FI58768C - FOERFARANDE FOER FRAMSTAELLNING AV RENT SYNTETISKT KALCIUMSULFATHEMIHYDRAT AV ETT ORENT, FOSFATHALTIGT OCH FUKTIGT KALCIUMSULFATSLAM. - Google Patents

Description

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JMTä l J '} UTLÄGGNINGSSKMFT Oö / Oo 2¾¾¾ (45) ^ v' (51) Kv.nc.3 / in * .a.3 C 01 F 11/46 FINLAND — FINLAND (21) 2158/73 (22) HtkemiipUv · - Ans6ki «lng * rf« c 05.07.73 (23) AlkiipiM — GIMgfc «tsd> g 05-07-73 (41) Interpreters | ulkls * lc * t - Bllvlt off« Mllg 07-01.7 ^ _ ^ * ( 441 NihttvUulpeoon Jt kuL | ullud * un pvm. - 0 rMant · ocn ragistarstyralaan '' Αμ (Μ <μ utUgd och utl.skrWtm puMkarad 31.12. Ö0 (32) (33) (31) Catched «chairs ·» »» —EUgird priority 06.07.72

Federal Republic of Germany Förbundsrepubliken Tyskland (DE) P 2233189-1 (71) Veba-Chemie Aktiengesellschaft, Dorstener Stpasse 227, h66 Gelsen-Kirchen-Buer, Federal Republic of Germany Förbundsrepubliken Tyskland (DE) (72) Hans-Friedrich Kurand Schliephake, Diisseldorf,

Federal Republic of Germany-Förbundsrepubliken Tyskland (DE) (7 ^) Oy Kolster Ab (5M Method for the preparation of pure synthetic calcium sulphate hemihydrate - Förfarande för framställning av rent synthetiskt kai c i umsulfathemihihydrate

The invention relates to the preparation of pure synthetic plasters obtained by the production of phosphoric acid by the wet process. In this case, the purification of the gypsum can be carried out both as a separate process from the production of phosphoric acid and also immediately during the phosphoric acid process. The process of the invention results directly in pure calcium sulfate hemihydrate.

Phosphoric acid processes give synthetic gypsum which, although more uniform in composition than natural gypsum, is not useful for many purposes due to impurities, in particular phosphoric acid in insoluble and water-soluble form. This applies in particular to the use of such gypsum in the construction and cement industries.

The amount of PgO 2 impurities increases in the standard phosphoric acid method to 1-2% by weight, based on the dried gypsum. 80% of the P 2 O 2 impurities are then usually in insoluble form and about 20% in soluble form. However, by further washing with water, soluble phosphoric acid can be removed, but not insoluble.

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The prior art further includes phosphoric acid processes which give purer gypsum in the form of calcium sulphate dihydrate crystals.

According to one such method, for example, the hemihydrate is first precipitated and then recrystallized to large dihydrate crystals, which are then filtered off. With this method of operation, the proportion of P 2 O 2 in the final product is reduced to about 0.1. However, such a P 2 O 2 content is still too high for the cement industry.

Direct hemihydrate is obtained in such phosphoric acid extracts using phosphorus-sulfuric acid mixtures. However, in such methods, due to the nature of the method, no very low P 2 O 2 content in the gypsum is to be expected.

Dependence of hemihydrate persistence range on nitric acid concentration. and temperature was determined experimentally. In this case, it was examined in each case whether the products formed under the given reaction conditions (nitric acid concentration and reaction temperature) were still capable of binding after the addition of water. In the experiments, the dilution effect of the water contained in the dihydrate was also taken into account when reporting the nitric acid concentration.

To present the experimental results, it proved expedient to present not only the concentration c at which the change occurs at the minimum temperature t as a function of this temperature (Fig. 1), but also the function 100 / c. As shown in the corresponding figure 2, the corresponding measuring points are arranged on practically the same line, for which the equation (1) —g— = mt + holds as a good approximation in the range 20-100 ° C and in the nitric acid concentration range (-—> 3.333) of less than 30% by weight. b, where the values of the constants m and b are: (2) m / (= tgc £) J = 0.02853 (3) b = 0.7001+

The stability range of the calcium sulfate hemihydrate is thus below these lines and the stability range of the dihydrate is above them.

