WO2001007370A1

WO2001007370A1 - Removal of sulphates from a feedstock

Info

Publication number: WO2001007370A1
Application number: PCT/ZA2000/000128
Authority: WO
Inventors: Richard Guy Batson; James Ian Ramsay
Original assignee: The Butts Ash Trust
Priority date: 1999-07-28
Filing date: 2000-07-27
Publication date: 2001-02-01
Also published as: AU6541800A

Abstract

A process for removing sulphates from a sulphate-containing aqueous feedstock comprises seeding the feedstock, which is at a temperature, Tf, where ambient <Tf<120°C, by adding thereto particulate calcium sulphate anhydrite. The seeded feedstock is heated to Tp at which precipitation of calcium sulphate anhydrite from the feedstock can take place, with Tp>Tf and Tp≥90°C. In a precipitation zone, solid calcium sulphate anhydrite is allowed to precipitate from the feedstock, and thereafter, in a separation zone, precipitated calcium sulphate anhydrite is separated from treated water. The precipitation and separation zones are maintained at a pressure sufficient to ensure that the heated feedstock does not boil.

Description

REMOVAL OF SULPHATES FROM A FEEDSTOCK

THIS INVENTION relates to the removal of sulphates from a feedstock. It relates in particular to a process for the removal of sulphates from a feedstock, and to a precipitation/separation reactor.

According to a first aspect of the invention, there is provided a process for removing sulphates from a sulphate- containing aqueous feedstock, which process comprises seeding a dissolved sulphate-containing aqueous feedstock, which is at a temperature, T_f, where ambient < T_f<120°C, by adding thereto particulate calcium sulphate anhydrite as a seeding component; heating the seeded feedstock to a precipitation temperature, T , at which precipitation of calcium sulphate anhydrite from the feedstock can take place, with T >T_f and T_p≥90°C; if necessary, adding a calcium component, which is at least slightly soluble in the heated seeded feedstock, to the heated seeded feedstock; allowing, in a precipitation zone, solid calcium sulphate anhydrite to precipitate from the feedstock, thereby to obtain treated water containing a lower level of dissolved sulphates than the feedstock; separating, in a separation zone, precipitated calcium sulphate anhydrite from treated water; maintaining the precipitation zone and the separation zone at a pressure sufficient to ensure that the heated feedstock does not boil; withdrawing a slurry comprising precipitated calcium sulphate anhydrite and water; and withdrawing treated water. In one embodiment of the invention, the temperature, T_f, may comply with 40°C<T_f≤120°C so that there is little or no conversion of the seeding component, ie the seed particulate calcium sulphate anhydrite, to gypsum (CaS0₄.2H₂0) irrespective of the contact time between the seeding component and the feedstock prior to the heating of the seeded feedstock to the precipitation temperature, T . Typically, in this embodiment, T_f may be less than 100°C, eg 55°C to 65°C.

However, in another embodiment of the invention, the temperature, T_f, may comply with ambient≤T_f≤40°C . The process may then include minimizing the contact time between the seeding component and the feedstock prior to the heating of the seeded feedstock to the precipitation temperature, T_, thereby to prevent or inhibit conversion of the seeding component to gypsum, which can occur at such relatively low feedstock temperatures if there is sufficient contact time between the seed calcium sulphate anhydrite and the feedstock. Thus, in the event that T_f is relatively low, the point of addition of the seeding component to the dissolved sulphate-containing aqueous or unseeded feedstock should be as close as possible to the point at which the heating of the seeded feedstock to T_p occurs, and the heating to T should be effected as rapidly as possible. In this embodiment, T_f may be about 20°C, eg about 24°C.

The process may thus include heating the unseeded feedstock to the temperature, T_f . This may be effected by subjecting the unseeded feedstock to heat exchange with treated water and/or with slurry. Similarly, the heating of the seeded feedstock to the precipitation temperature, T_p, may include subjecting it to heat exchange with treated water and/or with slurry. More particularly, in one embodiment of the invention, at least some of the treated water may, in a seeded feedstock heat exchange stage, be used to heat the seeded feedstock, and thereafter, in an unseeded feedstock heat exchange stage, be used to heat the unseeded feedstock. The addition of the particulate calcium sulphate anhydrite or seeding component, in the form of some of the slurry, to the feedstock may then be effected upstream of the seeded feedstock heat exchange stage.

In another embodiment of the invention, at least some of the treated water may, in a primary seeded feedstock heat exchange stage, be used to heat the seeded feedstock, and thereafter, in an unseeded feedstock heat exchange stage, be used to heat the unseeded feedstock, while, in a secondary seeded feedstock heat exchange stage, at least some of the slurry may also be used to heat the seeded feedstock, with the secondary seeded feedstock heat exchange stage being located upstream relative to the primary seeded feedstock heat exchange stage. The addition of the particulate calcium sulphate anhydrite or seeding component, in the form of some of the slurry, to the feedstock may then be effected upstream of the secondary seeded feedstock heat exchange stage.

In yet another embodiment of the invention, treated water may, in a seeded feedstock heat exchange stage, be used to heat the seeded feedstock, while slurry may, in an unseeded feedstock heat exchange stage, be used to heat the unseeded feedstock. More particularly, the unseeded feedstock may enter a feed vessel or tank, with unseeded feedstock being withdrawn from the feed tank, passing through the unseeded feedstock heat exchange stage, and being returned to the feed vessel or tank. The process may then also include withdrawing unseeded feedstock from the vessel, for feeding to the reactor after seeding thereof . The seeding of the feedstock may then be effected by adding the particulate calcium sulphate anhydrite or seeding component to the feedstock which has been withdrawn from the vessel. Some of the slurry from the unseeded feedstock heat exchange stage may be used for the seeding of the feedstock. The process may thus include feeding at least a portion of the slurry produced in the process into the unseeded feedstock which has been withdrawn from the feed vessel, to seed the feedstock .

The remainder of the slurry which is produced is withdrawn as a produc .

Each heat exchange stage will thus comprise at least one suitable heat exchange device, such as a plate or tube heat exchanger, with plate heat exchangers being more economical and effective for heating of the unseeded feedstock, and tube heat exchangers being more suited to handling the seeded feedstock and the calcium sulphate anhydrite slurry, although other designs of heat exchangers may also be used.

Tests have shown that to produce, within a few minutes, treated water having a final sulphate concentration close to that predicted from equilibrium tests, it is necessary to maintain a concentration of the particulate calcium sulphate anhydrite, in the precipitation zone, of at least 0.05% by mass of the feedstock, preferably at least 0.5% by mass of the feedstock, and that the reaction rate increases with temperature over the range considered. Thus, at least part of the calcium sulphate anhydrite which is precipitated from the feedstock in the precipitation zone, ie at least part of the slurry produced, may be thickened to a density such that, when added to the feedstock, the seeded feedstock contains at least 0.05%, and preferably at least 0.5%, calcium sulphate anhydrite, by mass. The thickened slurry may typically comprise from 5% to 40% by mass solids, i^'e precipitated calcium sulphate anhydrite. The thickening of the precipitated calcium sulphate anhydrite or slurry may be effected in the separation zone and, possibly, the precipitation zone.

