IE42222B1

IE42222B1 - Process for making calcium sulphate-semihydrate and apparatus used therein

Info

Publication number: IE42222B1
Application number: IE2746/75A
Authority: IE
Original assignee: Hoechst Ag
Priority date: 1974-12-18
Filing date: 1975-12-17
Publication date: 1980-07-02
Also published as: IE42222L; RO67270A; FR2294985B1; GB1474839A; DE2459739B2; DE2459739A1; LU74027A1; PL109429B1; BR7508349A; CH618146A5; NL172142C; NL172142B; NL7514656A; DK574175A; SE405844B; AT340445B; DK142026B; DK142026C; BE836714A; FR2294985A1

Abstract

In a process for preparing calcium sulphate alpha -semihydrate from calcium sulphate dihydrate, which arises as a waste product in chemical processes, for example in the production of phosphoric acid by digesting rock phosphate with sulphuric acid, the calcium sulphate dihydrate in an aqueous suspension is continuously pumped into an autoclave at temperatures above 110 DEG C and recrystallised therein. Calcium sulphate alpha -semihydrate suspension is continuously drawn from the autoclave and is introduced into a separation zone. Finally, the calcium sulphate alpha -semihydrate suspension has cold water passed thereinto on its way from the autoclave to the separation zone. A plant for carrying out the process may consist of a first regulating vessel (2), a rotary filter (5), a further regulating vessel (8), an autoclave (11) and a separation apparatus (16) which are connected in series in terms of flow. Between the autoclave (11) and the separation apparatus (16) a mixing section (12) is arranged, debouching into which, close to its end facing the autoclave (11), there is a cold water supply line (15).

Description

The present invention relates to a process for making calcium sulphate α-semihydrate, e.g. from a calcium sulphate dihydrate by-product obtained in ahemical production, especially in the production of phosphoric acid from sulphuric-acid-processed crude phosphate, of the kind wherein an aqueous suspension of calcium sulphate dihydrate, which may have been freed from organic contaminants and slimy inorganic contaminants by flotation, and from water-soluble contaminants by washing/ is continuously pumped into an autoclave and recrystallized therein at a temperature above 110°C, and the resulting calcium sulphate a-semihydrate suspension is continuously discharged from the autoclave and introduced into a separation zone.

The invention also relates to an apparatus when used in carrying out the present process.

It has already, been disclosed that a calcium sulphate by-product obtained in chemical production, - 2 42222 expecially in the production of phosphoric acid from sulphuric-acid-processed crude phosphate, can be subjected to flotation and counterflow washing and then delivered to an autoclave in which it is recrystallized to calcium sulphate α-semihydrate at an approximate temperature of 120°C and at a pH of from 1.5 to 3.

The resulting calcium sulphate α-semihydrate is continuously taken from the autoclave in the form of an aqueous suspension, which is cooled in a heat exchanger and then separated from the mother liquor in a separator (e.g. a centrifugetor filter). The separated mother liquor contains substantially all of the foreign constituents which were lodged in the crystal lattice, ; the calcium sulphate α-semihydrate crystallized out, containing about 15% of residual moisture, is either mixed with water and made into prefabricated gypsum articles, or dried with hot air and made into gypsum powder (cf. Chemie-Xngenieur Technik, 44 (1972), pages 969-972, and British Patent Specification No.992,468.

A disadvantage which we have encountered in this known process resides in the fact that the calcium sulphate α-semihydrate suspension coming from the autoclave is required to be subjected to indirect cooling before it is possible for it to be delivered to the separating means. Inasmuch as double tube heat exchangers, for example, are normally used to achieve this, it is necessary to select a minimal value for the width or cross-sectional area of the flowing suspension which is to undergo cooling. In other words, it is necessary for the suspension to be passed through the tubes at a relatively high speed. However, this means that the pipe bends and flanges, in &aaa~a particular, are subject to considerable erosion. In addition to this, we have found that the selection of a narrow cross-sectional area for a flowing medium often results in the heat exchanger tubes becoming clogged by scaly deposits left by the carried-over suspended material.

It is therefore an object of the present invention to provide a process of the kind first mentioned, and an apparatus for use in this process, which avoid the disadvantageous effects referred to above, and make it unnecessary for the calcium sulphate a-semihydrate suspension taken from the autoclave to be passed at relatively’ high speed through a cooling zone.