Surprisingly, it has now been found that pure synthetic calcium sulphate is immediately obtained in the preparation of phosphoric acid when impure starting materials suspended in a nitric acid solution having an HHO 2 content of 30 to 80% by weight, preferably 35 to 70% by weight, are treated with stirring at a temperature of 22 to 100 ° C. C, preferably 50-90 ° C, one of said reaction conditions being freely selectable in a given range, but the value of the other must be chosen to be in the stability range of calcium 3,58768 sulphate hemihydrate, which, simplified, is the area of the equation * mt + b (1) between the line defined by c and the diagram pp / t abscissa t between the ordinates plotted at 22 ° C and 100 ° C, leaving the range of values pp> 3.33 (corresponding to c ^ 30 by weight # HNO ^) and wherein for the reaction conditions without other side components and at atmospheric pressure, Equation (1) has the form ...

pp = 0,0285 t + 0,700 (2) that with this treatment the impurities dissolve, that at the same time the calcium sulphate recrystallises to calcium sulphate hemihydrate and that the product thus purified is separated from the liquid phase in a manner known per se and the precipitate is washed countercurrently with water.

Although the conversion of naturally occurring gypsum to hemihydrate with strong mineral acids and saline solutions at elevated temperature is known, it is completely surprising that the high cleaning efficiency observed with nitric acid according to the invention is found in gypsum obtained in the production of phosphoric acid. Furthermore, the good filterability of the hemihydrate formed by this treatment under the conditions of the invention was surprising.

When nitric acid is used, the cleaning efficiency is so high that the hemihydrate formed after water washing without additional dry matter can be directly cast into gypsum boards. When treating impure synthetic gypsum with other strong mineral acids or salts, no such quality improvement has been achieved. An additional advantage of this method of work is that the spent nitric acid, which contains gypsum impurities, can still be used in a fertilizer plant after the gypsum has been filtered off. Thus, it is possible to make full use of the contaminants of the filtered gypsum consisting mainly of P 2 O 2 as a nutrient in the fertilizer industry.

The purification of the impure calcium sulphate dihydrate according to the invention with nitric acid must be carried out at a sufficiently high concentration and at sufficiently high temperatures.

The experimental results summarized in Figure 2 show that the hemihydrate can be obtained both at very high nitric acid concentrations (low pp values) and relatively low temperatures (e.g. 80 # HNO 2 and 25 ° C) as well as at medium nitric acid concentrations and high temperatures (e.g. 35% by weight HNO 2). ^ and 90 ° C).

58768 k

In order to obtain highly permeable products, it has proved expedient to work at the highest possible temperatures. On the other hand, at too high temperatures, unwanted acid losses due to nitric acid evaporation could occur. In the experiments, the preferred temperature range has been 50-90 ° C and the most preferred nitric acid concentrations have been between 35-70%.

Equations 1-3 are suitable for calculating the required nitric acid concentration c at a given temperature. However, in order to calculate the required temperature t at a predetermined nitric acid concentration in the whole solution, it is advisable to modify Equation 1 accordingly. This gives (5) t „1. J20 -]> (° C) m cm In this case, the values of the constants are: (7) 1 / m (= ctg «) = 35.05 (8) b / m (= a) = 2k, 55 The validity of these equations is shown in Tables 1 and 2, in which the corresponding concentrations or temperatures for the predetermined temperatures t or the predetermined HNO 2 concentrations c have been calculated using the above equations and then compared with the values read from Fig. 1.

Minor deviations in the calculation of nitric acid concentration or temperatures can be eliminated with a corresponding safety margin. If the conversion to the hemihydrate were allowed to take place directly at the conversion point of Figure 1, then it would otherwise take longer than in cases where working in a "safe area", i.e. in the half-hydrate stability range, sufficiently far from the boundary lines. On the other hand, due to the associated increased corrosion and higher costs, it is natural that too much nitric acid or too high temperatures are not used for the conversion. In practice, it has proved expedient to use safety margins Δ c and Δ t of the order of 5-6 by weight? » HNO 2 and 5-6 ° C. Corresponding examples of appropriately preferred HNO 2 concentrations and temperatures t are shown in the last columns of each table.