The process will include, where necessary, raising the pressure of the seeded feedstock to the precipitation zone pressure to prevent the feedstock from boiling. This may be effected by passing the seeded feedstock through a pumping stage.

The process may include, where necessary, further heating the seeded feedstock, after it has been subjected to the heat exchange with treated water and/or with slurry, eg after it has passed through the seeded feedstock heat exchange stage(s), to raise its temperature to T_p . The further heating may be effected by any suitable heating means, such as steam heating through coils or by direct injection; electrical heating by tracing, electrodes, immersion heaters, radiant heaters, microwaves etc; or direct heating by flames. Thus, an external heat source, such as waste process steam, electricity or gas is used. The heating of the feedstock to T may be effected in the pipelines between stages, in special heating equipment or in the precipitation zone itself.

It will be appreciated that the seeding of the feedstock, the heating of the seeded feedstock to the precipitation temperature, and, where applicable, the addition of the calcium component, can, if desired, take place more-or-less simultaneously. For example, all these steps may be effected in the precipitation zone.

In one embodiment of the invention, the precipitation zone is thus separate from the separation zone.

The precipitation zone may then be provided by a reactor which includes a pressure vessel of substantially spherical or cylindrical shape, with the inside of the vessel providing the precipitation zone. Mixing means, such as a stirrer, circulating tank eductor, a mixing pump on a sidestream of seeded feedstock, or the like, may be provided inside the vessel, for keeping the particles of calcium sulphate anhydrite and the feedstock in continuous motion, thereby to enhance the kinetics of the precipitation process. In the reactor, the calcium sulphate anhydrite will precipitate, preferentially, on the solid seed, resulting in the formation of larger particles of calcium sulphate anhydrite, although some crystals form spontaneously .

Instead, the precipitation zone may be provided by a pipe reactor, with the diameter and configuration of the pipe reactor being such that the calcium sulphate anhydrite particles are kept in suspension in the feedstock passing through the pipe reactor.

Treated water containing the precipitated calcium sulphate anhydrite, passes from the precipitation zone to the separation zone. The separation zone may be provided by a separation vessel containing separation means, such as one or more parallel plate or cone stacks, a filter means, or the like, and by means of which separation of the slurry or the precipitated calcium sulphate anhydrite from the treated water is effected.

However, in another embodiment of the invention, the precipitation zone and the separation zone may be provided in a single precipitation/separation reactor having an upright elongate precipitation/separation reactor shell in which the separation zone is located above the precipitation zone so that the separated precipitated and/or seed calcium sulphate anhydrite drop back under gravity into the precipitation zone where it is mixed with incoming feedstock, such as by means of a stirrer or circulating tank eductor. The seeded feedstock then enters the shell at a position intermediate its lower and upper ends. Inside the shell, the calcium sulphate seed is thus able to react with dissolved sulphates present in the feedstock, thereby to cause the dissolved sulphates to precipitate from the feedstock solution. Precipitated calcium sulphate anhydrite accumulates on the particles of the calcium sulphate anhydrite seed, resulting in the formation of larger particles of calcium sulphate anhydrite, which settle to the bottom of the shell, from where they are withdrawn as the slurry which thus contains also some feedstock water. The resultant treated water is withdrawn from the top of the shell .

Separation means in the form of a parallel inclined plate stack through which rising water passes, may be provided in the separation zone, as hereinbefore described. The separation means may be provided above the point at which the seeded feedstock enters the shell, to assist in removal of calcium sulphate anhydrite particles from the water. In particular, the plate stack may comprise a plurality of inverted truncated conical plate separation members nestled one inside the other. Instead, any other suitable separation means, such as a filter, may be used.

The process may include, if necessary, subjecting the treated water from the separation zone to filtration, to remove traces of suspended calcium sulphate anhydrite remaining in the treated water.

The further heating, when necessary, may be effected in the reactor, eg by means of a reactor steam jacket, direct steam injection, or an electric immersion heater. Instead, or additionally, the further heating, when necessary, may be effected upstream of the reactor, eg by means of a suitable heater, such as an immersion heater located upstream of the reactor, electrical trace heating around a feedstock pipeline leading to the reactor, or the like. The temperature T_p will be selected by taking into account the desired sulphate level in the treated water. A typical specified dissolved sulphate level for potable water is 200mg/ . Bearing in mind that the solubility of calcium sulphate anhydrite in water is inversely related to the temperature of the water, this solubility level of sulphate in water is reached when the calcium sulphide anhydride is in equilibrium with water at about 125 °C. Thus, to produce potable water of this quality, T_D will need to be at least 125°C, and, in practice, about 150°C. To maintain the feedstock in the precipitation and separation zones at non- boiling conditions at this temperature, requires that the process be operated at a pressure greater than 400 kPa(g), and preferably greater than 500 kPa(g) .

It will be appreciated that, by means of the hydrothermal sulphate removal process according to the invention, dissolved sulphates in an aqueous feedstock can be precipitated, after the addition of a balancing concentration of calcium, as calcium sulphate anhydrite, rather than as gypsum (CaS0₄.2H₂0) , and the level of the dissolved calcium sulphate in the feedstock reduced by raising the temperature of the feedstock. This latter feature arises^' therefrom that the solubility of calcium sulphate anhydrite in water is inversely related to the water temperature, ie as the water temperature increases, the solubility level of calcium sulphate anhydrite decreases. The saturation concentration of calcium sulphate in water at 20°C is about 2000mg/£; however, water can also be supersaturated with dissolved calcium sulphate up to a level of about 5000mg/ .

Sulphates can also be present in water in other forms such as ferrous sulphate, when, under acid conditions, the solubility may be even higher, or as sodium sulphate, which is extremely soluble in water. Thus, by means of the present invention, dissolved sulphate levels can be reduced from concentrations of several thousands parts per million to very low levels.

The calcium component is added to the feedstock in those cases where there is a deficiency of calcium ions in the feedstock. Thus, having determined the concentration of sulphates to be removed from the feedstock, sufficient of the calcium component is added such that the dissolved calcium in the feedstock is stoichiometrically in balance with the sulphates to be removed. The calcium component or calcium compound which is added to the heated seeded feedstock may be a calcium salt which is soluble or slightly soluble in the heated seeded feedstock, and may be selected from lime, calcium chloride, calcium bicarbonate or the like. Acid feedstocks will normally be treated with lime which will simultaneously supply the calcium required to form calcium sulphate anhydrite and the alkalinity necessary to neutralize the free acids and to precipitate simultaneously other metals in the feedstock, such as iron, manganese and nickel, as hydroxides. These processes can take place concurrently with the precipitation and separation of calcium sulphate anhydrite in the same equipment. The metal hydroxides, carbonates and other species formed under the high temperatures and pressures of the hydrothermal sulphate removal process are crystalline, not amorphous, and settle and filter easily, yielding byproducts which are compact and easily handled.

According to a second aspect of the invention, there is provided a precipitation/separation reactor, which includes a normally upright cylindrical reactor shell having its respective ends closed off and providing an operatively lower precipitation zone and a separation zone above the precipitation zone and in direct communication therewith; a feed inlet leading into the shell intermediate its ends and located in the precipitation zone; a slurry outlet at the lower end of the shell; a treated water outlet at or near the upper end of the shell; and a separation member stack within the shell above the feed inlet and located in the separation zone, the separation member stack comprising a plurality of inverted truncated hollow conical parallel separation members nestled one inside the other, with the upper and lower ends of all the separation members being located at the same levels respectively, and with the upper end of the innermost separation member being closed off with a conical component, the lower end of the conical component thus having the same diameter as the upper end of the innermost separation member.

