DE4340839C1 - Process for the utilisation of waste sodium sulphate - Google Patents

Abstract

The invention relates to a process for the utilisation (valorisation) of waste sodium sulphate by reaction with potassium chloride and water to give potassium sulphate. The process is, according to the invention, characterised in that the reaction (potassium sulphate step) is carried out in one step, with potassium sulphate and a potassium sulphate mother liquor (conversion liquor) of the composition 11.5 - 15.0 kmol of NaCl 2.4 - 3.0 kmol of Na2SO4 23.0 - 43.6 kmol of KCl 1000 kmol of H2O being formed at 5-50 DEG C, that the potassium sulphate formed is separated off from the potassium sulphate mother liquor and that the potassium sulphate mother liquor is mixed by stirring with a KCl-NaCl mixture, in particular a KCl-NaCl crystallisate, in such a mixing ratio that an almost pure potassium chloride crystallisate and a solution almost saturated with KCl and NaCl (supernatant liquor) are formed.

Description

The invention relates to a method for recycling Waste sodium sulfate by reaction with potassium chloride Potassium sulfate in the presence of water.

Waste sodium sulfate is precipitated in various chemical processes By-product. For the recovery of waste sodium sulfate are in Chemie Ingenieurtechnik 61 (1989), No. 12, page 933-940, four Possibilities called:

• - Crystallization and calcination of sodium sulfate for sale
• - electrochemical processes
• - conversion processes
• - reduction processes

In addition to electrochemical, appears economically promising Processes especially the conversion of sodium sulfate to potassium sulfate, which is a coveted chloride-free potash fertilizer. The Implementation of Na₂SO₄ to K₂SO₄ according to the relationship

Na₂SO₄ + 2 KCl = K₂SO₄ + 2 NaCl

has always been a two-stage conversion with Glaserit as Intermediate stage executed. Most of the time, this process is with one Isolation of sodium chloride and / or sodium sulfate from the the mother liquors produced by the crystallization of glaserite. A preferred method for the production of potassium sulfate from sodium sulfate and potassium chloride sees next to a two-step conversion at 20-35 ° C another Glauber's salt crystallization at 0-5 ° C with subsequent separation and recycling of the crystallized Glauber's salt as necessary process steps. This creates one NaCl-rich mother liquor, which must be disposed of or evaporated.

From DE 28 20 445 B2 a method is known in which the anhydrous Sodium sulfate first with the mother liquor of potassium sulfate in Glaserite and glaserite mother liquor is implemented. After the separation of the glaserite crystals, the latter with potassium chloride solution Potassium sulfate and potassium sulfate mother liquor converted. The ones from crystallized potassium sulfate separated potassium sulfate mother liquor is used for the production of glazier. The potassium sulfate is dried. The Glaserite mother liquor is brought to about 0 ° C. in a further process step cooled, Glauber's salt crystallized out. After the separation of the Glauber's salt is the Glauber's salt mother liquor from 0 ° C Boiling temperature heated and the whole introduced into the process Water evaporates. After separation of the solid sodium chloride existing evaporation crystals, the hot evaporation solution with the  Glauber's salt from the cooling process and this mixture into the Process stage of the glaserite crystallization fed.

The main disadvantages of the process described are complicated process management involving a relatively large number of process steps, the high energy requirement for deep-freezing and reheating the Solution as well as for the complete evaporation of the water and the high Effort for checking and controlling the many among themselves to call coupled process stages. Other known methods Chem.-Ing.-Tech. 61 (1989), No. 12, pp. 933-940 also the disadvantage of the multi-level and high Energy requirements and are not suitable for waste sodium sulfate high and often fluctuating proportions of anhydrous and hydrated Sodium sulfate, possibly even completely as Glauber's salt (Na₂SO₄ · 10 H₂O) may be present. Each t (measured as K₂O) potassium sulfate about 4.4 to 5 t of water must be evaporated if it is anhydrous Sodium sulfate is used. When using Glauber's salt increases the amount of water to be evaporated again by another 2 t each t K₂O in potassium sulfate, which means an economic recovery of Not waste sodium sulfate according to the known conversion processes is possible or at least strongly questioned.

The invention has the task of waste sodium sulfate in anhydrous and / or crystal water-containing form at technically simple, cost and Energy saving way to convert to potassium sulfate.

This problem is solved with a generic method that the characteristic features of claim 1, advantageous Further developments of the invention are in the subclaims 2 to 4 specified.  