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Table 1 Calculation of the HNO 2 concentration to be used at a given reaction temperature

Output / 100, C c diagram (c - c) C

thermal 1 c 'calc. 7 According to 1. 'P A, o ..% by weight. “Weight-? when Ac = o tlla · c HNO, 55: "° - * HNO weight- * 3 Π ^ ° 3 3 HNO 3 20 70 1 + 0 1.81 + 2 5l +, 3 55.8 1.5 60 50 2.12 ^ 1 + 7.0 1 + 7.2 0.2 53 60 2.1 + 12 1 + 1.5 1 +0.5 -1.0 1 + 7 70 2.698 37.1 36.6 -0.5 1 + 3 80 2.983 33.5 32.8 -0.7 1 + 0 90 3.2681 30.6 30, 6 - 37 100 3,553 28.1 29.1+ 1.3 31+

Table 2

Calculation of the retained reaction temperature for a predetermined nitric acid concentration Starting concentration 100 t1. t diagram 1 (t-t) t

by weight- $ HKO ^ c as', ° C 0q & S A t = 5 ° C

30 3.333 92.3 9h 1.7 97 35 2.857 75.6 72.9 -2.7 81 1 + 0 2,500 63.1 60.8 -2.3 68 1 + 5 2.222 53.3 52.9 -0 , 1 + 58 50 2,000 1 + 5.6 1 + 6.1 + 0.8 51 55 1, 818 38.7 1 + 0.8 2.1 l +! + 60 1.667 33.9 35.8 1.9 39 65 1,538 29,1+ 31,2 1,8 31+ 70 1,1 + 29 25,5 26,8 1,3 31 75 1,333 22,2 22,9 0,7 27 80 1,250 19,3 19, 1 -0.2 2l +

The process according to the invention is therefore based on treating the crude sulphate precipitated in the phosphoric acid solution at temperatures of 588768 to 22-100 ° C with nitric acid having a concentration c such that the concentration of HNO 2 in the entire liquid phase is in the range of 30-80% by weight. hemihydrate stability range. In this case, the nitric acid concentration to be selected for dissolution at a given temperature t is determined by the equation (l *) = c + Δ c (w / w # HNO ^) or at a given nitric acid concentration c the reaction temperature must be t, pr obtained from equation (6) t = t + Δ t (° C) pr where the calculated quantities c and Constants in these equations are obtained from Equations 1-3, 5, 7 · The safety margins Ac or At in Equations 1 and 2 should be at least 5 ~ 6 (weight # HNO ^ or ° C) . However, it is not appropriate to use more than 20 (wt. ° or ° C) as a safety supplement, as this may expose the conditions for the formation of anhydrite and thus lose the economic advantages of the process.

The reaction temperature required for a given sulfuric acid concentration or the nitric acid concentration required for a given reaction temperature can also be determined graphically. In this case, it is necessary to work in the respective area of existence of the hemihydrate, i.e. below the lines drawn in Fig. 2. Values in the vicinity of a curve or line must be avoided in order for the change to occur as quickly as possible. Most preferably, the procedure in question is first determined graphically and then proceeded according to equations k and 6.

The conversion can be promoted by vigorously stirring the suspension.

However, it is not necessary to break the crystals because the phosphate inclusions and other impurities enter the solution without difficulty by the method according to the invention.

The action time of the acid on the sulphate precipitates depends on the HNO 2 concentration, the reaction temperature, the stirring strength, the aging after precipitation and the nature of the impurities, which action time depends in particular on the composition or origin of the crude phosphate in question. Generally, 1-6 hours are sufficient, but longer exposure times promote the growth of crystals and thus the filterability of the fines. Normally, however, the optimum crystal size is already formed after 3 ~ 1 * hours.

