DE1928771B1 - Process for the separation and recovery of fluorine compounds from gypsum obtained by sulfuric acid digestion of natural calcium phosphate - Google Patents

Description

I 928

The invention relates to a method for the separation and recovery of fluorine compounds by digestion of natural calcium phosphute with sulfuric acid and by filtration of the digestion mass obtained gypsum by treatment with sulfuric acid at elevated temperature.

Natural phosphate rocks usually contain substantial amounts, often 3 to 4 percent by weight, of fluorine. In the usual digestion of phosphate with sulfuric acid, the fluorine is divided into the solid, liquid and gaseous phases. In general, approximately 60 to 70 go _"o the fluorine in the phosphoric acid, 20 to 30", ₀ of the fluorine found in the gypsum, and 2 to 10% of the fluorine arrive already during decomposition in the gas phase. The disposal of the waste gypsum, which is obtained in an amount of 5 parts by weight per part by weight of P ₂ O ₅ , is often difficult and often associated with considerable costs.

Efforts are therefore increasingly being made to utilize the resulting gypsum economically. The conversion to sulfuric acid and cement is particularly worthy of the opportunities that arise special attention for low-sulfur countries, as in this way most of the for the Phosphate digestion required sulfuric acid can be recovered and only a small amount, through Loss-related share must be covered from new sulfur raw materials. The fluorine content of Waste gypsum is caused by the gypsum-sulfuric acid process when it is processed into sulfuric acid the formation of molten eutectics. which the solids reactions in cement production interfere in the hot furnace zone and attack the refractory furnace lining.

From patent specification 47 180 of the Office for Invention and Patents in East Berlin it is known Finally, wash the fluorine-containing plaster of paris with dilute sulfuric acid and then calcine it at 400 ° C to drive off the fluorine compounds. The resulting hydrogen fluoride, sulfuric acid and water vapor containing gases cause considerable in the downstream cooling and washing stage Corrosion, so that this process has not yet been used.

The object of the invention is to create a method for separation and recovery of fluorine compounds from fluorohydric gypsum by treatment with sulfuric acid at elevated temperature, corrosion by the gases escaping from the plaster of paris during the treatment being avoided and the fluorine contained in gypsum is obtained in the form of beneficial pure compounds.

According to the invention, this object is achieved in that the plaster of paris is _{mixed with 1.5 to 5 percent by weight of H 2} SO ₄ and heated to a temperature of 1 K) to 200 C, preferably 140 to 190 C, and the gases formed in the process Obtaining the fluorine compounds are washed. In this temperature range the fluorine compounds contained in the gypsum are _{decomposed with the formation of SiF 4} and HF without, as at higher temperatures, noticeable amounts of the excess sulfuric acid passing into the gas phase. In the gas scrubbing that follows the fluorine expulsion, an optionally HF-containing hydrofluoric acid solution which essentially contains no sulfuric acid is obtained. Pure hydrofluoric acid or its salts can be obtained from this solution without difficulty. If, on the other hand, the treatment of the gypsum with excess sulfuric acid is carried out at temperatures above 200 ° C., in particular in the range from 300 to 450 ° C., considerable volatilization of the excess sulfuric acid cannot be avoided. This not only makes it more difficult to obtain pure hydrofluoric acid and its salts, but the sulfuric acid content that gets into the washing solution is lost for the sulfuric acid recovery and requires an additional consumption of sulfur-containing raw materials to cover the losses.

While the undesirable volatilization of sulfuric acid with increasing temperature above 200 C increases sharply, our investigations showed that the fluorine rub-out from the plaster of paris is only increased significantly up to about 200 C. The further fluorine release above In contrast, around 200 C is only low and has no relation to that caused by increasing Sulfuric acid volatilization-related disadvantage.

According to the preferred embodiment of the invention, the gases are washed with an aqueous solution containing silicofluoric acid and hydrofluoric acid, part of the solution being removed from the circuit and replaced by an equivalent amount of water. By circulating the washing solution, a concentration of a maximum of about 30 percent by weight H ₂ SiF ,, is achieved. A pure hydrate of silicofluoric acid can be obtained from this solution by crystallization, optionally after further concentration by evaporation of water under vacuum. z. B. H ₂ SiF ,, · 2H ₂ O. can be obtained. The purity of this hydrofluoric acid is considerably higher than that of the acid which can be obtained from the absorption solutions of the phosphate lock.

According to the invention it is further provided that the plaster of paris by indirect heating with steam on the for the defluorization required temperature of 110 to 200 C is heated. The one with the indirect The gases formed by heating the waste gypsum are relatively rich in fluorine compounds and water vapor, so that a small amount of scrubbing liquid can be used for gas scrubbing. the The washing stage can accordingly be made small. The inventive treatment of plaster of paris with indirect heating is expediently carried out in a slightly inclined steam tube calcining drum. This apparatus consists essentially of one Tube with heating tubes that are arranged around the inner wall and acted upon by heating steam. According to the invention The plaster of paris to be treated is applied to one end of the pipe and through the rotation of the pipe conveyed to the other end while the gases or vapors escaping from the plaster of paris normally flow in the opposite direction through the pipe and removed at the plaster feed end and fed to the absorber.

The sulfuric acid can, for example, be sprayed onto the plaster of paris with constant circulation. In any case, the gypsum should be 0.5 to 1.5 equivalents to 1 equivalent of fluorine at the start of the thermal treatment Contain sulfuric acid. The apparent sulfuric acid deficiency is due to the fact that some of the fluorine in the gypsum as sodium silicofluoride is present, which thermally without sulfuric acid

consumption in silicon tetratluoride and nairium fluoride disintegrates. Only one third of the fluorine present in this form is reacted with sulfuric acid necessary.