The method of the invention not only avoids the complexity of the Litigation, but also the previously required high Energy expenditure for Glazier salt crystallization and Water evaporation. This is achieved in that the Process of sodium sulfate conversion coupled with one after the Potash fertilizer manufacturing and hot dissolving processes the conversion solution derived from the sodium sulfate conversion instead of water to selectively extract sodium chloride KCl-NaCl crystallization (covering process) is used. It was surprising found that in this process according to the invention Potassium sulfate production from sodium sulfate in one step both with high SO₄ yield as well as with a high KCl yield is possible and thus the Generation and subsequent decomposition of glaserite can be omitted. It showed that the primary low potassium yield of the one-step Conversion process is compensated by the following KCl production. By coupling the single-stage Na₂SO₄ conversion according to the invention a high potassium yield is achieved with the crystallizate covering process, because the dissolved potassium chloride in the conversion solution of the K₂SO₄ production by the dissolving sodium chloride in The cover process is displaced and almost in the cover liquor leaving the process NaCl saturation is reached. When processing the top liquor in KCl production (fertilizer) is one in this way Total potassium yield of practically 100% possible. This also eliminates the effort of cooling the solution and evaporating water. The process according to the invention is technically very simple since it is Ambient temperature can run off and neither cooling nor Evaporation requires. As raw materials can easily both anhydrous sodium sulfate and KCl as well as Glauber's salt and KCl as well Water can be used. The use of Glauber's salt or Na₂SO₄ Glauber's salt mixtures have no disadvantages anhydrous sodium sulfate with it. In principle it doesn't do any  Difference whether the necessary process water in the process immediately as liquid water or as crystal water or in one Potassium chloride solution is introduced. Because of the simplicity of the K₂SO₄- Manufacturing process without glaserite stage, freezing and Evaporation level, the expenditure on equipment is very low, the Process control and process control are simplified.

The inventive method of converting waste sodium sulfate to Potassium sulfate, which is shown schematically in the single figure, consists of the following process steps:

Stage I: discontinuous or preferably continuous reaction of Na₂SO₄ or Na₂SO₄ · 10 H₂O (stream b) in a K₂SO₄ reactor 1 with potassium chloride i and water m at 5-50 ° C, preferably at 20-30 ° C, to form a suspension c from potassium sulfate and potassium sulfate mother liquor, the composition of which, based on 1000 kmol H₂O, is approximately in the range 11.5-15.0 kmol Na₂Cl₂, 2.4-3.0 kmol Na₂SO₄ and for K₂Cl₂ temperature-dependent 23.0-43.6 kmol, where the highest KCl concentration corresponds to the highest solution temperature. The maximum permissible K₂Cl₂ content can be calculated according to the relationship K₂Cl₂ = (26.6 + 0.34 T) kmol, where T is the solution temperature in degrees Celsius; a lower limit of (21.6 + 0.29 T) kmol should not be undercut. Working in the vicinity of the upper limit is advantageous.

Stage II: separation of the crystallized potassium sulfate f from the K₂SO₄ mother liquor d in a separation unit 2 (z. B. centrifuge or filter) and drying (vapors l) of the moist potassium sulfate in a conventional dryer 3 to achieve a salable potassium sulfate k.

Stage III: stirring the potassium sulfate mother liquor d in a stirred tank 4 with a dry or moist mixture a of potassium chloride and sodium chloride (preferably a KCl-NaCl crystals from crude salt processing) in such an amount that the corresponding solution after the stirring process is only approximately at NaCl is saturated, ie at least slightly below the saturation limit.

This stirring process leads to a suspension g with a KCl crystal h, which is separated from the solution phase e in a separating unit 5 (eg centrifuge or filter). The corresponding KCl crystals h are either further processed to potash fertilizers n by customary processes or used in whole or in part as raw material i for the production of potassium sulfate in reactor 1 , for which purpose drying is not necessary.

In an advantageous embodiment of the process, the required process water can each be introduced directly from the outside into the K₂SO₄ stage I and into an additional cleaning stage (washing stage) switched on (not shown) between the K₂SO₄ separation 2 and the K₂SO₄ drying 3 , the washing solution of which then the K₂SO₄ stage I is supplied. This creates an even purer K₂SO₄ product, as is desirable for some special uses.

The required potassium chloride should be at least 95% KCl (based on have the anhydrous substance) and can either be in solid form or in Framework of the dissolving power of the imported process water as a KCl solution be used. Typically, such quality is in the Deck level III immediately generated in excess. The through the stirring process approximate solution saturated with KCl and NaCl e (top lye) becomes the solution cycle of a not shown  Potash crude salt processing fed. A corresponding Potash fertilizer factory, which also contains the KCl-NaCl mixture for the The method according to the invention should therefore be as close as possible stand in the plant in which the inventive method is operated.

