FI97375B - Process for treating, by using the heat from calcination waste gases, a residual acid which arises when dissolving titanium-containing raw materials in accordance with the filtrate process - Google Patents

Description

97375

Process for the treatment by the kraft formed role in dissolving the titanium--of raw materials in the spent acid using kalsinointijätekaasujen heat for 5 The present invention relates to a method for treating the sulfate process task resulting from the production of titanium dioxide in the spent acid to energy and material-saving manner by using the titanium dioxide 10 from the calcination of the formation of hot kalsinointijätekaasu of the heat of the spent acid concentration and the titanium dioxide contained in the kalsinointijätekaasujen recovery .

In a process for the preparation of titanium dioxide pigments in which titanium-containing raw materials, for example ilmenite and / or titanium-containing slags, are solubilized by sulfuric acid and also called the "sulphate process", the elements contained in the raw material are converted to basic and ferrous sulphates. From the clarified “solutions, titanium is hydrolytically precipitated as titanium di-” oxide hydrate, which is then separated by filtration, washed, bleached and further processed into a pigment in a known manner.

·. ·. The filtrate is a solution containing 20 to 24% by weight of H 2 SO 4 and dissolved metal sulphates, called "waste acid".

Table 1 shows the soaking of ilmenite; .V Typical composition of waste acid.

* 1 »* · 1 · • · * · 2 97375

Table 1 Composition of waste acid

Ingredient Concentration (g / 1)

Sulfuric acid 304.1 (= 22.9% by weight) δ Fe 53.6

Mg 6.3

Ti 3.4

Mn 0.95 AI 0.53 10 V 0.59

Ca 0.42

Cr 0.26

Trace elements <0.03 15 If titanium-containing slags are used as raw material, which can contain up to 85% by weight of TiO2 and thus correspondingly less iron, waste acids of similar composition but with a lower iron content are obtained.

20 In order to recover waste acid economically, methods have been developed for concentrating it, in which: it is referred to below as increasing the concentration to a maximum of. 34% by weight of H2SO4 for "pre-concentration" and raising it to a maximum of 71% by weight of H2SO4 for "concentration".

25 In connection with the concentration, a substantial part of the • · · «· · * precipitates. pre-iron and e.g. the optical properties of the pigment »·» * · "· 'from impurities, chromium and vanadium, in the form of their sulphates. After their separation, the concentrated sulfuric acid can be returned to the dissolution of the titanium-containing raw material and reused. If the titanium-containing raw material is to be dissolved requires, the acid can be concentrated: * · '· the acid can be further concentrated, for example to an H 2 SO 4 content of more than 80% by weight, before being returned to solution.

3,97375

Attempts have already been made to utilize the process heat generated as the heat content of the hot waste gases formed during the calcination of titanium dioxide hydrate. These waste gases are hereinafter referred to as "calcination waste gases". The energy required for the calcination of Ti-5 titanium dioxide hydrate is obtained by burning liquid or gaseous fuel in the presence of air in a combustion chamber connected in front of the calcination furnace. The calcination furnace is preferably a rotary tube furnace.

In the process of U.S. Patent No. 3,105,744, a portion of the hot untreated calciner waste gases is returned to the combustion chamber, thereby saving fuel and air, reducing gas volume, and reducing dust emissions from the rotary kiln. The use of heat from calcination waste gases for the concentration of waste acid is not possible with this method.

In the process according to EP-A-133 505, a metal sulphate-containing waste acid with a sulfuric acid content of 23 to 28% by weight is pre-concentrated in the first 20 steps by means of process heat to a maximum H2O content of 32% by weight. In this case, any precipitating iron (II) sulphate heptahydrate can be separated. The preconcentrated acid is concentrated to a H 2 SO 4 content of 62 to 70% by weight, cooled, and recycled to dissolve the precipitated metal. 25 after separation of additional salts (sulphates) and possibly li- «· · I. tonic concentration. The metal salts are thermally decomposed • · · ’· * · * and the resulting sulfur oxides are used to make sulfuric acid or fuming sulfuric acid. In this process, process heat is understood to be, for example, the heat released in the absorption of SO 3. The use of hot calcination waste gases with concentrated metal-containing waste acids -: * '· is not possible due to the intense spraying and acid scaling that occurs during the evaporation of the acids.