The nitric acid concentration determined from the equations or diagrams, respectively, is taken into account when determining the reaction temperature, means the entire liquid phase. The residual solution adhering to the fines must be taken into account when adding nitric acid (or lowering the temperature) and possibly increasing the concentration of 7 58768 acids to be added accordingly. When adding salts with a very high water content of the crystal, for example MgSO 4. T H ^ O, this water content should also be taken into account as a basis for calculating the required acid concentration throughout the liquid phase. However, after the change, the hemihydrate is so stable that the post-wash can be performed with hot water without immediate conversion back to the dihydrate.

The calcium sulphate hemihydrate obtained under the conditions of the invention has very good filterability. It does not contain any insoluble, i.e. water-insoluble, phosphorus pentoxide; it can therefore be freed of all adhering impurities by filtration in a countercurrent water wash and is therefore obtained in a very pure form.

Similar results are obtained if calcium sulphate hemihydrate is prepared in a way "in statu nascendi" in the production of phosphoric acid in the presence of nitric acid of equivalent concentration. Thus, in the nitric acid process, the otherwise usual intermediate phase, the preparation of the dihydrate, is abandoned. The sulfuric acid normally used in phosphoric acid leaching is also not required. In such a process, nitric acid acts simultaneously as a solvent and as a substance to obtain pure calcium sulfate hemihydrate. The sulphate ions required to obtain calcium sulphate hemihydrate in this mode of operation must be introduced in the form of soluble salts, preferably in the form of ammonium or potassium salts. In contrast to the above-mentioned method for separating the dihydrate from the phosphoric acid solution, this working procedure gives a hemihydrate with a P_0 content of the order of 0.2%, i.e. only 10-20% of the impurities common in other processes.

Due to the high concentration of impurities, the hemihydrate or dihydrate obtained from phosphoric acid processes in the uses so far has not been further applicable for construction purposes without special post-treatment methods. In most cases, it must be disposed of as waste or recovery areas must be provided. The semi-hydrate obtained, filtered and washed by the process according to the invention, on the other hand, can be processed directly into building boards. While gypsum species obtained from current phosphoric acid production processes must be cleaned, dried and re-converted into castable products by costly and often extensive process steps, thus necessitating the application of environmentally harmful substances such as dust and hydrogen fluoride. , because virtually no worthless waste products remain and at no additional cost a valuable product is obtained for the production of building boards or gypsum for other purposes.

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Example 1a

Standard finishing method

Phosphoric acid gypsum containing (calculated on the dry matter content) a total of 2.06% P 2 O 3 of water-soluble 0.9% by weight is mixed with 2 parts of water at 50 ° C for 3 hours, then filtered and washed with a filter cake. with water. The washed gypsum contained impurities as follows: P ^ O ^ iia total 1.26% P ^ O ^ ria water soluble 0.11+%

Example 1b

Method of purification with nitric acid The crude phosphoric acid gypsum used at a lower concentration or at a lower temperature in 1a is mixed with 2 parts of Uo%. nitric acid at 50 ° C for 3 hours, then filtered and washed with a filter cake. The washed gypsum contained impurities as follows: P ^ O ^ ^ a total of 0.96% P ^ O ^ ^ from water-soluble 0.08%

Example 1c

The gypsum used in the purification process 1a according to the invention is mixed with 2 parts of 65% HNO 2 at 50 ° C for 3 hours. The resulting hemihydrate is filtered and the filter cake is washed with 58768 9 water. The washed gypsum contained impurities as follows:

PgOj for a total of $ 0.09

PgOt-water-soluble 0.08% le (according to the invention) contains significantly less impurities compared to Lathan and lb. The calcium sulphate hemihydrate obtained in lct is then mixed without further addition of water and cast into a sheet which hardens after a short time.

