EP2396278A1

EP2396278A1 - Preparation of purified calcium chloride

Info

Publication number: EP2396278A1
Application number: EP10703471A
Authority: EP
Inventors: Claude Breton; Michel Hanse; Laurent Bessenay; David Savary
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 2009-02-12
Filing date: 2010-02-11
Publication date: 2011-12-21
Also published as: FR2941939A1; RU2011137433A; FR2941939B1; WO2010092117A1; CN102317212A

Abstract

The present invention relates to a process for preparing purified calcium chloride, in particular calcium chloride that is purified as regards the presence of magnesium and other metals present in the form of impurities, these impurities being precipitated by alkalinization, and the precipitate is separated from the calcium chloride solution. Another subject of the present invention relates to the reuse of by-products (or residues) from known processes in the process for preparing calcium chloride initially.

Description

Preparation of purified calcium chloride

Cross-reference to related applications

The present application claims the priority benefit of French patent application No. 0950868 filed February 12, 2009. TECHNICAL FIELD The present invention relates to a process for preparing calcium chloride that is stripped of magnesium and other metal impurities. Prior art

Calcium chloride is a chemical that has numerous applications in all fields of industry, including the food industry. Although calcium chloride is a by-product of the manufacture of certain products, especially sodium carbonate via the Solvay process, these by-products are usually relatively dilute solutions of CaCl₂, which is very soluble in water, but that contain often significant amounts of impurities, especially of sodium, magnesium and other metal compounds, such as iron, lead, etc. It is known that calcium chloride may be obtained by dissolving solid calcium carbonate using hydrochloric acid. For large-scale production, the production may be carried out directly starting from limestone. However, although limestone is predominantly composed of calcium carbonate (CaCOs), it usually also contains relatively large proportions of magnesium carbonate (MgCOs) and other impurities.

Consequently, independently of the origin of the raw materials, it is necessary, in order to meet the applicable product specifications, to avoid as far as possible the presence of impurities and in any case the necessary measures should be taken so that the acceptable values for each of the elements other than calcium and chlorine are not exceeded.

Generally, within the context of the production of saline solutions (NaCl brine, 36% CaCl₂ solution, etc.), the magnesium must be removed in order to meet the specifications for the end product. It is known that this step can be carried out by adding an alkaline agent to the medium to be purified, which agent results in the magnesium precipitating in the form of its hydroxide Mg(OH)₂. Nevertheless, if the precipitation OfMg(OH)₂ is poorly controlled (from the point of view of the chemistry of the reaction and/or its implementation), the liquid/solid separation which follows will be very difficult to achieve.

Indeed, there are many parameters that influence the nucleation and growth of the crystals and if the particles of precipitate obtained are too fine or not dense enough, the settling velocity and the filterability will be insufficient to allow exploitation on an industrial scale. OBJECT OF THE INVENTION

One object of the present invention is consequently to propose a controllable and reliable process for preparing purified calcium chloride, in particular calcium chloride that is purified as regards the presence of magnesium and other metals present in the form of impurities. It is also desirable that the process permits the preparation of calcium chloride starting from high quality by-products of known processes, in order to reduce the amount of residues formed and to be able to efficiently and economically reuse products which, without such a treatment, could not be used in certain fields of industry. General description of the invention

In order to solve the aforementioned problem, the present invention proposes a process for preparing a purified calcium chloride solution comprising a step (d) of precipitating magnesium and other impurities from a calcium chloride solution containing magnesium and other impurities in solubilized form via alkalinization of this solution by addition of calcium hydroxide, preferably in the form of milk of lime, followed by a step (e) of separating the precipitate from the calcium chloride solution, preferably by settling and/or by filtration, so as to obtain a purified calcium chloride solution. The liquid/solid separation may be carried out by any other suitable liquid/solid separation method such as, for example, a simple settling, or a centrifugation, or a filtration, or else by combining various known methods.