The reactor may include mixing means in the precipitation zone for effecting mixing of a body of fluid in the precipitation zone. The mixing means may comprise a stirrer or a circulating tank eductor.

The lower end of the shell may be closed off with an inverted conical lower end piece in which, in use, precipitated matter collects, with the slurry outlet being provided at the apex of the conical end piece. The upper end of the shell may be closed off with a domed upper end piece, with the treated water outlet being provided in the domed end piece.

The angle of inclination of the separation members to the vertical is typically about 30°, but this can vary depending on the parameters affecting the reactor performance in use. The separation members may ^'be supported on a support which spans the reactor shell. The separation members may be located in position relative to each other by means of elongate spacers extending from the upper ends of the separation members to their lower ends, and with the spacers being spaced circumferentially apart. The angle of inclination of the conical component to the vertical preferably is the same as that of the separation members .

The invention will now be described by way of example with reference to the accompanying drawings .

In the drawings,

FIGURE 1 shows a simplified flow diagram of a process for removing sulphates from a feedstock containing dissolved sulphates, according to a first embodiment of the invention;

FIGURE 2 shows a longitudinal cross -sectional view of the precipitation reactor of Figure 1, with the spacers omitted for clarity;

FIGURE 3 shows a sectional view through III -III in Figure 2 ;

FIGURE 4 shows a simplified flow diagram of a process for removing sulphates from feedstock containing dissolved sulphates, according to a second embodiment of the invention;

FIGURE 5 shows a simplified flow diagram of a process for removing sulphates from a feedstock containing dissolved sulphates, according to a third embodiment of the invention;

FIGURE 6 shows a longitudinal cross-sectional view of the settler of Figure 5, with the spacers omitted for clarity;

FIGURE 7 shows a side view of one of the separation components of the settler of Figure 6; and

FIGURE 8 shows a simplified flow diagram of a process for removing sulphates from a feedstock containing dissolved sulphates, according to a fourth embodiment of the invention.

Referring to Figures 1 to 3, reference numeral 10 generally indicates a process for removing sulphates from a feedstock containing dissolved sulphates, according to a first embodiment of the invention.

The process 10 includes an incoming feedstock line or conduit 12, fitted with a control valve 14. The line 12 leads into a mixing tank 16. The tank 16 is fitted with level control means 17 connected to the control valve 14.

A feedstock line or conduit 18 leads from the tank 16 to a pump 20. The line 18 is provided with a shut-off valve 22.

A seeded feedstock line or conduit 24 leads from the pump

20, and is fitted with a pressure release valve 26, a pressure gauge 28, a temperature gauge 30 and a shut-off valve 32. The line 24 leads into a seeded feedstock heat exchanger 34, with a seeded feedstock line or conduit 36 leading from the heat exchanger 34. The line 36 is provided with a temperature gauge 38, a further temperature gauge 40, and leads into a precipitation/separation reactor, generally indicated by reference numeral 50. Electrical trace heating 42 is provided around the line or conduit 36 between the temperature gauges 38, 40.

The reactor 50 comprises an upright elongate cylindrical shell, generally indicated by reference numeral 52. The shell 52 comprises a first cylindrical portion 54 to the lower end of which is fitted a conical end piece 56. A shell component 58 flares upwardly from the upper end of the shell portion 54, with a cylindrical skirt 60 being attached to the upper end of the flared component 58. A domed end cap 62 closes off the upper end of the skirt 60. Circumferentially extending flanges 64 on the skirt 60 and the end cap 62 serve to secure the end cap 62 to the skirt 60.

A feed inlet 66 is provided in the skirt portion 54 and terminates in an upwardly directed axially aligned nozzle or eductor 68 having a reduced diameter. A port 72, for receiving a temperature gauge 74, is provided in the shell portion 54.

The shell portion 54 provides a lower precipitation zone 70, while the shell component 58 provides a separation zone 71 above the precipitation zone and in direct communication therewith .

A slurry outlet 76 is provided at the apex of the conical end piece 56.

Within the flared shell portion 58 is located an inverted truncated conical separation plate member stack, generally indicated by reference numeral 80. The stack 80 comprises a plurality of inverted truncated separation plate members 82 nestled one -within the other and extending parallel to each other. The lower ends of the separation members 82 are located at the same level, as are their upper ends. The lower ends of the separation members 82 are located on a support, generally indicated by reference numeral 84. The support 84 comprises two diametrically extending support members 86 spanning the shell and on which the lower ends of the separation members 82 rest.

The separation members 82 are located in position relative to each other by means of spacers 88 extending upwardly from the lower ends of the separation members to their upper ends. The spacers 88 are located circumferentially apart, and three or more, for example six, spacers are provided for each separation member. The spacers associated with each separation member are attached, eg welded, to the rear side of the separation member.

The angle of inclination of the separation members 82 to the vertical is 30°. A conical component 90 is mounted to the top of the innermost separation member 82 of the stack 80. The diameter of the lower peripheral edge of the component 90 is thus the same as the diameter of the upper peripheral edge of the innermost separation member 82. A small aperture (not - shown) is provided at the apex of the component 90, to permit air escape from the component 90, eg at start-up.

A connection 94, for receiving a temperature gauge 96, is provided in the shell skirt 60, as is a connection 98 for receiving a pressure gauge 100.

A treated water outlet 102 is provided in the domed end cap 62.

A conduit or line 104 leads from the treated water outlet 102 to the heat exchanger 34, with a treated water line or conduit 106 leading from the heat exchanger 34. The line 106 is fitted with a valve 108 for controlling the pressure in the reactor 50. A portion of the line 104 is also provided with electrical trace heating 42. The line 104 is also fitted with a pressure gauge 105.

A slurry line or conduit 110 leads from the slurry outlet 76 of the reactor 50 to an unseeded feedstock heat exchanger 112, with a slurry line or conduit 114 leading from the heat exchanger 112. The line 114 is provided with a peristaltic pump 116 which operates so as to release the pressure in the slurry line 114 from the process pressure to about atmospheric pressure. A line or conduit 118 leads from the discharge of the pump 116 to the line or conduit 18 immediately upstream of the pump 20, and is fitted with a valve 120. An excess slurry withdrawal line 122, fitted with a valve 124, leads from the line or conduit 118. The process 10 includes a flow line 126, fitted with a shu -off valve 128 and a pump 130, which leads from the vessel 16 into the heat exchanger 112. The heat exchanger 112 can typically be a concentric tube heat exchanger. A conduit 128 leads from the heat exchanger 112 back to the feed vessel 16.

The electrical trace heating 42 which is strapped to the pipeline or conduit 104, is provided to heat treated water withdrawn from the reactor 50 further, in the event that additional heat is required for heat exchange in the heat exchanger 34, such as during start-up.

The process 10 also includes a seed slurry start-up line 132 for providing start-up seed.