Example 1

15.0 kmol of anhydrous sodium sulfate and a KCl solution from Composition (kmol) 46.0 K₂Cl +1000 H₂O are continuously Electricity entered into a stirred vessel and at a temperature of 20-30 ° C and a residence time of about 3 hours Brought reaction. A suspension of 12.0 kmol K₂SO₄ is formed and 0.5 kmol Na₂SO₄ as a solid in a K₂SO₄ mother liquor Composition (kmol) 34.2 K₂Cl₂, 12.0 Na₂Cl₂, 2.5 Na₂SO₄, 1000 H₂O, the mixing vessel through an overflow or a bottom drain nozzle leaves and by thickening and centrifuging into a solid and a liquid phase is separated. The potassium sulfate formed dried. The separated K₂SO₄ mother liquor is with filter damp from a hot dissolving process vacuum crystals Composition (kmol) 86.5 K₂Cl₂, 36.0 NaCl₂, 210 H₂O continuously stirred at about 25 ° C in a second stirred vessel. This will solve it Sodium chloride, and there remains solid potassium chloride and an almost NaCl-saturated solution phase by thickening and centrifuging is separated. It forms 98.4 kmol K₂Cl₂, of which 46.0 kmol Potassium sulfate conversion are supplied. The KCl excess of 52.4 kmol is further processed to potassium chloride with 60% K₂O. As immediate usable products thus result in a total of 64.4 kmol K₂SO₄ and K₂Cl₂. The potassium yield is 74.5% and the SO₄ yield 83.3%.  

Example 2

The process is carried out essentially as in Example 1. There will be only carried out the following modifications: Instead of anhydrous Sodium sulfate is Glauber's salt (Na₂SO₄ · 10 H₂O) as a sulfate raw material used, and the potassium chloride is not as a KCl solution, but as Moist salt (consisting of 46 kmol K₂Cl₂ and 20 kmol H₂O) was introduced. Furthermore, 830 kmol H₂O are given as hot water in the process. The The temperature of the hot water is chosen so that the negative The heat of fusion of the Glauber's salt does not drop below a temperature 18 ° C leads. The process control and the material yields are the same otherwise the example 1.

The main advantages of the method according to the invention are its technical simplicity, the usability of Glauber's salt and any Glauber's salt-Na₂SO₄ mixtures instead of anhydrous Sodium sulfate without disadvantages or increased energy expenditure and in one high potassium and sulfate yield, where the potassium yield by Integration of the resulting top liquor into the solution cycle of a KCl factory can theoretically be increased to 100% if Appropriate freedom from water in the KCl factory (e.g. through Carnallite processing) is present. The process can be either carry out continuously or discontinuously. There are no environmental impacts.

Claims (4)

1. A process for recycling waste sodium sulfate by reaction with potassium chloride to give potassium sulfate in the presence of water, characterized in that
that the reaction takes place in one stage at a temperature of 5 to 50 ° C, in particular 20 to 30 ° C, to a suspension of potassium sulfate and potassium sulfate mother liquor, the composition of which, based on 1000 kmol H₂O, 11.5 - 15.0 kmol Na₂Cl₂
2.4 - 3.0 kmol Na₂SO₄
23.0 - 43.6 kmol corresponds to K₂Cl₂,
that the potassium sulfate formed is separated from the potassium sulfate mother liquor and dried and
that the potassium sulfate mother liquor is stirred with a KCl-NaCl mixture, in particular a KCl-NaCl crystallizate, in such a proportion that a pure potassium chloride crystallizate and a solution saturated with KCl and only approximately saturated with NaCl are formed, where appropriate the KCl- All or part of the crystals as raw material of the potassium sulfate stage and / or the solution formed saturated with KCl and only approximately saturated with NaCl are fed to the solution circuit of a processing plant for crude potassium salts. 2. The method according to claim 1, characterized, that the potassium sulfate formed by the reaction of the raw materials at least part of what is required for litigation Water is washed and the resulting washing solution after their separation from potassium sulfate crystals of the potassium sulfate stage is fed. 3. The method according to claim 1, characterized, that the potassium chloride required for the process and necessary water either separately or entirely or partially in the form of a potassium chloride solution in the Potassium sulfate level will be introduced. 4. The method according to any one of claims 1 to 3, characterized, that the potassium chloride required for the process as damp salt fed to the process with at least 95% KCl content in the solid becomes.