One major advantage of the process according to the invention is that the magnesium precipitates in a basic medium in the form of magnesium hydroxide particles which enable a good solid/liquid separation that can be exploited on an industrial scale. The calcium chloride solution obtained via this process has reduced contents of magnesium, but also of other impurities, such as in particular phosphorus, iron, aluminium, and also other metals such as cobalt, chromium, copper, manganese, nickel, lead, tin, titanium, vanadium and zinc. In a preferred embodiment of the invention, the precipitation in step (d) of magnesium and other impurities from a calcium chloride solution containing magnesium and other impurities in solubilized form via alkalinization of this solution by addition of calcium hydroxide, preferably in the form of milk of lime, is carried out in a continuous mode.

In a variant embodiment of the invention, the step (d) is carried out in a batch or semi-batch mode.

The expression continuous mode is understood to mean a mode of realization of the precipitation step (d), according to which a reaction device is used, into which the calcium chloride solution and the calcium hydroxide are continuously or quasi-continuously introduced, and from which the solution and products of the reaction are continuously or quasi-continuously withdrawn. The reaction device comprises advantageously one or more reactors sufficiently mixed to avoid the settling of the calcium hydroxide, when in solid form, and the settling of precipitate.

The continuous or quasi-continuous introduction of the calcium chloride solution and the calcium hydroxide can be realized with any suitable means such as for instance with centrifugal pumps or peristaltic pumps. The introduction of calcium hydroxide, when in solid or powder form, in the reaction device can be carried out with any suitable means such as, for instance, with screw feeding systems, vibrating feeders, or loss-in weight feeding systems. The continuous or quasi-continuous withdrawal of the solution and products of the reaction from the reaction device can be carried out with any suitable means such as, for instance, with centrifugal pumps or peristaltic pumps for a continuous withdrawal or with gravitational draining controlled sequentially by means of electric or pneumatic valves for a quasi-continuous withdrawal. The expression quasi-continuous introduction or withdrawal is understood to mean an introduction or withdrawal of solution, reactants, or products of reaction that, although non continuous in a strict sense, enable the same function, as for instance sequential introductions or sequential withdrawals in which the quantities of solution, reactants, or reaction products, are small compared to the total quantity contained in the reaction device.

Contrary to the continuous mode, to work in batch mode means to introduce the calcium chloride solution and the calcium hydroxide into the device of reaction, then to let the reaction be carried out, and finally to - A -

completely or almost completely withdraw the solution and the products of reaction from the device of reaction.

Also contrary to the continuous mode, when the calcium chloride solution is introduced beforehand into the device of reaction and then the calcium hydroxide is introduced at a constant flow rate over a certain time period, before totally withdrawing the content of the reaction device, the present description of the invention uses the term of precipitation of magnesium and other impurities in semi-batch mode.

In the preferred embodiment of the invention, in which the stage (d) is carried out in continuous mode, the mean residence time of the calcium chloride solution in the device of reaction is advantageously equal to or greater than 15 minutes, preferably equal to or greater than 30 minutes. It is in general less than or equal to 5 hours, preferably less than or equal to 2 hours.

The concentration of the aqueous calcium chloride solution is generally equal to or greater than 10 weight % of calcium chloride, preferably equal to or greater than 30 weight %, more preferably equal to or greater than 32 weight %, and most preferably equal to or greater than 35 weight % of calcium chloride. The concentration of the aqueous calcium chloride solution is generally less than or equal to 60 weight %, preferably less than or equal to 50 weight %, more preferably at less than or equal to 40 weight %, and most preferably less than or equal to 38 weight % of calcium chloride.

A suitable choice of certain parameters of the above process makes it possible to further improve the purification of calcium chloride solutions.

In particular, in step (d), the temperature is generally equal to or greater than 20⁰C, preferably equal to or greater than 40⁰C, more preferably equal to or greater than 50⁰C, and most preferably equal to or greater than 55°C. The temperature is advantageously less than or equal to 80⁰C, preferably less than or equal to 70⁰C, more preferably less than or equal to 65°C. In particular the temperature is about 60⁰C. The inventors have indeed determined that a temperature below 40⁰C generally leads to a degradation of the settling parameters, and a judicious choice of the temperature within the above limits is therefore beneficial for an optimal precipitation and an optimal separation (see Example A.4). And a too high temperature leads to severe corrosion of materials and to higher iron content in calcium chloride solution. According to one advantageous variant of the invention, the precipitation in step (d) is carried out by adjusting the pH measured at 60⁰C, denoted by pH(60°C), to a value between 7.3 and 9.0, preferably between 7.7 and 8.7. Indeed, experiments have shown that pH variations affect the process on several levels. By lowering the pH(60°C) from 8.2 to less than 7.3, the magnesium precipitation yield becomes low due to the low basicity of the medium (see Example A.3). Surprisingly, the inventors have been able to observe that the settling rate and the suspension density of thickened sludges at 2 hours remain high however. By increasing the pH(60°C) from 8.2 to more than 9.0, the magnesium precipitates almost entirely, due to the more advanced alkalinization of the medium, but unexpectedly the settling and fϊlterability characteristics deteriorate.