In use, dissolved sulphate containing feedstock or feed water, such as mine effluent, enters the process 10 along the line or conduit 12. It will be appreciated that, if necessary, the feedstock can be treated to contain dissolved calcium sulphate. For example, the water may initially contain sulphate ions in a form other than calcium sulphate, eg in the form of sodium sulphate. The pretreatment will then include treating the water with a source of calcium ions, eg by adding calcium chloride or lime (calcium hydroxide) thereto, so as to form the dissolved calcium sulphate, with the purpose of the process thus being to remove the sulphate ions . In water at ambient temperatures, ie at around 20°C, the saturation concentration of calcium sulphate is about 2000mg/^; however, due to oversaturation, the actual calcium sulphate concentration may be up to 5000mg/ . It is typically necessary to lower this to about 300mg/-? which is an acceptable level of calcium sulphate in potable water. The incoming feedstock is thus at ambient temperature, typically about 20°C, and it may be introduced into the process 10 at a flow rate of, for example, 240 /hour.

In the heat exchanger 112, the feedstock is heated by heat exchange with slurry from the reactor 50, while at the same time the temperature of the slurry is reduced to below 100°C so that it can be safely discharged via the excess slurry withdrawal line 122.

By means of the heat exchanger 112, the feed vessel 16 is maintained at a temperature of about 24°C, ie T_f is about 24 °C. Typically, about 10£/hour of slurry is seeded into the feedstock along the flow line 118, so that the total flow rate along the line 24 is about 25θ /hour at a solids

(calcium sulphate anhydrite) concentration of about 0,5% by mass. The slurry is at a temperature of about 60°C and the temperature of the seeded feedstock is raised to about 25 °C after the slurry has been added thereto. The point of addition of the slurry to the feedstock is close to the heat exchanger 34, and the heat exchanger 34 is designed such that the temperature of the seeded feedstock is rapidly raised above 40°C. The pump 20 raises the process pressure to 600 to 700 kPa(g). The seeded feedstock temperature is typically raised, in the heat exchanger 34, to about 150°C by heat exchange with hot treated water from the reactor, with the reactor temperature being about 155°C, ie T_p is about 155°C. By means of the trace heating 42 attached to the conduit or pipe 104, the temperature of the treated water from the precipitation reactor is raised to about 160 °C, with the treated water exiting the heat exchanger 34 at about 30°C.

At 160°C, the equivalent saturation pressure for water is 535 kPa(g). Thus, operating the reactor 50 at a pressure of 6 to 7 bar, will ensure that the solution within the reactor 50 is non-boiling. In the shell portion 54 of the reactor 50, the calcium sulphate anhydrite seed particles have sufficient residence time to allow them to react with dissolved calcium sulphate in the feedstock, with this calcium sulphate also being converted to the anhydrite form and precipitating out onto the seed particles. In this fashion, larger particles are formed. The intimate contact between the hot feedstock and the anhydrite seed causes precipitation to occur. In particular, the high velocity, in an upward direction, of the seeded feedstock emerging from the nozzle or eductor 68 'pulls' liquid and solids upwardly in the region of the nozzle, creating upward/downward circulation of the liquid and solids in the region of the nozzle. As the particles grow due to the precipitation of calcium sulphate anhydrite, they reach a point where they are sufficiently large so that they are no longer entrained by the upwardly moving liquid, and settle downwardly. These larger particles settle out in the conical end piece 56 of the reactor 50 from where they are continuously withdrawn as a thickened slurry, typically having a solids concentration of about 20%(m/v), along the line 110. Thus, the slurry is at a pressure of 600 to 700 kPa(g) and at the same temperature as the reactor, ie 150°C to 160°C. Typically the flow rate of the slurry will be about 50<?/hour when the feedstock flow rate is about 240£/hour.

Treated water within the shell portion 54 of the reactor 50 passes upwardly along the inclined separation members 82 which thus function as a parallel plate separator to remove solid particles from the treated water which thus accumulates in the upper portion of the reactor, ie above the separation plate stack 80. Fine anhydrite particles, which have a lower settling velocity than the general upflow velocity in the reactor shell portion 54, flow into the separation stack 80 where they are captured and agglomerated. The large agglomerates slide off the separation members 82, and drop to the bottom of the reactor .

The conical component 90 ensures that the particles which accumulate in the upper portion of the reactor slide back into the upper ends of the separation members 82, and then back into the lower portion of the reactor.

Treated water, ^' containing substantially no solid calcium sulphate particles and around 300mg/£ dissolved calcium sulphate which is the saturation level of calcium sulphate anhydrite in water at 150 °C, is withdrawn along the line 104 and passes through the heat exchanger 34. Typically, the water exits the heat exchanger 34 at a temperature of about 30°C, and it is withdrawn from the process along the line 106 at this temperature.

The hot slurry withdrawn along the line 110 passes into the heat exchanger 112 and exits the heat exchanger along the line 114, typically at a temperature of 70°C to 80°C. The slurry passes through the pump 116 where its pressure is reduced to about atmospheric pressure, with the slurry then being routed along the line 118 as hereinbefore described. Excess slurry, ^' ie excess calcium sulphate anhydrite which is not required for seeding purposes, is withdrawn along the flow line 122.

Referring to Figure 4, reference numeral 150 generally indicates a process according to a second embodiment of the invention for removing sulphates from a sulphate-containing aqueous feedstock.

Parts of the process 150 which are the same or similar to those of the process 10 hereinbefore described with reference to Figures 1 to 3 , are indicated with the same reference numerals. The line or conduit 24 of the process 150 is additionally fitted with a flow indicator/controller 152 and a sampling point 154.

A seeded feedstock concentric tube heat exchanger 156 is provided in the line or conduit 36. The conduit 36 is also provided with a pressure gauge 158, a sampling point 160 and a valve 161 upstream of the heat exchanger 156, as well as with a pressure gauge 162 and a shut-off valve 164 downstream of the heat exchanger 156 ahead of the reactor 50.

The reactor 50 of the process 150 is similar to the reactor 50 of Figures 1 to 3 apart therefrom that it is provided with a mixer 166 driven by an electric motor 168 and which provides continual mixing in the precipitation zone 70. It is also fitted with electric immersion heaters 170 for raising the seeded feedstock temperature to the precipitation temperature, T_n . The top of the reactor 50 is provided with a pressure indicator/controller 172.

The treated water line or conduit 104 leading from the top of the reactor 50 is fitted with a sampling point 174 and a shut-off valve 176 and, instead of leading into the plate heat exchanger 34, leads into the heat exchanger 156. Thus, the hot treated water from the reactor 50 is, in the first instance, used to heat up seeded feedstock in the heat exchanger 156; thereafter, by means of a flow line 178 which leads from the heat exchanger 156 to the heat exchanger 34, it is used to heat up unseeded feedstock in the plate heat exchanger 34. The line or conduit 178 is provided with a shut-off valve 180, a pressure gauge 182 and a temperature gauge 184. The treated water line or conduit 106 leading from the heat exchanger 34 is provided with a pressure gauge 180, a temperature gauge 182 and a flow control valve 184. A branch line or conduit 188, fitted with a valve 190 leads from the line 106 to the feed tank or vessel 16. This line is used during start-up whilst the seeded feed water temperature is being raised to

Tp .