The invention is described in more detail below using several exemplary embodiments.

example 1

Every hour KM) parts by weight of natural calcium phosphate with 3.8 percent by weight of fluorine in a reaction tank with 94.8 parts by weight of sulfuric acid per hour. The reaction sludge is filtered on a tilting cell filter. 230 parts by weight of moist gypsum with 1.2 percent by weight of fluorine, based on dry, are obtained per hour Calcium sulfaldihydrate.

The plaster of paris is mixed with 9.85 parts by weight of 70 "sulfuric acid per hour in a steam tube sizing drum and heated to 180 ° C. The residence time of the plaster of paris at this temperature is 60 minutes. The plaster of paris treated in this way contains only 0.23 percent by weight of fluorine and is suitable for SO ₂ - Well suited for extraction by the gypsum-sulfuric acid process.

The gases withdrawn from the drum consist essentially of water vapor as well as silicon telrafluoride and hydrogen fluoride. The gases are _{washed in a washer with an H 2} SiF "solution: the temperature of the solution is 90 C. The H ₂ SiF" solution is circulated, the concentration increasing over time to 28 percent by weight of H ₂ SiF, , increases. In order not to let the fluorine losses become too great as the vapor pressure increases with the increasing H ₂ SiF, concentration, an average of 7.K5 parts by weight of the 28 "H ₂ SiF" solution are withdrawn from the circuit every hour when this concentration is reached and replaced by fresh water. In this way, 96 ""

ίο the fluorine compounds contained in the gases washed out, while at the high washing liquid wedge siempcralur practically no water vapor condenses from the gases.

By vacuum evaporation and crystallization

• 5 can be obtained from the withdrawn washing liquid pure Ii ₂ SiF ,, · 2H ₂ O.

Example 2

I'm the limit of the temperature and the amount the addition of sulfuric acid on the fluorine expulsion / u investigate was a series of experiments at 1 K). 150.2 (K). 300 and 400 C with different Sulfuric acid additives carried out. The one

Plaster of paris that was set for 2 seconds contained 1.0 percent by weight F. The recovery time was 1 hour. A denotes the F content in percent by weight. based on the dry DiIn dial, η the degree of expulsion of the in F in "". The results are summarized in the following table.

vj mcngcstclli and shown in the diagram.

Fluorine content A. based on dry gypsum, and degree of expulsion of with sulfuric acid added plaster samples after heating in a drying cabinet

H ₂ SQ ₄
in the dry

plaster

Weight percent

110 C

1.7
2.7
3.7
4.7

Weight I

percent

0.67
0.52
0.45
0.80

33
48
55
20th

150 C

A I

Weight percent

0.59
0.41
0.31
0.22

4!
59
69
78

201) C A. Ί O 1" 300 A. C. ·> 400 C. A "ι it O O Weight- ^: 0 i Weight Weight -29 Percent | 50 ^: percent ■ percent 56 IiM)! 0.71 ».50 0.44

0.32

0.20
0.15

68
80
85

0.26
0.22

74
78

0.22
0.20
0.11

From the diagram and the table it can be _{assumed that the F content of the gypsum decreases with all H 2} SO ₁ additives with increasing temperature. Above 200 C, however, considerable temperature increases are necessary for a further reduction in the F-content, which at the same time cause undesired H ₂ SO ₄ evaporation from the gypsum, which is made of sulfuric acid. According to this, defluorination above 200 C is possible, but the absorption solution then contains considerable amounts of sulfuric acid. An addition of sulfuric acid above 5.0 percent by weight will increase the expulsion of fluorine, but such a high addition of sulfuric acid impairs the economic viability of the process.

With sulfuric acid additions of less than 1.5 percent by weight, the defluorination of the gypsum in the temperature range up to 2 (M) C is generally too low. Only in the case of gypsum from the digestion of low-fluorine phosphates can the residual fluorine content in the gypsum, which is permissible for further processing in the gypsum-vinylic acid process, _{be achieved with lower H 2} SO _{4 additions.}

The calcium sulfate defluorinated according to the invention is heated further in direct heat exchange with the hot exhaust gases from the cement kiln. The _{excess H 2} SO ₄ is volatilized and washed out in a downstream scrubbing stage together with the sulfur trioxide already present in the exhaust gases. This washing acid is expediently mixed with the gypsum to be defluorinated according to the invention.

Claims (4)

Patent claims: 1. Process for the separation and recovery of fluorine compounds au ·, by digestion -.on natural calcium phosphate with sulfuric acid and filtration of the digestion mass) gypsum obtained by treatment with Schwcfclsäuic increased temperature, thereby ken 11- draws that the gypsum with an amount of sulfuric acid from 1.5 to 5.0 percent by weight, based on on calcium sulfate dilrate. mixed and is heated to a temperature of 110 to 200 C and the resulting gases for recovery the gaseous fluorine compounds are washed. 2. The method according to claim!. Characterized in that the gases are washed with a circulating aqueous kieselfluorwassersioffsäurehalligen solution and part of the solution is removed from the circuit and replaced by an equivalent amount of water. 3. The method according to claim 1 and 2, characterized in that the .Mixture of plaster and Sulfuric acid is heated to a temperature of 140 to 190 C. 4. The method according to any one of claims 1 to 3, characterized in that the plaster of paris through indirect heating is heated with steam. 1 sheet of drawings