In order to further improve the liquid/solid separation in step (e), other measures are possible, especially the addition of flocculants. In one particular embodiment of the present invention, the separation of the magnesium precipitate and of the other impurities in step (e) is therefore carried out in the presence of one or more suitable flocculant(s), preferably a flocculant of anionic polyacrylamide type, such as Prodefloc A2107 (CAFFARO), or a copolymer of acrylamide and of sodium acrylate such as Magnafloc 10 (CIBA). Indeed, the inventors have been able to demonstrate that the addition of a flocculant makes it possible to substantially increase the initial settling rate (see Example A.2). In principle, the realkalinization of step (d) may be carried out in batch mode, in semi-batch mode or in continuous mode. Advantageously, the realkalinization of step (d) is however carried out in continuous mode given that the inventors have observed that it is more effective, in terms of settling and filtration, than a realkalinization in semi-batch mode (see Example A.1). Indeed, by adding calcium hydroxide, for example in the form of milk of lime, in continuous mode, the growth of the particles OfMg(OH)₂ is favoured to the detriment of their nucleation, that is to say to the detriment of the formation of very fine particles. Furthermore, the suspension density at the end of the test is improved with an alkalinization in continuous mode. After the precipitation of the magnesium, the separation thereof is carried out. Although the separation in step (e) can be carried out by any suitable method, for example a simple settling, or by a centrifugation, one particular embodiment of the present invention relates to the separation in step (e) by settling, followed by a pressurized filtration (see Example B.I). Although the implementation of continuous process is in principle more difficult, they have the advantage of being able to be automated, provided that all the important parameters thereof are controlled. Furthermore, a continuous process makes it possible to largely avoid the "batch" effect observed in batch or semi-batch mode.

During the separation in step (e), the precipitated particles of magnesium hydroxide and of other impurities must have characteristics that are suitable as a function of the chosen separation method. For example, if the particles do not have a suitable size, they may pass through the filter or else rapidly clog it up. As already indicated, it has been shown that the size of the precipitated particles may be favorably influenced, inter alia, by the mode of addition of the calcium hydroxide (see Example A.1). Indeed, by favoring the growth of the Mg(OH)₂ particles by the addition of milk of lime in continuous mode, the separation in step (e) is improved, especially if it is carried out by settling and/or by filtration. The particle size of the solid formed may be measured, for example, using a laser particle size analyzer according to one procedure described in the tests of the present document. Thus, it has been determined that the magnesium precipitated in step (d) advantageously has a diameter d90, that is to say a cut-off diameter such that 90% of the weight of the sample is constituted of particles having a diameter below this value, between 28 μm and 65 μm, preferably between 30 μm and 60 μm, and a mean diameter d50, that is to say a cut-off diameter such that 50% of the weight of the sample is constituted of particles having a diameter below this value, between 8 μm and 20 μm, preferably between 11 μm and 16 μm.

Although the steps (d) and (e) described above propose the purification of calcium chloride solutions that contain magnesium and other impurities in solubilized form, it is possible to start from a large number of other raw materials or secondary materials by providing suitable preparation steps.

Thus, in one particular embodiment of the present invention, the calcium chloride solution to be purified is obtained starting from solid calcium carbonate, for example directly starting from limestone. The process according to this preferred embodiment comprises, before step (d), a step (c) of dissolving solid calcium carbonate with a concentrated solution of hydrochloric acid. A suitable solution of hydrochloric acid has, for example, an HCl concentration between 30% and 38% by weight, preferably of around 36% by weight.