The excess slurry withdrawal line or conduit 122 leads directly from the slurry line 110 leading from the bottom of the reactor 50, while the seeding component conduit 118, which is fitted with a pump 192, leads into the conduit 36 upstream of the heat exchanger 156.

The process 150 also includes a lime slurry tank 194. A conduit 196 fitted with a valve 195, a peristaltic pump 197 and a further valve 198 leads into the reactor 50.

In use, the process 150 is similar to the process 10, and its reactor 50 functions in substantially the same manner as the reactor 50 of the process 10 save that the continuous mixing of lime, feedstock and seed calcium sulphate anhydrite is ensured by means of the mixer 166 in the precipitation zone 70, while heating up of the seeded feedstock to the precipitation temperature T in the zone 70 is effected by means of the electric immersion heaters 170.

Sufficient lime is added along the line 196 to ensure that dissolved calcium and sulphate iron concentrations in the heated seeded feedstock in the precipitation zone 70 are stoichiometrically in balance.

Referring to Figures 5 to 7, reference numeral 200 generally indicates a process according to a third embodiment of the invention for removing sulphates from a sulphate-containing aqueous feedstock.

Parts of the process 200 which are the same or similar to those of the processes 10, 150 hereinbefore described with reference to Figures 1 to 4, are indicated with the same reference numerals.

The process 200 is similar to the process 150, except that the precipitation zone 70 and separation zone 71 are separate from each other, and are provided by separate items of equipment.

The process 200 thus includes a precipitation reactor, generally indicated by reference numeral 202. The precipitation reactor 202 includes a substantially spherical pressure vessel 204, fitted with a mixer 206 driven by an electric motor 208. The precipitation reactor

202 is fitted with a temperature indicator/controller 210 and a pressure indicator/controller 212. Heating of the seeded feedstock to its precipitation temperature, T is effected within the pressure vessel 204 by means of live steam injection through a conduit 214 which leads into the bottom of the pressure vessel 204.

A transfer line or conduit 216 leads from the pressure vessel 204 to a settler, generally indicated by reference numeral 220.

The settler 220 includes an upright cylindrical shell 222 fitted with a connection 224 to which the line or conduit 216 is connected. A skirt 226 is fitted to the upper end of the shell 222, and is provided with a peripheral flange 228. A domed top 230, provided with a complementary peripheral flange 232, closes off the upper end of the skirt 226. A connection 234, to which the line 104 is connected, is provided on the domed top, as are connections 236, 238 to which are mounted temperature and pressure indicators/controllers 260, 262 respectfully. The lower end of the shell 222 is closed off with an inverted conical bottom 240 having, at its apex, a connection 242 to which the line 110 is connected. Separation means in the form of a stack or array 250 of hollow truncated conical separation components 252, are provided inside the shell 222. The members 252 nestle one within the other, are spaced vertically apart, and extend parallel to each other. The lower ends of the separation members are thus staggered vertically, as are their truncated upper ends. The stack 250 includes clockwise separation components 252, as hereinafter described, as well as anticlockwise separation components 252, as also hereinafter described. In the stack 250, the clockwise separation components 252 and the anticlockwise separation components 252 are arranged in alternating fashion. Thus, apart from the uppermost separation component and the lowermost separation component, each clockwise separation component 252 is sandwiched between two anticlockwise separation components, while each anticlockwise separation component 252 is sandwiched between two clockwise separation components .

As indicated in Figure 7, each clockwise separation component 252 comprises a hollow truncated conical separation member 254 having a lower end or base 256 and an upper end 258 which is thus of smaller diameter than the base 256. The separation member 254 has an upper separation surface 260, as well as a lower separation surface 262.

From the upper separation surface 260 protrudes a plurality, typically about 9, elongate spacers 264. Each spacer 264 extends from the base 256 to the upper end 258 of the separation member 254, and is in the form of a strip protruding from the separation surface 260. The thickness of each strip is typically in the order of about 3mm, while the width of the strip, ie the height that the strip protrudes from the surface 260, is typically about 15mm. Each spacer 264 is mounted perpendicularly to the surface 260, and has an upper edge 265 which is spaced from the surface 260. Each spacer 264 is curved, ie it does not extend linearly from the base 256 to the upper end 258 of the separation member 254. In particular, each spacer 264 is arranged such that the angle of the valley 266 formed between the spacer 264 and the surface 260, ie the line of intersection of the spacer with the separation surface, has a constant set value along the entire length of the spacer. In other words, the valley angle or the true angle that the line of intersection 266 of the spacer with the separation surface makes with the horizontal, is a constant set value along the length of the spacer. Thus, the valley apex or the line 268 representing the intersection of the spacer 264 with the separation surface 260 is in the form of a helical spiral.

The spacers 264 slope upwardly to the left when the separation component is viewed in elevation, as seen in Figure 7. In other words, when the separation component 32 is viewed from the top (not shown) , the spacers 264 spiral or turn in a clockwise sense. The anticlockwise separation components 252 are similar to the clockwise separation components except that their spacers 264 slope upwardly to the right when the anticlockwise separation components are viewed in elevation. In other words, when the anticlockwise separation components 252 are viewed from the top, their spacers 264 spiral or turn in an anticlockwise sense .

The stack 250 also includes a central support pipe 270, with the lowermost separation component 252 being attached to the lower end of the pipe 270, eg by means of welding. The lowermost support component 252 is provided with a plurality of spaced elongate stiffeners 272, as well as stiffening rings 274.

The upper end of the pipe 270 is attached to a support 276 A tubular component 278 leads from the truncated upper end of the uppermost separation component 252. The component 278 is mounted in a circular plate 280 which is sandwiched between the flanges 228, 232 such that the inside of the domed top 230 is separated from the inside of the skirt 226 by the plate 280. Water can pass from the upper ends of the separation components 252, through the tubular component 278, and out of the upper end of the tubular component into the inside of the domed top 230, ie into the zone above the plate 280.

All the separation components 252 are thus supported on the lowermost support components 252, with one support component nestling within the other. The spacers 264 serve to space adjacent separation members apart, to carry the weight of superior separation components, and impart rigidity to the separation members. In particular, each spacer is supported by at least two spacers, in zones of cross-over, of the support component below it, through the separation member of the separation component below it. In other words, each spacer 264 is supported in at least two places along its length, thereby obviating any tendency for it to tilt and to distort the separation member to which it is attached.

Thus, in the stack 250, the separation components 252 extend parallel to each other and provide parallel inclined separation surfaces. The angle of inclination of the upper and lower separation surfaces of the separation components to the horizontal is 60° .

In use, the suspension of precipitated calcium sulphate anhydrite in treated water, which enters the separator or settler 220 through the inlet connection 224, moves downwardly, and enters the spaces between adjacent separation components 252. The suspension thus moves upwardly between the separation components. As the suspension moves upwardly between the separation components, solid particles separate out on the upper separation surfaces 260 of the separation members 254, while clarified water moves upwardly along the spacers 264 between the adjacent separation components into the annular space defined, around the pipe 270, by the upper truncated ends 258 of the separation members. From there, the clarified water passes into the zone above the plate 280, and out of the settler 220 through the outlet connection 234.

When sufficient solid particles have gathered on the upper separation surfaces of the separation members, they slough off and collect in the bottom 240 of the settler 220.

