride is treated, in a second reactor 43, with sodium carbonate 44 to improve its purification calcium cations which are removed in the form of a precipitate 45 calcium carbonate. Sodium chloride solution 46 collected from the reactor 43 can be used in various industries, especially for the manufacture of sodium carbonate by the ammonia process, or for the manufacture of chlorine by electrolysis. In particular, the installation of Figure 4 may be incorporated into the plant of Figure 2.