Method of preparation of calcium sulphates dehydrated starting from phosphogypsums.

The industrial manufacture of phosphoric acid " by attacking natural phosphates to sulfuric acid, results in the formation as a by-product of large amounts of gypsum (about 5 tons of gypsum per ton of p2o5 product) T-

(4 Ρ θ) 2^AC 3 +³ SO4 HR₂ + 2H₂ 0 ->3 AC s04, 2 hr₂ 0 + 2 PO4 HR₃

The artificial gypsums phosphogypsums or particle size have a structure which distinguishes it from the natural gypsums and depends on original phosphate, the method and conditions of walking in the workshop. For a workshop operating in driving conditions with phosphate constants determined, the quality of the gypsum at the filter output is homogeneous. The crystals of phosphogypsum are always fine, their particle size is generally between 20 and 200 γ; they have a high residual moisture (between 20 and 30 %) and are soiled a number of impurities which make them unsuitable, raw, preparing gypsum of good quality.

The use of gypsum for the manufacture phosphogypsums therefore requires, prior to any heat treatment, scrubbing aqueous medium, known to the art, designed to eliminate or neutralize harmful impurities. After purification, the phosphogypsum is in the form of a filter cake formed of very fine crystals retaining 15 to 20 % moisture. Its transformation into plaster then requires a drying step for removing water moisture followed by a calcination step for removing a part or the entire water of hydration of the gypsum.

The method according to the invention only relates to the drying of the phosphogypsum previously cleaned and dried.

Generally, the heat treatment of the gypsum (so4 AC, 2 hr₂ 0) in atmosphere unsaturated with water vapor results in increasing temperature the following successive phases:

the hemihydrate - ] W (So4 AC, 1/2 hr₂ 0)

- 1' ill anhydrate (so4 AC, MC₂ 0) also known as soluble anhydrite

- 1 '•anhydrate ll (so4 AC) also known as anhydrate insoluble or lime

- 1' anhydrate of I (so4 AC■■î,0 + AC)

The hemihydrate alone and 1' and anhydrite II are commonly used, alone resulted in a mixture, in the composition of casts typically used in the construction industry. The occurrence of^! the anhydrite III in the platers is however, except when very particular, carefully avoided since because of its high avidity of water, 1' and anhydrite III which provides plasters containing this engagement very steep, phenomenon known as "whip" by men skilled artisan. Anhydrite III rehydrates also spontaneously hemihydrate in the presence of moisture of hate atmospheric so that the plasters containing them does not retain stable properties over time.

The cooking method according to the invention, particularly well adapted for treating phosphogypsums, makes it possible to prepare pure hemihydrate or gypsum plasters or mixing hémydratej? the anhydrite II-d-free + anhydrate lll.

The methods of baking or pot traditionally employed in 1^: chalk pit industry, perfectly suited for treatment of natural gypsums coarsely divided are very difficultly transposable at phosphogypsum because of its extreme fineness. This state of dividing the original phosphogypsum constitutes yet a benefit from the thermodynamic point of view. In effect, two factors limit the rate of dehydration of the gypsum:

- its low thermal conductivity which causes a large temperature gradient within each grain, and then within the layer of grain when the gypsum is disposed in deep bed

- the low diffusion speed of water initially within the grain to and possibly through the layer to make maximum thermodynamically, the division state the phosphogypsum yet is suitable to il use a cooking technique which makes it possible to develop a maximum effective exchange between the gypsum and the heat source. This object is completely achieved by using the technique of baking gypsum in a stream of hot gas. Until now, this technique was not wearing the fireworks to less costly obtain the phase hemihydrate without simultaneously manufacture of 1' and anhydrite III is very harmful to the quality of the plaster the US Patent 3,648 994 discloses a procedure which reduces the proportion of anhydrite III formed, without being able to completely eliminate the above however, in précliauffant'd first the gyp -: e with temperature as close as possible a temperature you're phase change gypsum hemihydrate>fa - - -; m has to be able to carry out the transformation with gases at Tg. oérature aa.ssi lower as possible. It is evident that in this ο - -, -: the cal,-to-.ries required to transform gypsum hemihydrate endothermiqut read must be made by the volume "- bout large gas, which is harmful for 1" procèo economics "

We have found that it is. possible to prepare VBE1~plasters finally free anhydrite III by treatment thei framework policy gypsum in a hot gas stream by operating as described below:

- is carried out in a first phase dehydration of the phosphogypsum in a stream of hot gas without taking precautions to avoid stringent anhydrite III

- in a second phase, now rehydrate hemihydrate state all of 1' and anhydrite III formed by treatment of the product previously dehydrated in a stream of cold air or warm and wet.