The separation components 252 thus serve to separate precipitated calcium sulphate anhydrite from treated water.

Referring to Figure 8, reference numeral 300 generally indicates a process according to a fourth embodiment of the invention, for removing sulphates from a sulphate- containing aqueous feedstock.

Parts of the process 300 which are the same or similar to those of the processes 10, 150 and 200 hereinbefore described with reference to Figures 1 to 7, are indicated with the same reference numerals.

The process 300 includes a secondary seeded feedstock concentric tube heat exchanger 302 in the line or conduit

36. The hot slurry line 118 leads into the heat exchanger 302 with a cooler slurry withdrawal line 304 leading from the heat exchanger, into the unseeded feedstock line 36 upstream of the heat exchanger 302.

The line 36, between the heat exchangers 302, 156 is fitted with a temperature gauge 306 and a pressure gauge 308. The secondary concentric tube heat exchanger 302 is thus located upstream of the other or primary concentric tube heat exchanger 156, relative to the direction of flow of the feedstock along the line 36.

Instead of leading into the precipitation reactor 202 of the process 200, the line 36 in the process 300 leads into an electric immersion heater 310, with a transfer line 312 leading from the immersion heater 310 to a pipe reactor 314. The diameter and configuration of the pipe reactor are such that calcium sulphate anhydrite seed is kept in suspension in the feedstock passing through the pipe reactor .

A transfer conduit 316 leads from the pipe reactor 314 to the settler 220.

A hot treated water withdrawal line 318 leads from the treated water outlet 234 of the settler 220 into a sand filter 320, with the hot treated water line 104 then leading from the sand filter 320.

In the processes 150, 200, the seed slurry addition can instead be effected directly into the reactors 50, 202 respectively by means of a flow line 119, indicated in broken line in Figures 4 and 5.

It has been found that, to prevent excessive scaling of the pipelines, the heat exchange surfaces and the reactor providing the • precipitation zone, it is necessary to maintain a concentration of seed calcium sulphate anhydrite throughout the sections of the processes where precipitation of calcium sulphate anhydrite can take place. It has also been determined that the calcium ions which are added to the feedstock to balance the sulphates in the feedstock, should be added as late as possible in the process, ie only in the precipitation zone. Since, in many feedstocks, the sulphates are not present solely as calcium sulphate but often as more soluble forms such as sodium sulphate or ferrous sulphate at low pH values, if the calcium is only added in the precipitation zone, there is much less tendency to scale the pipelines and heat exchangers leading to the precipitation zone, even where the temperatures are high, since the concentration of calcium sulphate present in the feedstock is likely to be below the solubility thereof at those temperatures.

In order to produce treated water with a very low concentration of sulphates at a low cost it is necessary to conserve heat within the processes. This can be accomplished by insulating pipelines and each piece of equipment which operates at a high temperature relative to the ambient air temperature. It is also essential to maximize exchange of usable heat in the treated water and in the slurry with the incoming feedstock whilst balancing the inlet and outlet temperatures on both sides of the heat exchange surfaces. The availability of low grade, waste heat, normally in the form of steam, in some industries, means that treated water can be produced by this hydrothermal sulphate reduction process at very little cost .

In the process of the invention, the seeding of the feedstock with the calcium sulphate anhydrite takes place either when the feedstock temperature is at a sufficiently high level, ie at at least 40°C, to ensure that the seed particles do not convert to gypsum, ie CaS0₄.2H₂0, or in such a fashion that, after the seeding has taken place, the contact time between the seed particles and the feedstock, prior to heating of the seeded feedstock to a temperature of at least 40°C, is minimized, thereby also to ensure little or no conversion of the seed particles to gypsum. Additionally, in the process of the invention, the seed is added before the heating up of the feedstock to the precipitation temperature, T_p, of 150°C to 160°C takes place. It is believed that this is essential since if this heating up were to be effected prior to the seeding, then the dissolved calcium sulphate which precipitates out, will precipitate onto the equipment such as the internals of the reactor and the surfaces of the heat exchanger, rather than onto the seed particles .

If necessary, the process may include adding, on start-up, calcium sulphate anhydrite particles from an external source as seed, until sufficient precipitated calcium sulphate anhydrite particles have been formed to provide for the full seeding needs of the process. In the process 10 of Figure 1, this addition is effected along the flow line 132. Additionally, in the process 10, mains water will normally be used on start-up, until the feedstock water temperature from the heat exchanger 34 is at least 40°C, preferably 55°C-65°C, at which stage the calcium sulphate feedstock flow, and the external seed addition is commenced.

Instead of providing the electrical trace heating 42 in Figure 1, any other suitable heating means may be used to raise the seeded feedstock to the precipitation temperature, T_p, of 150°C to 160°C, eg indirect heat exchange with a thermal fluid, indirect heat exchange with condensing steam, direct stream injection into the seeded feedstock.

It is believed that any sulphate containing effluents such as acid mine drainage, sulphuric acid containing industrial effluents etc can be treated in the process of the invention. If necessary, the feedstock can be further pretreated before being treated in the processes. For example, if the feedstock contains heavy metals, then it can first be pretreated to remove such heavy metals .

It was unexpectedly found that, by means of the process of the invention, calcium sulphate levels in water can be reduced to acceptable potable water levels in a cost effective and relatively simple fashion. Additionally, in this process, energy recovery is maximized so that external energy requirements are very low.

The process of the invention has relatively low capital costs, relatively low running costs, does not result in the creation of further problems, eg as regards by-product disposal, and produces by-products having commercial value, such as calcium sulphate anhydrite.

It is also believed that the process of the invention is safe to operate, since the treated water is withdrawn from the process at approximately ambient temperature, the seed is added to the feedstock at low pressure, ie at about atmospheric pressure, and the excess calcium sulphate particles are removed at low pressure, ie about atmospheric pressure .

The process of the invention is thus based on the principle that anhydrous calcium sulphate, ie calcium sulphate anhydrite, exhibits a inverse solubility trend with respect to temperature, ie as the temperature increases, the solubility thereof in water decreases .

It is often difficult to remove calcium sulphate from aqueous feedstocks since saturated and supersaturated solutions of calcium sulphate in water can remain stable for long periods of time. However, by means of the process of the invention, in excess of 85% of dissolved sulphates present in the feedstock, can be removed, with both sulphate and calcium concentrations in the treated water easily meeting most potable water specifications.

The process of the invention was evaluated on laboratory batch scale in the following non-limiting examples. These examples were carried out in a 1,5 stainless steel pressure vessel, fitted with temperature and pressure measurement means, heating means, pressurizing means and agitation means. A sampling device, fitted with an in-situ prefilter, was also installed so as to obtain reproducible results. A further valve permitted pressurization of the vessel before each experiment.

The experimental procedure in all the Examples was relatively standardized, unless stated differently. In each case, the test liquid (l,3f) was heated in the vessel, with the top open, to over 50°C. Seed was added to the solution, whereafter the vessel top was closed, pressurized to 1 bar gauge and agitation commenced. The temperature of the vessel was increased to approximately 150°C to 155°C, while the pressure was increased to about 6 bar. About 100m of the solution was then removed into a beaker of water, so as to flush the sample line. Another 100m£-150m of sample was then removed into a beaker of cold water, with a known volume. This determined the dilution of the sample. The vessel was then removed from the .heating means, and allowed to cool.