The use of hydrochloric acid makes it possible not only to dissolve the calcium, but the main advantage is that new and unwanted chemical species are not introduced. Furthermore, by choosing a concentrated solution of hydrochloric acid, the liquid contribution is minimized and the dilution effect is thus reduced.

It is also desirable that the process permits the preparation of calcium chloride starting from by-products (or "residues") of known processes, in order to be able to efficiently and economically reuse products which, without such a treatment, could not be used in certain fields of industry. One major advantage of this embodiment is therefore the versatility of the dissolving step (c), the precipitating step (d) and the separating step (e). Indeed, owing to a prior step (c), it is also possible to prepare an equally purified calcium chloride solution starting from sludges (or slurry) containing calcium in various forms, especially calcium carbonate.

The present process can therefore easily be adapted depending on the origin of the initial raw or secondary materials.

Another possible application of the process according to the invention relates to the brine purification sludges originating, for example, from the

SOLVAY process. Indeed, the sodium chloride extracted from the subsoil (raw brine) cannot be used directly in the SOLVAY process, but must undergo a pre- purification of the calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). Without this treatment, the equipment downstream would be subjected to significant deposits of calcium carbonate or of magnesium hydroxide, which would prevent a stable and continuous operation of soda ash plants.

The raw brine is therefore brought into contact with: - a source of hydroxide ions (OH^") which will precipitate Mg²⁺ in the form of magnesium hydroxide Mg(OH)₂; - a source of carbonate ions (CO₃ ² ) which will precipitate Ca²⁺ in the form of calcium carbonate CaCO₃.

The two reactions are complete and the Ca²⁺ and Mg²⁺ contents of the purified brine entering the soda ash plant are very low. The brine purification sludges (ES) which result therefrom are therefore a mixture of liquid (brine) and solid (CaCO₃ and Mg(OH)₂) as the main constituents.

Besides the sodium chloride, ammonia (NH₃) is used in the SOLVAY process. This ammonia flows in a loop: it is absorbed in the brine upstream of the process and its absorption permits the subsequent or concomitant absorption of CO₂. Downstream of the process, the NH₃ is regenerated by distillation. This distillation brings into contact milk of lime (suspension based on slaked lime Ca(OH)₂) and filtered liquid (based on NaCl and on NH₄Cl). Besides the regeneration OfNH₃, sludges are formed. These distillation sludges form a liquid/solid mixture, the liquid of which is essentially constituted of Ca²⁺, Na⁺ and CI^" and the solid of which is principally composed Of CaCO₃, Ca(OH)₂, Mg(OH)₂ and CaSO₄.xH₂O. Both the brine purification sludges (ES) and the distillation sludges (DS) containing Ca²⁺, Na⁺ and Cl^" may be used in a variant of the process according to the invention for reusing one or the other sludge, or both sludges concomitantly.

Consequently, a reuse variant proposed by one particular embodiment of the present invention consists in using the ES sludges and the DS liquid. These soda ash plant residues can thus be reused in a process that additionally comprises, before step (c), the steps:

(a) of dissolving the magnesium and other impurities and of precipitating the calcium in the form of calcium carbonate via carbonation by injection of carbon dioxide: Mg(OH)₂ (s) + (Ca²⁺ + 2Cl^") + CO₂ (g) → (Mg²⁺ + 2Cl^") + CaCO₃ (s) + H₂O

(b) of separating and washing the precipitate of calcium carbonate from the solution by settling and/or by filtration, washing the cake of solid obtained with water, either directly on the filter or by putting it back into suspension in water and filtering, in order to remove the traces of sodium chloride, so as to obtain a finely divided solid calcium carbonate that exhibits good reactivity for the production of calcium chloride by acid attack.