Example 2

Purification method according to the invention

From normal phosphoric acid stock (dihydrate gypsum slurried in phosphoric acid) the gypsum is separated by centrifugation, dissolved in 2 parts of 65 # nitric acid and treated for 3 hours at 65 ° C. Here, too, the conversion to puo-hydrate takes place. When the PgO 2 content of the unwashed gypsum before the addition of nitric acid was 15 * 6 i »%, the washed hemihydrate still contains the following impurities:

Po0c total 0.25 3 ^ 25

Po0c water soluble 0.10 # 2 5

Example 3 &

Precipitation of potassium sulphate dihydrate with ammonium sulphate (HNO 2 concentration and reaction temperature in the dihydrate range)

In a continuous process, 10 kg of crude phosphate is dissolved hourly in 36 kg of $ 50 HNO 2, which was diluted to $ 35 with acid in the most painful washed wash water, and gypsum is precipitated by the simultaneous addition of 13 kg of ammonium sulfate. After a residence time of 5 hours at 65 [deg.] C., the crystals consisting of CaSO4 x 2 → 0 are filtered off and washed. The filter cake contains the following impurities:

PgO ^ !! a total of 1 * 32 i »- 8.5 fan wasted imported

PGO ^ to

PgO 2 from water-soluble 0.58 and NH 4 -N 0.32 £ NO 2 -N 0.57 i> H 2 O 45 *

Example 3b

Precipitation of potassium sulphate hemihydrate with ammonium sulphate (method according to the invention)

In a continuous process, 10 kg of crude phosphate are dissolved hourly in 25 kg of 70 # HNO 2, diluted with washed gypsum wash water to about 50 # HNO 2, and 13 kg of ammonium sulfate and stirred for 5 hours at 65 ° C. . In this case, CaSO 4 x 1/2 HgO crystals are formed, which are filtered off and washed. The filter cake consists of CaSO 4 x 1/2 EHO. It contains the following impurities:

PgO 2 totaling $ 0.21 <$ 1 wasted P 2 O 2 P 3 O 2 water soluble 0.11 NH 3 -N 0.11 i NO 2 -N 0.06 i H 2 O 35 i

The CaSO 2 x 1/2 E 2 O obtained is brought to a flowable state by stirring and sheets are cast therefrom by means of a mold, which hardens after a short time.

A comparison between 3a * n and 3b shows that the hemihydrate obtained by the process (3b) according to the invention contains considerably less impurities than the CaSO 4 x 2H 2 O obtained in Experiment 3 &.

Example 4

Precipitation of calcium sulphate hemihydrate with potassium sulphate

In a continuous process, 10 kg of crude phosphate are dissolved hourly in 45 kg of HNO at $ 40, while 18 kg of potassium sulfate are added at 80 ° C. After a residence time of 3 hours, the crystals consisting of CaSO 4 x 1/2 HgO are filtered off. After washing with hot water, the filter cake still contains the following impurities:

PgO 2 adds up to 0.30

PgOj. water soluble 0.10 and NO -N 0.10 i K 2 O 0.20 i H 2 O $ 35.

Claims (6)

A process for the preparation of pure synthetic calcium sulfate hemihydrate for the production of cast plaster or plaster mortar or other building materials of an impure, moist calcium sulphate slurry, characterized in that impure starting materials are suspended in a nitric acid solution of an HNO solution. #, preferably 35 ~ 70% by weight, is treated with stirring at a temperature of 22-100 ° C, preferably 50-90 ° C, one of the said reaction conditions being freely selectable within the range indicated, but the value of the other must therefore be chosen to be within the range of calcium sulphate hemihydrate, which is simplified by the area that becomes between a straight line defined by the equation - = mt + b (1) c and the abscissa ti diagram / t between the ordinates drawn from points 22 ° C and 100 ° C, where the range of the values 3, 3.33 (corresponding to c <30 wt% HNO where atmospheric pressure equation (1) is in the form pp = 0.0285 t + 0.700 (2), by this treatment the impurities are released, that the calcium sulphate is simultaneously recrystallized into calcium sulphate hemihydrate, and that the purified product is separated from the liquid phase in a manner known per se and the precipitate is washed countercurrent with water. Process according to claim 1, characterized in that the nitric acid concentration required at a specified temperature t for conversion to hemihydrate is determined by means of equations 1 and 2 respectively, and in practice nitric acid is selected with an HNO a security amount Δ c greater than C = c + Ac (3) pr where fcc is of the order of ς c ^ 20 wt-Jf. 3. A process according to claim 1, characterized in that where the nitric acid concentration in the aliquot phase is c, the temperature required for conversion to hemihydrate is first determined by the equation (5) t.im.ifc] (5) m c m J