U.S. Pat. No. 4,97375, EP 97 259, which is the starting point for this application, saves energy in the production of titanium dioxide by the sulphate process by forming hot (about 400-500 ° C) sulfur oxides containing sulfur oxides and dust oxides for example at a temperature above 320 ° C as a filter or by electrostatic gas cleaning and recycle these purified gases completely or partially into the combustion chamber or contact them with the acid to be concentrated and recover their heat, for example by concentrating about 65% by weight sulfuric acid up to 85% H 2 SO 4. -%, possibly followed by the additional step of evaporating the waste acid containing about 23% by weight of H 2 SO 4 15 now by means of gases cooled to 150-240 ° C.

The use of the heat of crude, dust-containing hot calcination waste gases to concentrate the metal sulfate-containing waste acid by bringing the waste gases directly into contact with the acid is not practical in this process either.

The task was thus to develop a process for the production of titanium dioxide by the sulphate process, ···. to treat the waste acid formed in an energy and material saving manner by utilizing titanium • · «*. . the heat of hot unrefined * · * · ’calcination waste gases from the calcination of carbon dioxide for the concentration of waste acid and the recovery of titanium dioxide contained in the calcination waste gases.

The object was solved according to the invention by a process in which the waste acid is pre-concentrated, optionally cooled and the ferrous sulphates containing water of crystallization in the case of ferrous ores are concentrated, concentrated to a sulfuric acid content of 50 to 71% by weight and the concentrated sulfuric acid 97375, the process is characterized in that the calcination waste gases are contacted, unpurified, with a suspension having a sulfuric acid content of 26 to 29% by weight, the concentration increase due to evaporation of the water being offset by the addition of a waste containing 20 to 24% by weight of sulfuric acid. and the residue formed in the suspension is separated off and, after washing, recycled to the production of titanium dioxide.

A first embodiment of the invention comprises a combination of the following steps: a) Waste acid containing 20 to 24% by weight of H 2 SO 4 is in direct contact with hot unpurified calcination waste gases in a circulation and is pre-concentrated to a H 2 SO 4 content of 26 to 29% by weight, and the concentration of this acid is kept constant by recirculating more residual acid and crude calcination waste gases to form a residue containing mainly titanium dioxide and calcium sulfate.

b) Separate the pre-concentrated acid from the residue and wash the residue with water.

c) The washed residue is recycled to the titanium dioxide • manufacturing process.

: d) The pre-concentrated acid separated from the residue is cooled to a temperature of 10 to 25 ° C, whereupon metal sulfates containing water of crystallization precipitate and the H 2 SO 4 content of the acid rises to a value of 30 to 35% by weight.

*. '· (E) Metal sulphates containing water of crystallisation are separated from acid containing 30 to 35% by weight of H2SO4.

f) The acid containing 30 to 35% by weight of H 2 SO 4 is concentrated to a concentration of H 2 SO 4 to 30 to 71% by weight, whereby more metal sulphates precipitate.

• M

; '! * G) The acid concentrated to a H 2 SO 4 content of 60-71% by weight is separated from the precipitated metal sulphates and recycled to dissolve the titanium-containing raw material.

• * 6 97375

Another embodiment of the invention comprises a combination of the following steps: a) The waste acid containing 20 to 24% by weight of H 2 SO 4 is in direct contact with the hot unpurified calcination waste gases in circulation and is pre-concentrated to a H 2 SO 4 content of 26 to 29% by weight and kept the acid concentration is constant by feeding more residual acid and crude calcination waste gases into the circuit to form a residue containing mainly titanium-10 dioxide and calcium sulfate.

b) Separate the pre-concentrated acid from the residue and wash the residue with water.

c) The washed residue is recycled to the titanium dioxide production process.

D) The pre-concentrated acid separated from the residue is concentrated to a H 2 SO 4 content of 60-71% by weight, whereby metal sulphates precipitate.

e) The acid concentrated to a H 2 SO 4 content of 60 to 71% by weight is separated from the precipitated metal sulphates and recycled to dissolve 20 titanium-containing raw materials.

.Relatively rich in iron, e.g. : in the case of waste acids derived from ilmenite. ’the first embodiment must be chosen. It freezes the pre-concentrated acid separated from the residue at a temperature of 10 to *** 25 to 25 ° C, whereupon a mixture of metal sulphates containing water of crystallization precipitates and is separated (steps d and e) consisting of ·. *. essentially of ferrous sulphate heptahydrate, known in the art as "green salt".