At the end of step (a), the solid obtained contains predominantly calcium carbonate, for example more than 90%. The magnesium is solubilized almost completely, for example, more than 90%, and is thus removed from the process. The quantity of washing water of the precipitate of calcium carbonate implemented at the step (b) is chosen to have advantageously a sodium content in the final washed solid equal to or less than 10 weight %, preferably equal to or less than 5 weight %, more preferably equal to or less than 2 weight %, and most preferably equal to or less than 1 weight % of sodium expressed on dried solid. The solid fraction from step (b) is then treated successively in steps (c), (d) and (e) that correspond to the steps described previously:

(c) Acid attack of the cake Of CaCO₃ by concentrated HCl: CaCO₃ (s) + 2 HCl(aq) → CaCl₂ (aq) + CO₂ (g) + H₂O The acid brine of CaCl₂ is thus generated. (d) Realkalinization in order to precipitate the traces of residual magnesium and of other impurities by addition of calcium hydroxide, for example milk of lime:

MgCl₂(aq) + Ca(OH)₂(s/aq) → Mg(OH)₂(s) + CaCl₂(aq) (e) Separation of the solution of CaCl₂ from the solid fraction by settling and/or filtration, preferably through a filter press in order to obtain a solution of CaCl₂ that is as clear as possible.

In this variant, a large portion of the magnesium is already removed from the process at the end of step (a) by a liquid/solid separation, for example using a known method or combination of methods, in particular those described in relation to step (e).

The carbon dioxide used in step (a) may come from the process itself, namely from step (c) and/or for example from a process for producing quicklime by calcination of calcium carbonate. In the event that step (c) does not produce enough carbon dioxide to carbonate the ES sludges, it is thus possible to top it up with carbon dioxide originating from other sources such as for example the carbon dioxide produced in the SOLVAY process, the advantage of such a recycling being the reduction in the net emission of CO₂. In the opposite case where step (c) produces an excess of carbon dioxide for carbonating the ES sludges, the extra carbon dioxide may be reused for other industrial applications. Indeed, the carbon dioxide obtained is of high purity and has a concentration generally equal to or greater than 95 volume %, preferably equal to or greater than 97 volume %, more preferably equal to or greater than 98 volume % of dry gas. It could be subjected to a washing operation with optionally alkalinized water in order to rid it of traces of hydrochloric acid vapour and other soluble pollutants. This washing operation could be carried out in one or more steps, with industrial equipment used for this type of usage, such as for example spray scrubbers, equipped with mist eliminators in order to rid the gas of droplets of washing liquid. It could also be carried out on one or more scrubbers provided with random or structured packing. The purified carbon dioxide thus obtained could be reused in one of the sectors of the soda ash plant by being transported by gas pipeline, or be subjected to a cooling operation and sold in liquid form for technical or food applications. Finally, if at the end of the process it is desired to have solid purified

CaCl₂, an additional variant provides, after step (e), a step (f) of desiccating the purifϊed calcium chloride solution, for example by evaporation or any other suitable method, so as to obtain solid purified CaCl₂.

Examples

Various variants of the process are illustrated in a non- limiting manner by the following examples, the results of which appear in the tables below.

The pH of the slurries is measured with a Yokogawa 400-BE-2 pH-meter equipped with a Mettler Toledo HA405-BXK gel electrode. The apparaus is pre- calibrated with a Radiometer Sl 1-M013 pH standard buffer solution at the measurement temperature of the slurry. The measurement of the settling velocity and the estimation of the volume of clear liquid after 2 hours of settling are carried out using 2-litre graduated flasks that is thermostated at the test temperature. In order to carry out these tests, it is possible to proceed, for example, as described in the encyclopaedia "Les Techniques de l'lngenieur, Traite de Genie Chimique, Chapitre Decantation, J3450, Determination experimentale des vitesses de sedimentation, Editions TI, Mars 1999, Paris [Engineering Techniques, Treatise on Chemical Engineering, chapter on Settling, J3450, Experimental determination of settling velocities, published by Editions TI, March 1999, Paris]. The settling velocity is taken from the settling curve in its linear section as a function of time, before compaction or compression of the sludges occurs. The realkalinization is carried out in a 2-litre reactor equipped with a 6- pitched-blade impeller rotating at 500 rpm.

A) Examples relating to the precipitation and settling of a solid based on Mg(OH)₂ A.I : Influence of the mode of addition of calcium hydroxide (e.g. milk of lime) Characteristics of the milk of lime: The milk of lime (LCH) used contains: 284 ± 4 g Ca(OH)₂ / kg LCH 9 ± 3 g CaCO₃ / kg LCH Test conditions:

- Slurry to be realkalinized: 36% acid (pH < 0) solution of CaCl₂ containing between 4 and 10 g/kg of solid (typically constituted of 90% of CaSO₄-XH₂O and 10% of SiO₂) and around 1O g MgCl₂/kg slurry to be precipitated

- Realkalinization temperature (T) = 60⁰C - Realkalinization pH (60⁰C) = 8.2 - Realkalinization agent: LCH diluted with a CaCl₂ lye in the proportions: 63% by weight of LCH and 37% by weight of CaCl₂ lye containing 36% Of CaCl₂.