EXAMPLE 1 This test was carried out on a saturated solution of calcium sulphate. 50g of anhydrite, ie calcium sulphate anhydrite, was added to the solution. At 135°C (4,3 bar) the sulphate had reduced from 1430mg/£ S0₄ to 309mg/£ Ξ0₄. At 140°C, the sulphate in the solution had reduced to 223mg/ S0₄. This experiment was repeated, with the temperature being allowed to rise to 152 °C (6 bar pressure) . The sulphate concentration of the final solution was 171mg/£ S0₄ , with the calcium concentration of 73mg/£ Ca . The experiment took 2 hours to complete, from the time of seed addition.

EXAMPLE 2

The need for seed was then investigated in Example 2. The experiment of Example 1 was repeated, except no seed was added. The initial sulphate concentration was 1376mg/ S0₄ , with the final concentration being 330mg/ S0₄. The final temperature was 155°C, and the pressure 6,5 bar. This showed that the precipitation of calcium sulphate is not totally dependent on seed, but for optimum results, seed is required. White precipitation (anhydrite) was noticed on the side of the vessel. The ultimate need for seed would, it is believed, be dictated by the requirement to prevent the precipitation and scale build-up on the surfaces of the heat exchangers and reactor.

EXAMPLE 3 Example 1 was repeated, using a saturated solution of calcium sulphate, which was heated to a temperature of 155°C at a pressure of 6 bar. Less seed was added, with only a 2% seeding taking place. The sulphate concentration reduced to 159mg/-f? S0₄. The vessel was then left for almost 30 minutes at that temperature, and analysis repeated. The final sulphate concentration was 150mg/ S0₄.

EXAMPLE 4

A mixed solution of sodium sulphate and lime was prepared. The initial concentrations of calcium and sulphate were 343mg/ Ca, and 1018mg/ S0₄ respectively. The final concentrations of calcium and sulphate were 57mg/^ Ca and 635mg/£ S0₄ respectively. This showed that the lime may have a slow dissolution and dissociation, and therefore was not optimumly in the free calcium form to combine with the sulphate. This experiment was repeated, with the addition of excess lime, and stirred for over 2 hours. The pH was above 12. The initial concentrations of calcium and sulphate were 457mg/ Ca and 947mg/ S0₄. This reduced to 67mg/ Ca and 374mg/£ S0₄. This calculated to a 60% removal of sulphate. This experiment showed that lime required time to react/dissociate. The higher than usual sulphate concentration may be due to a pH limiting phenomena, as sodium hydroxide would be produced.

EXAMPLE 5 Calcium chloride was added to a sodium sulphate solution, and subjected to the procedure of Example 1. Only 2g of seed was added. The initial sulphate concentration of 1020mg/ S0₄ was reduced to 176mg/f S0₄ , using a temperature of 155 °C and a pressure of 6,2 bar. This experiment showed that calcium chloride can be used to exchange the sulphate from the sodium sulphate, producing a final calcium sulphate precipitate and a sodium chloride solution.

EXAMPLE 6

Water from Grootvlei Mine having a sulphate value of 2100mg/ S0₄ and a pH of 11,7, was used. The pH after the process was 8,7, with the sulphate concentration reduced to 500mg/ S0₄ and the calcium to 60mg/ Ca . The remainder of the sulphate, after that combined with the calcium in the water, is in the form of sodium sulphate.

EXAMPLE 7

A synthetic Grootvlei water was created and experiment 6 repeated, except for the pH being raised to above 12,0. The sulphate concentration reduced from 165Omg/I? as S0₄ to 394mg/ as S0₄. The calcium concentrations reduced from 500mg/£ Ca to 50mg/ Ca. This also proves that the remainder of the sulphate is present as sodium sulphate. EXAMPLE 8

The pH of Grootvlei Mine water was raised to 12,3, with the initial sulphate concentration being 2138mg/f S0₄. This reduced through the process to 161mg/ S0₄, with a calcium of 133mg/ Ca .

EXAMPLE 9

Withoek water having a sulphate concentration of 1950mg/£

S0₄, but mainly as magnesium, with a little calcium, was used. Magnesium was removed by precipitation with lime at a pH of 11,7. The solution was filtered, and the procedure of Example 1 carried out. The sulphate concentration reduced from 1950mg/ S0₄ to 350mg/ S0₄. This was repeated with the raise in pH to form a stoichiometric relationship between calcium and sulphate. The thermal process was also left for over 30 minutes. The sulphate concentration reduced to 240mg/ S0₄.

EXAMPLE 10 A landfill leachate from a class 1 hazardous waste disposal site and having a sulphate concentration of 19567mg/£ S0₄ , was used. The straight leachate was subjected to the procedure of Example 1. The sulphate concentration reduced to 17630mg/ S0₄. This was only a 10% reduction. Speciation of the waste showed that the calcium sulphate component of the waste is only 10%. Speciation also showed that the waste also contained a carbonate content of about 20000mg/^ C0₃. Accordingly, alkalinity was then removed by pH adjustment to 4, and thereafter neutralization with lime to pH 8 took place. The procedure of Example 1 was repeated, with the sulphates reducing to 4222mg/ S0₄. The same de-alkalizing process was then carried out, but with no pH neutralizing. Calcium chloride was added to the solution, and the procedure of Example 1 repeated. The sulphate concentration reduced to 2032mg/ S0₄, calculating to a 89,6% removal of sulphate. Thereafter, straight leachate was used, with a stoichiometric amount of calcium chloride. The sulphate concentration reduced to 4310mg/ S0₄, which calculates to a 78% removal of sulphate.

The seed used in Examples 1 to 10 was obtained by calcining gypsum (CaS0₄.2H₂0) at a temperature in excess of 500°C, typically in the range 500°C-700°C. This form of calcium sulphate anhydrite is typically referred to as insoluble anhydrite or anhydrite II. When the anhydrite is prepared by calcining gypsum at a temperature below 500 °C, typically in the range 250°C-300°C, so-called soluble anhydrite is obtained. In direct comparative tests to some of the Examples given hereinbefore, it was found that when soluble anhydrite was used as seed, the resulting residual sulphate levels in the solutions after treatment were generally higher than when using anhydrite II as seed.

EXAMPLE 11

The process of Figure 5 was also evaluated at pilot plant scale .

Heat exchanger 34 was of 5 pass plate-and-frame construction whilst heat exchanger 156 was a single pass tube-in-tube exchanger. Precipitation reactor 202 was a 500mm diameter mild steel stirred reactor whilst the separation reactor 220 had a main shell diameter of 800mm and was also constructed from mild steel.