Derived from relatively low-iron titanium ores and / or slags, for example 30; For the waste acids present, a second embodiment must be chosen. It lacks an intermediate separation of the green salt, and the pre-concentrated acid obtained in step b, separated from the residue, is concentrated directly to a H2SO4 content of 60 to 71% by weight.

»· · 7 97375

It is easy for a person skilled in the art to decide, on the basis of the iron content of the waste acid, which embodiment is to be used in each case.

During the calcination of titanium dioxide hydrate, the flue gases flowing through the rotary kiln carry dusty titanium dioxide, so that the hot untreated calcination waste gases leaving the furnace contain, in addition to the flue gases, an average of about 15 to 30% by volume of N oxides and in addition about 0.8 g / m3 (NTP) TiO2.

Some of the untreated calcination waste gases at 320-400 ° C are returned to a rotary kiln and used to preheat the combustion air and the major part is brought into direct contact in a direct gas cooler, for example in a scrubber or venturi scrubber, with 20-24% by weight H2SO4 in circulation. this is pre-concentrated to a H 2 SO 4 content of 26 to 29% by weight.

This acid is passed in a cycle and held in direct contact with the crude calcining waste gases, whereby the same is fed into the circuit. . early such an amount of waste acid that the acid content remains constant, in the range of 26 to 29% by weight of H 2 SO 4. At the same time, the volume is kept constant by removing part of the: 25 acid in the circulation.

• · · * ... In both embodiments, expensive hot cleaning of uncleaned calcination waste gases is avoided.

After the entrained condensate has been separated from the calcination waste gases and further purification steps have been taken to reduce the sulfur oxide content to a level approved by the authorities, it is released into the environment.

The acid is spontaneously exchanged under the action of hot untreated calcination waste gases, which, when the calcination gases cool to 80-90 ° C: strong evaporation of water and 8 97375 residues form . Dissolved metal sulphates, which the waste acid contains up to about 13 to 15% by weight, in particular ferrous sulphate, do not precipitate at this stage.

The residue is filtered off and washed with water; instead of water, a weakly acidic aqueous washing solution can also be used. The residue is washed in two washing steps following each other without a clear boundary point, each lasting about 60 minutes.

In washing step 1 (prewash), the aqueous solution containing sulfuric acid and ferrous sulphate adhering to the residue is washed away, and in washing step 2 (final washing) most, about 80-90%, of the precipitated calcium sulphate.

15 Table 2 shows typical compositions of washed and annealed residue.

Table 2

Ingredient Analysis of the annealed residue (% by weight) 20 After washing step 1 After washing step 2

TiO 2 79.5 96.1

CaSO 4 19.0 2.4; SiO, 1,5 1,5 • · · · '' • · • «♦ · · ··«: 25 The TiO2 component of the residue consists mainly of partially calcined titanium dioxide from fly ash from the calcination waste gases and to a lesser extent titanium dioxide hydrate, which waste acid is contained in suspension from the hydrolysis step. The calcium and silicic acid content of calcium sul-30 is derived mainly from titanium and other starting materials.

The titanium dioxide dissolved in the waste acid remains: · in solution and is separated in the subsequent processing steps together with the metal sulphates precipitated by concentration.

In prior art methods, waste acid is contacted with purified, i.e. with the calcination waste gases released from the titanium dioxide-5, so that no precipitation containing titanium dioxide can occur at this stage, and therefore no filtration is performed at this process step. Therefore, the known method also does not provide the possibility to separate and recover titanium dioxide suspended in a pre-concentrated acid.

In contrast, the process according to the invention makes it possible to recover both the titanium dioxide which has entered the acid with the calcination gases and that which is already suspended in it.

In addition, it is possible to remove a substantial part of the calcium sulphate contained in the pre-concentrated acid from the process with the residue and thus considerably reduce undesirable calcium sulphate precipitates in the subsequent treatment steps; the precipitated calcium sulfate tends to form deposits in the pipes and tanks, which reduce the effective diameter and thus the flow of material. The deposits detach after reaching a certain thickness, which allows the detached bodies to clog the pumps and nozzles in the piping system. The pre-concentrated acid is separated from the residue by a filter, for example a filter press and, in a preferred embodiment, a membrane filter. Some of the calcium sulphate precipitates before the press, which is separated, preferably in an intermediate storage tank, and pumped to a membrane filter press, filtered at 50 to 80 ° C. The residue separated from the pre-concentrated acid is washed with water at • · 20 to 60 ° C. preferably 30 to 50. The washing takes place in a membrane filter press.