- Mode of addition of the milk of lime to achieve pH 8.2:

^■ Addition over 5 minutes (min) in semi-batch mode (Test 64-3)

^■ Addition over 30 min in semi-batch mode (Test 64-4)

^■ Continuous test (Test 65-4): injection of LCH and of acid slurry and drawing-off of the realkalinized slurry in continuous mode; residence time = 30 min, sampling of the slurry after at least 3 renewals of the reactor

Results of settling operations after realkalinization:

Table 1 : Influence of the mode of addition of LCH during the realkalinization on the settling characteristics (tests carried out without flocculant)

Results of the particle size analysis after realkalinization:

Table Ia: Influence of the mode of addition of LCH on the laser particle size of the solid obtained

The particle size analyses of the solids given in the present table were carried out in the following manner: the solid is separated from its mother liquors by filtering through filter paper, then washed with deionized water in order to remove most of the impregnated chlorides. The solid is then dried overnight at 60⁰C, then after coarse deagglomeration, is dispersed in alcohol that has been previously saturated with a similar solid. The dispersion is carried out on a Malvern MS 17 autosampler subjecting the sample to ultrasound treatment with a power index of 100% for 170 seconds. The particle size analysis is then carried out on a Malvern Mastersizer S laser particle size analyzer with a focal length of 300 mm. The diameter d90 corresponds to the diameter such that 90% of the weight of the sample is constituted of particles having a diameter below this value, the diameter d50 of the particles corresponds to the diameter such that 50% of the weight of the sample is constituted of particles having a diameter below this value, and the diameter dlO corresponds to the diameter such that 10% of the weight of the sample is constituted of particles having a diameter below this value. The span of the particle size curve is defined as the ratio: (d90 - dl0)/d50. Conclusion: As shown in Table 1, a realkalinization carried out in continuous mode is more effective in terms of settling than a realkalinization in semi-batch mode: that is to say the initial settling velocity and the percentage of clear volume relative to the volume of sludges to be treated are higher during a realkalinization in continuous mode. The fact that Mg(OH)₂ has a higher d90 diameter during a realkalinization in continuous mode, as shown in Table Ia, is favourable to a better solid/liquid separation, in particular in the case of a settling operation and a filtration.

A.2: Influence of the use of a flocculant on the settling characteristics

Test conditions: - Acid slurry based on 36% CaCl₂ realkalinized, in continuous mode at T = 60⁰C; pH(60°C) = 8.2; residence time (ts) = 30 min, by LCH as is (not diluted) described previously - Flocculants used for the settling operations:

^■ MAGNAFLOC 10 (acrylamide -based anionic copolymer; CIBA) ■ PRODEFLOC A2107 (anionic polyacrylamide; CAFFARO) Results:

Table 2: Results of the settling tests with or without MAGNAFLOC 10

(mg/L : mg commercial flocculant as is/ L realkalinized slurry)

Table 2a: Results of the settling tests with or without PRODEFLOC A2107 (mg/kg: mg commercial flocculant as is/ kg realkalinized slurry)

Conclusion:

The addition of a flocculant makes it possible to improve the settling characteristics, probably by increasing the size of the particles OfMg(OH)₂ (formation of floes) thus enabling a better separation. A.3 : Influence of the realkalinization pH Objective:

To simulate a fault in the supply of LCH or of acid slurry into the realkalinization reactor and to evaluate the impact on the settling characteristics. Test conditions: - realkalinization T = 60⁰C; pH(60°C) = 7.3; 8.2 (nominal); 9.0

- Realkalinization by LCH as is in continuous mode with a mean residence time ts = 30 min Results:

Table 3: Influence of the realkalinization pH on the settling characteristics and on the clear volume obtained

Conclusion:

By realkalinizing the slurry to a pH(60°C) below 7.3, the Mg(OH)₂ precipitates poorly. The settling velocities and the volume of clear liquid at the end of the settling operation remain at high values. At a pH of 8.2 and above, the Mg(OH)₂ precipitates greatly. At the pH of 8.2 the settling characteristics are good, but they deteriorate significantly when the pH is increased up to a pH of 9.0.