According to the present invention, we provide a process for making calcium sulphate a-semihydrate, wherein an aqueous suspension of calcium sulphate dihydrate is continuously pumped into an autoclave and recrystallized therein at a temperature above 110°C, and the resulting calcium sulphate α-semihydrate suspension is continuously discharged from the autoclave and introduced into a separation zone, in which process, water at a temperature of up to 40°C is introduced into the calcium sulphate α-semihydrate suspension travelling from the autoclave to the separation zone.

Preferred features of the present process provide: (a) for the calcium sulphate α-semihydrate suspension to contain from 200 to 600 g of solid matter per litre; - 4 42222 (b) for the water to be used at a temperature of from 10 to 40°C; and (c) for the water to be used in a proportion of from 0.3 to 1.0 part by volume per part by volume of calcium sulphate α-semihydrate suspension.

The present invention also includes an apparatus, when used in a process as specified in the last preceding paragraph but one, in which a first agitator-provided receptacle, a rotary filter, a second agitator-provided receptacle, an autoclave and a separator are arranged in series, in the order given, in which apparatus the autoclave and the separator are connected together by means of a mixing zone provided with a water supply line opening thereinto near that end of the mixing zone which is the nearer the autoclave.

Preferred features of the present apparatus provide: (d) for a temperature control means to be provided near that end of the mixing zone which is the nearer to the separator; (e) for the water supply line to be provided with a flow-meter and an automatically actuated valve; (f) for the flow-meter to comprise a diaphragm; and for the automatically actuated valve to be arranged to supply the water using a cascade control, and for the temperature control means to be operatively coupled with the flow-meter, the temperature taken by the former permitting the desired quantity of water to - 5 42222 be passed through the flow-meter to‘ be determined.

The arrangements adopted in the process and apparatus of the present invention enable the flow velocity of material travelling through the line connecting the autoclave to the separator to be so reduced that the erosion of pipe bends and flanges, in particular, becomes ignorably small. Thus, in the process and apparatus of the invention, We contemplate the use of a line so large in diameter that it is not likely to become clogged even by pieces of scaly material that may find their way thereinto.

A further advantage of the present process resides in the fact that the addition of the water to the calcium sulphate α-semihydrate suspension results in the formation of a more dilute mother liquor, from which the calcium sulphate q-semihydrate is easier to separate in the separation zone.

In a particularly useful arrangement falling within the scope of the invention, the water is introduced into the mixing zone using a cascade control, whereby the temperature of the calcium sulphate α-semihydrate suspension can be determined within the mixing zone near that end of the latter which is the nearer to the separator, the temperature so determined being communicated to a control unit which is operatively coupled with a water flowmeter and which is adapted to establish the correct desired quantity of water to be passed therethrough, this being a quantity corrected to allow for time and temperature variations. This makes it possible substantially to eliminate variations of pressure which might tend to occur, for example, near the inlet of the mixing zone, being caused by scaly deposits, or by pressure variations 42323 in the autoclave, or by variations in the solids content of the calcium sulphate a-semihydrate.

One exemplary form of apparatus in accordance with the present invention is shown diagrammatically in the single Figure of the accompanying drawing, ι In the drawing: A line 1 for calcium sulphate suspension is arranged so as to pass down through the cover of a first agitator-provided receptacle 2 which is connected by means of a line 3 to the trough 4 of a rotary filter 5.

The gypsum filter cake taken from the drum 6 of the rotary filter 5 is delivered via a chute 7 to a second agitatorprovided receptacle 8, into which opens a line 9 for supplying water. In the second receptacle 8, the gypsum cake together with this water is made into a suspension of predetermined density, pH-value and temperature. Connected to the lower portion of the second receptacle 8 is a line 10 running to,through the cover of, and into, an autoclave 11. A further line runs from the cover of the autoclave 11 to a mixing zone 12, terminating beyond the latter in a centrifuge 16, which has a discharge means 17 for water and a discharge means 18 for CaSO^ semihydrate. Arranged so as to open into the mixing zone is a water supply line 15 having a flowmeter 14 and an automatically actuated valve 13 included therein, the valve being downstream of the flow-meter 14. Also a temperature control (TC), which takes the temperature towards the end of the mixing zone 12, is electrically connected to a quantity recorder FC which in turn is operatively coupled with the flow-meter 14 and the automatically actuated valve 13.

The following Examples illustrate the invention - 7 42222 and its technically beneficial effects.