- In the first phase of the treatment according to the invention, it is thus possible to use hot gas streams at relatively high temperature, thereby increasing the rate of dehydration and provide the necessary calories for the transformation by a volume minimum air. The second phase of the treatment further provides 1 by eliminating the anhydrite III is harmful to the quality plaster of recovering in the gas stream part of the heat from the sensible heat of the product and the calories of the exothermic reaction and anhydrite III-+ water -}héroihydrate and recycle them to thermal treatment (stage cooking or better drying in the case of the phosphogypsum) according to the desired end product, the process conditions of the first phase of the treatment will be different:

a) thus bringing, into contact gypsum particles dry for the O, 8 to 3 s with a gas stream temperature of between 260 and 350° c containing 20 to 500 grams of the d^: water per kg of dry gas, will get to the staked dewatering stage 1 + hemihydrate mixtures including anhydrite III in variable proportions containing at least 25% by weight anhydrite III

b) bringing gypsum particles in contact during 0.8 to 3 s with a dry gas stream temperature of between 350 and 450 °c, obtained 1' and anhydrite III-pure.

c) putting the gypsum particles in contact during 0.8 to 3 s with a stream of gas, temperature of between 600 and 800° C., containing 200 to 500 g of water per kg of dry gas, obtained at the output of the cooking stage mixtures including anhydrite III of the anhydrite II + ratios vaviables but a minimum 25% anhydrite III.

In the second processing phase, the products previously calcined next steps described previously are contacted during 0.8 to 3 s with an air stream of temperature between 20 and 50° c and relative humidity between 60 and 95 %, As a result of the second phase, this results in the following products:

- starting from a mixture hemihydrate - + - anhydrate

Lll (case a) of 1' which is excellent as pure ^ hëmîhydrate plaster prefabrication,

from the d -¹ the anhydrite III-(case b) of hémihydratejâ very early setting 1.

- starting from a mixture including anhydrite III of the anhydrite II-T-(case c) and anhydrite III can even sex ' the vir base for the composition of plaster D.¹enduetien.

Figures 1 and 2 represent a scheme to apparatus in which the present invention may be performed;

figure 1 - is shown diagrammatically a pneumatic dryer,

figure 2 - is shown diagrammatically an entire facility with several pneumatic driers phosphegypsepourlécher then for subjecting the dry-obtained cooking process and rehydration according to the invention.

- For carrying out the invention, use can be made of any equipment for efficient contact between the gas and solid particles. But the apparatus as a preferred pneumatic conveying such as that shown in Figure 1. It consists essentially of a vertical column (1) traversed by a stream of air optionally previously heated by a generator (2) hot gas and drawn by a fan (3) main. Phosphogypsum is fed from the bottom of the column via a rotary lock (4) or a sluice pressure balanced followed by an injector (5) disperser which makes a very efficient scattering the phosphogypsum in the gas stream. The top of the column, the solid product is separated from the gas stream in a cyclone (6). The solid product recovered at the tip of the cyclone via a rotating sluice valve (7) can then be discharged by a dispenser flap to the discharge pipe (8) or recycled at the base of the column.

It is possible, in the air dryer -. processing the phosphogypsum by humid air by injecting e.g. 1" steam (91

The residence time of the solid product in the column in ES *, F. ixë by the height of the column and by the gas velocity qul ' can be varied by varying the flow rates of the person operating the ventilated.

- Starting from the wet gypsum previously cleaned, the phase of processing a polarizer (drying, dehydration, except réhydrata) may be carried out sequentially and in■CR39 tinu in a system constructed of 3 dryers tires mounted in series as shown in Figure 2.

The first dryer (10) (11) is supplied by the ate; wet gypsum from a station D.^: purification and in (1the j) by dry air properly heated (13) to the crimped the dryer the particles of phosphogypsum dry its ", separated from the air in a cyclone (14) and in VBE1^{the R} dAA - oyées exits, a storage hopper (15) either at the next dryer.