A.4: Influence of the realkalinization and precipitation temperature Objective:

To determine the influence of a low realkalinization temperature (40⁰C instead of the nominal 60⁰C) on the settling parameters. Test conditions:

- realkalinization T = 40⁰C; pH(40°C) = 8.85; (which is equivalent to pH(60°C) = 8.2)

- Realkalinization by LCH as is in continuous mode with a mean residence time ts = 30 min

Results:

Table 4: Results of the settling tests with or without PRODEFLOC A2107

(mg/kg : mg flocculant/kg realkalinized slurry) - T = 60⁰C

Table 4a: Results of the settling tests with or without PRODEFLOC A2107 (mg/kg: mg flocculant/kg realkalinized slurry) - T = 40⁰C

Conclusion:

A realkalinization in continuous mode, carried out at a temperature of 40⁰C, gives rise to a reduction of the percentage of clear volume relative to the initial volume and of the suspension density of the compacted sludges relative to a realkalinization in continuous mode, carried out at a temperature of 60⁰C. In the presence of a flocculant, the decrease in temperature from 60⁰C to 40⁰C gives rise to a significant drop in the initial settling velocity, the percentage of clear volume relative to the initial volume and the suspension density of the compacted sludges being scarcely modified. B) Examples relating to the filtration of a solid based on Mg(OH)₂ after settling

In the tests for characterizing the filterabilities of cakes of solid presented below, reference may be made to the definitions and to the procedures described in the encyclopaedia "Les Techniques de l'lngenieur, Traite de Genie Chimique, Chapitre Filtration sur support, J3501, Editions TI, Decembre 1997, Paris [Engineers Techniques, Treatise on Chemical Engineering, chapter on Cake filtration, J3501, published by Editions TI, December 1997, Paris]. The higher the specific resistance of the cake of solid to the passage of the clear solution, the less easily the solid is filtered. And consequently, the worse is the productivity of the filter, expressed as kg of filterable solid per unit of time and of filter area. B.I : Influence of the mode of addition of milk of lime Test conditions:

- Test 61-8 and 61-8a: realkalinization of the acid slurry by LCH in batch mode (conditions of addition of LCH not controlled) at pH(60°C) = 8.2; settling and filtration under 3 bar of pressure

- Test 65-2: realkalinization of the acid slurry by LCH in 30 min semi-batch mode at pH(60°C) = 8.2; settling and filtration under 3 bar of pressure

- Test 66-5 and 68-4: realkalinization of the acid slurry by LCH in continuous mode at pH(60°C) = 8.2; settling and filtration under 3 bar of pressure

Results of the filtration tests:

Table 5: Results of the filtration tests

Conclusion:

The results from Table 5 show that the addition of milk of lime in continuous mode leads to specific resistances of the cake of solid that are nearly 100 times lower, and to a productivity of the filtration equipment that is 10 to 150 times greater, than a realkalinization by the milk of lime in batch mode or in semi-batch mode.

B.2: Influence of the realkalinization pH Test conditions:

- realkalinization T = 60⁰C; pH(60°C) = 7.3; 8.2 (nominal); 9.0

Table 6: Influence of the realkalinization pH on the filtration characteristics

Conclusion:

The results show that the fϊlterability of the realkalinized solids is 10 times lower at pH(60°) of 7.3 and 9.0 than at a pH(60°C) of 8.2.

C) Examples relating to the effectiveness in terms of the chemical purification of the calcium chloride solution during realkalinization

Table 7 below describes the results of trace element measurements of the calcium chloride solution obtained by acid attack of carbonated brine purification sludges before realkalinization, then after realkalinization for various tests. The calcium chloride solution obtained is settled then filtered under vacuum firstly through a rapid laboratory filter, then filtered through a cellulose ester MILLIPORE membrane having a porosity of 8 μm. The trace elements were measured by ICP (Inductively Coupled Plasma) atomic emission spectrometry, apart from arsenic and mercury, which were measured by atomic absorption spectrometry, and fluorine, which was measured by anion chromatography.