EXAMPLE 1 (Prior art example) m3/h of a calcium sulphate a-semihydrate suspension was continuously taken from a 25 m -autoclave 5 containing a charge of prepurified calcium sulphate dihydrate suspension, and conveyed through an ascending pipe and a double heat exchanger 40 mm wide to a centrifuge. The suspension had a temperature of 125°C upstream of, and of 85°C downstream of, the heat exchanger. The cooling zone had an approximate overall length of 32 m, with bends therein, bent through 180°, which were enamelled to provide protection against erosion.

After an approximate operation period of 100 hours at a flow velocity of about 2 metres per second, the 180° bends and neighbouring flanges were found to have been rendered leaky by abrasion. During this 100 hour period, it was necessary to interrupt operation eight times, in order to permit the removal of scaly deposits left by the carried-over suspended material, these deposits clogging the cooling zone, or to permit the replacement of eroded pipe portions, EXAMPLE 2 (Process of invention) m /h of a calcium sulphate α-semihydrate suspension at a temperature of 125°C was continuously taken from a 25 m3-autoclave which was continuously charged with prepurified calcium sulphate dihydrate suspension, and delivered through an ascending line and a mixing zone metres long and 80 mm wide to a centrifuge. A T-junction o was used to introduce 5 m /h of water at a temperature of 29°C into, the semihydrate suspension near the inlet of the mixing zone. The suspension had a temperature of 87°C downstream of the mixing zone.

At the resulting flow velocity of 0.8 metres per second, it was possible for the apparatus to be continuously operated over a period of several months.

The water adhering to the calcium sulphate a-semihydrate, after separation of the mother liquor, was found to contain water-soluble contaminants in a proportion reduced by 38%, by virtue of the above-mentioned dilution with water.

Claims

1. A process for making calcium sulphate α-semihydrate, wherein an aqueous suspension of calcium sulphate dihydrate is continuously pumped into an autoclave and recrystallized therein at a temperature above 110°C, and the resulting calcium sulphate a-semihydrate suspension is continuously discharged from the autoclave and introduced into a separation zone, in which process water at a temperature of Up to 40°C is introduced into the calcium sulphate a-semihydrate suspension travelling from the autoclave to the separation zone.

2. A process as claimed in Claim 1, wherein the calcium sulphate dihydrate employed comprises a calcium sulphate dihydrate by-product obtained in the production of phosphoric acid from sulphuric-acid-processed crude phosphate.

3. A process as claimed in Claim 1 or 2, wherein the aqueous suspension of calcium sulphate dihydrate employed is one which has been freed from organic contaminants and slimy inorganic contaminants by flotation, and from water-soluble contaminants by washing.

4. A process as claimed in Claim 1, 2 or 3, wherein the calcium sulphate a-semihydrate suspension contains from 200 to 600 g of solid matter per litre. 10 42222

5. A process as claimed in Claim 1, 2, 3 or 4, wherein the water is used at a temperature of from 10 to 40°C.

6. A process as claimed in any of Claims 1 to 5, wherein the water is used in a proportion of from 0.3 to 1.0 part by volume per part by volume of calcium sulphate “-semihydrate suspension.

7. An apparatus, when used in carrying out a process as claimed in Claim 1, in which a first agitator-provided receptacle, a rotary filter, a second agitator-provided receptacle, an autoclave and a separator are arranged in series, in the order given, in which apparatus the autoclave and the separator are connected together by means of a mixing zone provided with a water supply line opening thereinto near that end of the mixing zone which is the nearer to the autoclave.

8. An apparatus as claimed in Claim 7, wherein a temperature control means is provided near that end of the mixing zone which is the nearer to the Separator.

9. An apparatus as claimed in Claim 7 or 8, wherein the water supply line is provided with a flow-meter and an automatically actuated valve.

10. An apparatus as claimed in Claim 9, wherein the flow-meter comprises a diaphragm.

11. An apparatus as claimed in Claim 9 or 10, when read with Claim 8, wherein the automatically actuated valve is arranged to supply the water using a cascade control, and the temperature control means is operatively coupled with the flow-meter, the temperature taken by the former permitting the desired quantity of water to be passed 5 through the flow-meter to be determined.

12. 'A process for making calcium sulphate a-semihydrate conducted substantially as described in Example 2 herein.

13. An apparatus when used for carrying out a 10 process as claimed in Claim 1 substantially as described herein with reference to the accompanying drawing.

14. Calcium sulphate q-semihydrate whenever made by a process as claimed in any of Claims 1 to 6, or in Claim 12.

15. 15. Calcium sulphate α-semihydrate whenever made with an apparatus as claimed in any of Claims 7 to 11, or in Claim 13.