The feed flow rate was maintained within the range 200^/hr to 300^/hr. The feed was a synthetic calcium sulphate solution. Results

Claims

1. A process for removing sulphates from a sulphate- containing aqueous feedstock, which process comprises seeding a dissolved sulphate-containing aqueous feedstock, which is at a temperature, T_f, where ambient < T_f<120°C, by adding thereto particulate calcium sulphate anhydrite as a seeding component; heating the seeded feedstock to a precipitation temperature, T , at which precipitation of calcium sulphate anhydrite from the feedstock can take place, with T >T_f and T_p≥90°C; if necessary, adding a calcium component which is at least slightly soluble in the heated seeded feedstock, to the heated seeded feedstock; allowing, in a precipitation zone, solid calcium sulphate anhydrite to precipitate from the feedstock, thereby to obtain treated water containing a lower level of dissolved sulphates than the feedstock; separating, in a separation zone, precipitated calcium sulphate anhydrite from treated water,- maintaining the precipitation zone and the separation zone at a pressure sufficient to ensure that the heated feedstock does not boil; withdrawing a slurry comprising precipitated calcium sulphate anhydrite and water; and withdrawing treated water.

2. A process according to Claim 1, wherein 40°C<T_f≤120°C so that there is little or no conversion of the seeding component to gypsum (CaS0₄.2H₂0) irrespective of the contact time between the seeding component and the feedstock prior to the heating of the seeded feedstock to the precipitation temperature, T .

3. A process according to Claim 1, wherein ambient≤T_f≤40°C, with the process then including minimizing the contact time between the seeding component and the feedstock prior to the heating of the seeded feedstock to the precipitation temperature, T , thereby to inhibit conversion of the seeding component to gypsum, with the point of addition of the seeding component to the feedstock being close to the point at which the heating of the seeded feedstock to T occurs, and with the heating to T being effected rapidly.

4. A process according to any one of Claims 1 to 3 inclusive, which includes heating the dissolved sulphate- containing aqueous or unseeded feedstock to the temperature, T_f, by subjecting it to heat exchange with treated water.

5. A process according to any one of Claims 1 to 4 inclusive, wherein the heating of the seeded feedstock to the precipitation temperature, T , includes subjecting it to heat exchange with treated water and/or with slurry.

6. A process according to Claim 5, wherein at least some of the treated water is, in a seeded feedstock heat exchange stage, used to heat the seeded feedstock, and thereafter, in an unseeded feedstock heat exchange stage, is used to heat the dissolved sulphate-containing aqueous or unseeded feedstock, with the addition of the seeding component, in the form of some of the slurry, to the feedstock being effected upstream of the seeded feedstock heat exchange stage.

7. A process according to Claim 5, wherein at least some of the treated water is, in a primary seeded feedstock heat exchange stage., used to heat the seeded feedstock, and thereafter, in an unseeded feedstock heat exchange stage, is used to heat the dissolved sulphate-containing aqueous or unseeded feedstock, while, in a secondary seeded feedstock heat exchange stage, at least some of the slurry is also used to heat the seeded feedstock, with the secondary seeded feedstock heat exchange stage being located upstream relative to the primary seeded feedstock heat exchange stage, and with the addition of the seeding component in the form of some of the slurry, to the feedstock, being effected upstream of the secondary seeded feedstock heat exchange stage.

8. A process according to Claim 5, which includes maintaining, in the precipitation zone, a concentration of the particulate calcium sulphate anhydrite of at least 0.05% by mass of the feedstock, and wherein at least part of the calcium sulphate anhydrite which is precipitated from the feedstock in the precipitation zone is thickened to a density such that, when added to the feedstock as the seeding component, the seeded feedstock contains at least 0.05% by mass calcium sulphate anhydrite.

9. A process according to any one of Claims 5 to 8 inclusive, wherein the precipitation zone is provided by a reactor which includes a pressure vessel of substantially spherical or cylindrical shape, with the inside of the vessel providing the precipitation zone, and with mixing means whereby the particles of calcium sulphate anhydrite and the feedstock are kept in continuous motion thereby to enhance the kinetics of the precipitation process, being provided in the vessel, with treated water containing the precipitated calcium sulphate anhydrite passing from the reactor to the separation zone, which is thus separate from the precipitation zone.

10. A process according to any one of Claims 5 to 8 inclusive, wherein the precipitation zone is provided by a pipe reactor, with the diameter and configuration of the pipe reactor being such that the calcium sulphate anhydrite particles are kept in suspension in the feedstock passing through the pipe reactor, with treated water containing the precipitated calcium sulphate anhydrite passing from the reactor to the separation zone, which is thus separate from the precipitation zone.

11. A process according to any one of Claims 5 to 8 inclusive, wherein the precipitation zone and the separation zone are provided inside an upright elongate precipitation reactor shell in which the separation zone is located above the precipitation zone so that the separated precipitated and/or seed calcium sulphate anhydrite drop back under gravity into the precipitation zone where it is mixed with incoming feedstock.

12. A process according to any one of Claims 9 to 11 inclusive, wherein the separation zone includes separation means by means of which the separation of the precipitated calcium sulphate anhydrite from the treated water is effected.

13. A process according to Claim 12, wherein the separation means is in the form of a parallel inclined plate stack through which the calcium sulphate anhydrite- containing treated water passes.

14. A process according to Claim 12, wherein the separation means is in the form of a filter through which the calcium sulphate anhydrite-containing treated water passes.

15. A process according to any one of Claims 9 to 14 inclusive, which includes, if necessary, further heating the seeded feedstock, after it has been subjected to the heat exchange with treated water and/or with slurry, to raise its temperature to T .

16. A process according to Claim 15, wherein the further heating is effected upstream of the reactor.

17. A process according to Claim 15, wherein the further heating is effected in the reactor.

18. A process according to any one of Claims 1 to 17 inclusive, wherein the calcium component is added to the heated seeded feedstock, and is a calcium salt which is soluble or slightly soluble in the heated seeded feedstock.

19. A process according to any one of Claims 1 to 18 inclusive, which includes subjecting the treated water from the separation zone to filtration, to remove residual suspended calcium sulphate anhydrite remaining in the treated water.

20. A precipitation/separation reactor, which includes a normally upright cylindrical reactor shell having its respective ends closed off and providing an operatively lower precipitation zone and a separation zone above the precipitation zone and in direct communication therewith; a feed inlet leading into the shell intermediate its ends and located in the precipitation zone; a slurry outlet at the lower end of the shell; a treated water outlet at or near the upper end of the shell; and a separation member stack within the shell above the feed inlet and located in the separation zone, the separation member stack comprising a plurality of inverted truncated hollow conical parallel separation members nestled one inside the other, with the upper and lower ends of all the separation members being located at the same levels respectively, and with the upper end of the innermost separation member being closed off with a conical component, the lower end of the conical component thus having the same diameter as the upper end of the innermost separation member.

21. A reactor according to Claim 20, which ,includes mixing means in the precipitation zone for effecting mixing of a body of fluid in the precipitation zone.

22. A reactor according to Claim 20 or Claim 21, wherein the lower end of the shell is closed off with an inverted conical lower end piece in which, in use, precipitated matter collects, with the slurry outlet being provided at the apex of the conical end piece, and wherein the upper end of the shell is closed off with a domed upper end piece, with the treated water outlet being provided in the domed end piece.

23. A reactor according to any one of Claims 20 to 22 inclusive, wherein the angle of inclination of the separation members to the vertical is about 30°, and wherein the separation members are supported on a support which spans the reactor shell, with the separation members being located in position relative to each other by means of elongate spacers extending from the upper ends of the separation members to their lower ends, and with the spacers being spaced circumferentially apart.