The results indicated in the following table mentioned with a < symbol mean that their levels are below the detection thresholds of the equipment. Test conditions

- Slurry to be realkalinized: acid (pH < 0) solution of calcium chloride having a concentration of 36% by weight of CaCl₂ obtained by acid attack of carbonated, filtered and washed brine purification sludges, and containing between 4 and 10 g/kg of solid (typically constituted of 90% of CaSO₄-XH₂O and 10% of SiO₂) and around 1O g MgCl₂/kg slurry to be precipitated

- Realkalinization T = 60⁰C; pH(60°C) = 8.2

- Realkalinization by LCH having the same characteristics as in Example Al, carried out in batch mode or in continuous mode (residence time of 30 min and at least three renewals). The sample of liquid is filtered through a cellulose nitrate membrane with a filtration threshold of less than 8 μm before analysis of the dissolved elements. Results:

Table 7: Effectiveness of the stripping of trace elements from the calcium chloride lye after realkalinization and filtration

Conclusion:

It is observed that for the two modes of realkalinization, in batch mode or in continuous mode, both at a pH (60⁰C) equal to 8.2, the purification is signifϊcant for the following elements: magnesium, phosphorus, iron, aluminium, cobalt, chromium, copper, manganese, nickel, lead, tin, titanium, vanadium and zinc.

Claims

C L A I M S

1. Process for preparing a purified calcium chloride solution comprising the steps:

(d) of precipitating magnesium and other impurities from a calcium chloride solution containing magnesium and other impurities in solubilized form via alkalinization of said solution by addition of calcium hydroxide, preferably in the form of milk of lime;

(e) of separating the precipitate from the calcium chloride solution, preferably by settling and/or by filtration, so as to obtain a purified calcium chloride solution.

2. Process for preparing a purified calcium chloride solution according to Claim 1, wherein the precipitation in step (d) is carried out in a continuous mode.

3. Process for preparing a purified calcium chloride solution according to Claim 1 or 2, wherein the precipitation in step (d) is carried out by adjusting the pH(60°C) to a value between 7.7 and 8.7.

4. Process for preparing a purified calcium chloride solution according to any of Claims 1 to 3, wherein the temperature in step (d) is greater than or equal to 40⁰C and less than or equal to 70⁰C.

5. Process for preparing a purified calcium chloride solution according to any one of the preceding claims, in which the separation of the magnesium and of the other impurities in step (e) is carried out in the presence of a flocculant, preferably a flocculant of anionic polyacrylamide type.

6. Process for preparing a purified calcium chloride solution according to any one of the preceding claims, in which the separation in step (e) is carried out by settling, followed by a pressurized filtration.

7. Process for preparing a purified calcium chloride solution according to any one of the preceding claims, which makes it possible to prepare a purified calcium chloride solution starting from solid calcium carbonate, additionally comprising, before step (d), the step (c) of dissolving solid calcium carbonate with a concentrated solution of hydrochloric acid.

8. Process for preparing a purified calcium chloride solution according to Claim 7, in which the concentrated solution of hydrochloric acid has an HCl concentration between 30% and 38% by weight, preferably of around 36% by weight.

9. Process for preparing a purified calcium chloride solution according to either one of Claims 7 and 8, which makes it possible to purify calcium chloride solutions that contain magnesium and other impurities in solid form, especially in the form of their hydroxides, especially for reusing residues from a soda ash plant, additionally comprising, before step (c), the steps

(a) of dissolving the magnesium and other impurities and of precipitating the calcium in the form of calcium carbonate via carbonation by injection of carbon dioxide;

(b) of separating the precipitate of calcium carbonate from the solution, by settling and/or by filtration, so as to obtain solid calcium carbonate.

10. Process according to Claim 9, in which the carbon dioxide used in step (a) comes from step (c) and/or from a process for producing quicklime by calcination of calcium carbonate.

11. Process according to any one of the preceding claims, in which the magnesium precipitated in step (d) has a diameter d90 between 28 μm and 65 μm.

12. Process according to any one of the preceding claims, additionally comprising, after step (e), the step

(f) of desiccating the purified calcium chloride solution by evaporation so as to obtain solid purified CaCl₂.