4 4 4 10 97375

The residue separated from the pre-concentrated acid and washed is recycled to the TiO 2 production process.

In a preferred form of the process, the residue separated and washed from the pre-concentrated acid is recycled to a process step of bleaching titanium dioxide hydrate.

Bleaching is a treatment with reducing agents to bring heavy metals in the form of impurities of titanium dioxide hydrate to lower oxidation states, the compounds of which can be washed off with aqueous solutions. The uncoloured impurity silica, together with the titanium dioxide hydrate, remains in the residue and is treated together with it as a pigment. There is no disturbance in the amounts present, since silicon compounds are in practice conventional substances used for the treatment of TiO 2 pigments.

Since the TiO2 portion contained in the residue is recovered and utilized in the recovery of titanium dioxide, the process according to the invention is very material-saving.

The preconcentrated acid containing 26 to 29% by weight of H 2 SO 4 separated from the residue is cooled, when required by the iron content, to a temperature of 10 to 25 ° C, whereby the above-mentioned green salt is precipitated, which is separated. It only contains about 0.1% by weight of calcium sulphate, forms clear aqueous solutions and is available for technical purposes, such as the treatment of municipal and industrial waste water, without further purification.

If titanium dioxide and calcium sulphate were not pre-separated in the form of a residue, the green salt 30 would still contain about 1% by weight of calcium sulphate and about 3% by weight of TiO2 and could not be used for waste water treatment but should be further treated, mainly by thermal decomposition or neutralization.

As a result of the separation of the green salt containing crystalline water and the evaporation of the water, the H2SO4 content of the pre-concentrated acid rises to 30 to 35% by weight.

11 97375

According to the embodiment in question, the pre-concentrated acid is concentrated, after possible separation of the green salt, to a H 2 SO 4 content of 60 to 71% by weight in a known manner on a vacuum rotary evaporator, precipitating metal sulphates, mainly a mixture of low-water sulphates. ) sulphate monohydrate and is referred to in the art as the "filter salt". The resulting filter salt salt is preferably allowed to cool ("mature") in a tank where it remains for some time, for example in several successively connected mixing vessels, whereby in addition to enrichment of the colorant impurities in the solution, precipitated sulfate crystals grow.

15 The separated concentrated acid is recycled to dissolve the raw material. The filter salt can be thermally decomposed, the sulfur dioxide contained in the decomposition gas oxidized to sulfur trioxide and treated to concentrated or fuming sulfuric acid. The residue consisting of sparingly soluble oxides 20 can be removed as waste.

Alternatively, the filter salt may be neutralized with basic compounds, for example calcium compounds.

Calcium sulphate dissolved in waste acid has, in addition to the tendency to form sticky deposits, another detrimental property: it tends to form supersaturated * ... solutions, even in the presence of sulfuric acid and / or dissolved metal sulphates. Thus, for example, even when pre-concentrating the waste acid to a H 2 SO 4 content of about 28% by weight, the saturation content of the calcium sulphate is considerably exceeded, but only a part of the calcium sulphate exceeding the solubility limit precipitates. Supersaturation depends on a number of factors which, in some cases, cannot be further defined, for example the residence time in a pipeline and equipment system, '·. the concentration of acid, the number of seed crystals that promote the precipitation of CaSO 4, as well as the number of seeds in the apparatus, such as the roughness of the inner surfaces of the tubes and the amount of turbulence in the flow.

The problems caused by the formation of deposits in vessels and pipelines during the concentration step can be reduced to some extent by selecting suitable vessel materials (e.g. polypropylene, polyvinylidene fluoride, Teflon, etc.) and smoothing the inner walls of the pipes. In order to keep the amount of calcium sulphate recycled to the solubilization of the raw material dissolved in the subsequent precipitates and the concentrated acid, small amounts of anhydrite seed crystals can be added to the pre-concentrated acid, for example before it enters the filter press.

These seed crystals for anhydrite formation are prepared by reacting a solution of the calcium salt with sulfuric acid, stirring, heating and filtering, washing and drying the precipitate formed.

The invention is illustrated by the following examples, in which it should be noted that no anhydrite seed crystals were added to the solutions of Examples 1-5.

Example 1

364 m3 »h-1 (NTP) of heating gas was required for the calcination of one tonne of TiO2. By suitable heat exchange between the reaction products, i.e. calcined titanium dioxide and the unrefined calcination waste gases, for example by preheating the combustion air and circulating part of the untreated calcination waste gases to a rotary kiln, V. m3 «h * 1 (NTP) per tonne TiO2. This corresponds to a 15.1% saving in hot-30 energy.