The second dryer (16) is supplied with phcsp ' (17) and in ncgypse with hot air, possibly wet,

(18) in; in this dryer the dehydration is carried out the phosphogypsum; obtained, after separation in the cyclone (19) on the one hand air can be still hot, recycled, and an at least partially dehydrated phosphogypsum déctit as above that in sends the next dryer rehydratation "

The third dryer is supplied with (20) by the solid product from the cyclone (19) and by cold air wet (21). The solid product exiting said dryer is separated from the air in the cyclone (22) and stored (23) in various hoppers; air can be recycled into various painted installation and preferably, as it is nearly dry air, into the dryer (10),

Figure 2, is indicated in dashed line of paths and preferential air used.

The gas recirculation is. furthermore designed to yield, without supplying vapor, 1.' greatest possible moisture in the second stage and the optimal thermal efficiency in the set of 1^{the R} plant

The following non-limiting examples illustrate 1' disclosure:

Used as starting material a phosphogypsum washed and neutralized containing 16 % water humidity from a Florida phosphate. The gypsum is crystallized in the form of spherical particles having an average diameter of about

This phosphogypsum was subjected in three dryers tires mounted in series to treatment according to the invention R is the process conditions in the three successive dryer were the following:

- - Stage drying;

. gas temperature at the inlet of the column: 2, 50^{the I -} C.. gas temperature at the outlet of the column: 90 °c, residence time of the gypsum in the column: 1.2 seconds. temperature of the gypsum to the column outlet; 65 °c. humidity gypsum to the column outlet:<0.5 %

- Stage of dehydration:

. temperature of the gypsum to the column inlet: 60^C C.. gas temperature decay to 1 input of the column: 340 ° C.

. moisture hot gas to the inlet of the column:

200 g of water per kg of dry gas

. time séncur product in the column; the I ≤. gas temperature at the outlet of the cyclone: ibCO. product temperature to the output of the cyclone:

130° C.

- Stage rehydration:

. product temperature to the column inlet;

120° C.

, air temperature: 40^I C.; 90% relative humidity

. temperature of the plaster to the output of the cyclone; o0 °C the content of water of crystallization of the product exiting the cooking stage is 1.8%; water content scream tallisation the product exiting the stage réhydrataticn is.

of 6.4 % and its content of 0.5 gypsum %. The hemihydrate thus obtained has the following characteristics determined according to standard NFB 12,401

- quantity of gypsum in 100 g of water at saturation mixing: 160 grams

- setting time for mixing in a ratio

water Start=0.8%: 2 min end: min plaster

- mechanical strength obtained on test pieces dry 4 x 4 x 16 I® to I-mixing in a week for an

ratio water =0, 8

gypsum

in compression: 110 bar bending: 40 bar

The characteristics of this plaster make it parfai•body particularly adapted for use as plaster prefabrication for making tiles (wallboard as blocks) or plasterboard (wallboard and hockey).

Floor for the dehydration we have rigorously as in the example I is:

- Stage of dehydration:

. product temperature at the inlet of the column:

60° C.

. warm and moist gas temperature at the inlet of the column: 100^IC

. moisture hot gas to the inlet of the column:

400 g of water per kg of dry gas

. product residence time in the column: I-e. temperature of the plaster to the output of the cyclone 320° C.

The rehydration step is conducted as in 1' such as I:

- the product exiting the dewatering stage is a mixture composed of 70 % anhydrite III and 30 % anhydrite III. The output of the stage of rehydration, the mixture consists of 70 % hemihydrate and 30 % anhydrite III.

- After adding a low dose of a conventional retarder above mixture, we obtained a plaster having the following characteristics:

- quantity of gypsum in 100 g of water at saturation mixing: 200 grams

- begin engagement time: 7 min

- job completion time: 17 min

- final setting time: 25 min

- mechanical strengths:

. 4 x 4 x 16 mm test tubes to 2 H on wet

compressing: 24 bar bending: 12 bar

. 4 x 4 x 16 mm test pieces on days 7 to dry compressing: 75 bar bending: 30 bar this plaster fills therefore all conditions required by 12,301 NFB plaster casts on-grade construction PFC2. It can thus be used as plaster coating.