Example 2 Using the energy saving measures according to Example 1, 2.2 • t ^ h-1 calcined titanium dioxide was formed in a rotary tube furnace, which required a heating amount of 680 m3 «h * 1 (NTP) corresponding to the energy amount I <I« t li 13 97375 9.83 · 109 J per tonne of TiO2. The amount of calcinerant gases formed was 21,000 m3 <h * 1 (NTP) (wet) and their moisture content was 27.5% by volume H 2 O.

The hot crude calcination waste gases were blown into a scrubber tower and contacted there with H 2 SO 4 to 28% by weight of waste acid from the production of preconcentrated titanium dioxide. The washing tower was a jet washer with 28% by weight of acid removed from the bottom, which was recycled back to the jet washer by means of a circulating pump through an axial nozzle at the upper end. 10.1 m3 * h_1 of waste acid with a H2SO4 content of 23% by weight and a temperature of 60 [deg.] C. were fed into this circuit, while at the same time 7.8% by weight of pre-concentrated 15% acid were added to a H2SO4 content of 7.8 m3 / h <1. The amount of H 2 SO 4 removed was slightly greater than the amount fed as 23% by weight of acid, which is explained by the fact that the untreated calcination waste gases contained SO 3 / H 2 SO 4, which entered the acid cycle during washing.

The amount of water evaporated was 2.7 t * h_1, of which 2.4 t * h_1 20 came from waste acid and 0.3 t * h_1 from a condensate separator connected after the tower. The temperature of the pre-concentrated acid 28% by weight of the effluent, H2SO4 content, ·, · tea was 88 ° C, «·! and the waste gas temperature when removed from the jet washer was 94 ° C. This exhaust gas contained 35.4% by volume of water,

I

25 which corresponds to a dew point of about 70 ° C. In this way, condensation of water vapor was avoided in the subsequent • tl 2 2 further purification of the waste gas.

«· ·. ·. The amount of heat released was 7.1 · 106 kcal «h'1, which corresponds to 3.36 · 109 J per ton of TiO2. This results in an energy saving of 34.2% when the waste acid is pre-concentrated according to Example 2 with hot untreated calcination waste gases.

Thus, the procedures of Examples 1 and 2 • saved a total of 35 49.3% of the thermal energy used for calcination.

14 97375

Example 3 This example provides an overview, using Table 3, of the amounts of calcium precipitated during the preconcentration of waste acid with untreated calcination waste gases.

Table 3

Acid Concentration of calcium dissolved in acid, calculated as Ca Sampling point H2SO4 Weight% g Ca / kg 10%% H2SO4 Waste acid 23.0 0.0315 1.37

Circulating 15 pre-concentrated acid 27.5 0.036 1.30

Preconcentrated acid just before filtration 27.5 0.025 0.89 20

The amount of calcium precipitated in the pre-concentration until filtration in this example was 0.48 g / kg H 2 SO 4, i.e. 35% of the amount of calcium initially dissolved in the waste acid. Example 4 25 Table 4 shows the separation balance of sodium sulfate dissolved in waste acid from the beginning of the pre-concentration.

• M

V to the end of the operation.

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The table reads as follows:

The 1030 t of waste acid coming to the pre-concentration contained 1100 kg of dissolved CaSO 4. It precipitated 60 kg during pre-concentration. 325 kg of CaSO 4 (1040 kg) remaining in the solution precipitated on the way from the pre-concentration to the filtration press (residue separation). It then separated with 44 kg of green salt and another 527 kg with filter salt. Thus, to a H 2 SO 4 content of 70% by weight of concentrated acid, 144 kg of CaSO 4 remained still dissolved, which was recycled with the acid 10 to dissolve the titanium raw material.

According to this, during the pre-concentration and concentration, 87% by weight of the calcium sulphate originally contained in the waste acid was separated.

Compared to the filter salt, the chicken of the green salt then precipitated a small amount of CaSO 4, so that the green salt contained relatively few impurities.

Example 5 In this example, Table 5 provides an overview of the change in density as well as the change in sulfuric acid and iron-20 tap content during partial iron separation by separation of the green salt during cooling of the pre-concentrated acid.

Table 5 25 • · · • 9

Preconcentrated Temperature Density Concentration (w / w) acid (° C) (g / cm3) H2SO4 Fe (solubilized)

Before the separation of iron (II): .i .: 30 tusta 30 1,395 27,0 4,70 • · «·. · After the separation of iron (II) 20 1,371 31,9 2,71

The amount of iron dissolved in the pre-concentrated acid was thus reduced by 42% by separating the green salt. These reported values are 17,97375 from a number of individual determinations.

Example 6

Table 6 provides a review, based on laboratory experiments, of how supersaturation with dissolved CaSO 4 decreases in preconcentrated acids to which anhydrite seed crystals have been added after stirring for 4 or 200 hours at different temperatures.

In each experiment, 1700 g of waste acid 10 (23% by weight H 2 SO 4) was pre-concentrated to a H 2 SO 4 content of 28% by weight, the temperature of the solutions was adjusted to the values indicated in Table 6, 1 g of anhydrite seed crystals were added, the solutions were kept at the indicated temperature for 4 or 200 hours and separated. .

The initial calcium concentration in the pre-concentrated acids was 300 ppm, and the equilibrium concentrations are given in Table 6.

Table 6

· .. · 20 Equilibrium Temperature ° C

concentration 55 75 90 ppm Ca after 4 hours 190 170 150 ppm Ca after 200 hours 120 140 130. 25 • ·

The preparation of anhydrite seed crystals is described in Example 7.

Example 7

To the calcium chloride solution (0.2 mol / l, 1 L) was added 730 ml (1343 g) of 96% · · · *.% W / w sulfuric acid with slow stirring, and the mixture was sexed for 5 hours at a temperature of 70 ° C. , stirred for half an hour, filtered, the filter cake was washed with water and dried at 110 ° C. According to structural analysis, it consisted of 35 anhydrites. It was suitable as a seed crystal material for precipitating anhydrite from preconcentrated waste acid.

Claims (12)

A process for treating a residual acid arising in the dissolution of titanium-containing raw materials according to the sulfate process by utilizing the heat from hot calcination gases generated by titanium dioxide calcination, the residual acid being preconcentrated, optionally being cooled and separated by iron sulfate containing iron sulfate containing , is concentrated to a sulfuric acid content of 60-71% by weight, and the concentrated sulfuric acid is returned to the solution when the metal sulphate which has arisen at the concentration is first removed, characterized by the calcination of the calcination exhaust gases. in contact with a suspension whose sulfuric acid content is 26 to 29% by weight, the concentration increase due to water evaporation being smoothed out by the addition of a residual acid containing 20-24% by weight of sulfuric acid and the suspension formed the ether residue is separated and led back to the titanium dioxide preparation. ·. ·. 2. Process according to claim 1, characterized in that the separation of the ether supported in the 26-29% by weight containing sulfuric acid containing the suspension is carried out by means of a filter press, preferably a membrane filter press. The method according to claim 1 or 2, characterized in that the separation is performed at 50 - 80 ° C. Method according to any one of claims 1-3, characterized in that it is removed from the suspension. The paired ether is washed with water, the temperature of which is 20 - 60 ° C, preferably 30 - 50 ° C. 5. Process according to any of claims 1-4, characterized in that the separated ether is washed in a membrane filter press. 6. Process according to any of claims 1-5, characterized in that it is separated from the suspension. · Li 21,973,75 paired and washed ether is circulated to a process step where the titanium dioxide hydrate is bleached. Process according to any one of claims 1-6, characterized in that it comprises a 60 - 71% by weight. H 2 SO 4 content of concentrated precipitated metal sulfate containing acid is cooled before these sulfates are separated to mature the mixture in a series of interconnected mixing vessels. Process according to any one of claims 1-7, characterized in that the metal sulfates separated from the concentrated acid are thermally decomposed and the gases produced during the decomposition are processed into high-concentrated sulfuric acid. Process according to any one of claims 1-7, characterized in that the metal sulfates separated from the concentrated acid are neutralized. Process according to any one of claims 1-9, characterized in that at least 80% by weight of the calcium sulphate initially included in the residual acid is precipitated and separated during the pre-concentration and concentration. Process according to any one of claims 1 to 10, characterized in that anhydrite type crystals are added to the preconcentrated acid. 12. A process according to any one of claims 1 to 11, characterized in that the anhydrite type crystals are prepared by reacting a calcium salt solution with sulfuric acid by heating with stirring, by separating the formed precipitate by filtering. Ϊ. And washing and drying. • · • · ·