EP2934760A1

EP2934760A1 - Method for separating calcium carbonate and gypsum

Info

Publication number: EP2934760A1
Application number: EP13811473.1A
Authority: EP
Inventors: Lev Léon FILIPPOV; Inna KUVAEVA ÉPOUSE FILIPPOV; Michel Jean-Marie HOTTIER
Original assignee: Solvay SA; Centre National de la Recherche Scientifique CNRS; Universite de Lorraine
Current assignee: Solvay SA; Centre National de la Recherche Scientifique CNRS; Universite de Lorraine
Priority date: 2012-12-19
Filing date: 2013-12-18
Publication date: 2015-10-28
Also published as: US20150328645A1; WO2014095980A1; FR2999455B1; CN105307774B; FR2999455A1; CN105307774A

Abstract

The invention relates to a method for separating a suspension containing gypsum granules and calcium carbonate granules and an aqueous solution, characterized in that the aqueous solution comprises at least one sodium salt and in that the aqueous solution has a sodium ion concentration of at least 3 g/l and in that said method comprises a step of separation by floatation using a collector for the gypsum granules, wherein the collector comprises at least one heteropolar organic surfactant of formula RX, in which: - R denotes a hydrocarbon-based chain comprising from 2 to 50 carbon atoms, and the hydrocarbon-based chain is chosen from: a saturated or unsaturated, linear alkyl chain and a saturated or unsaturated, branched alkyl chain, - X denotes at least one ionizable group chosen from the group consisting of: a carboxylic group, a sulfonate group, a sulphate group, a phosphonate group, or a phosphate group, or a hydroxamate group.

Description

Method of separating calcium carbonate and gypsum

TECHNICAL AREA

The present application claims the priority of the French patent application No. 1262309, filed December 19, 2012, and whose entire content is incorporated herein by reference for all purposes.

The invention relates to a method for separating a slurry containing gypsum granules and calcium carbonate granules and an aqueous solution comprising at least one sodium salt, said method comprising a flotation separation step using a collector for Hetero-polar organic surfactant type gypsum granules. It also relates to the use of granules of calcium carbonate or gypsum granules treated according to the method of the present invention, in cement, or in civil engineering.

In fact, in many processes granules of calcium carbonate and gypsum (di-hydrated calcium sulphate) are generated and are found in suspension as a mixture. These mixtures are difficult to recover, for example in civil engineering, because of the difference in the type of use possible for one and the other of the minerals.

Calcium carbonate is often used as an inert filler in cements or concretes because of its low solubility in water; for example, the solubility product of calcite in water is K _s = 10 - ^" 8 3 (mol / L) ^.2 The other crystalline species of calcium carbonate, such as aragonite, or vaterite, are less frequent in the water. nature, have solubility products slightly different but close.

Calcium sulphate is more soluble in water: approximately 0.2 g of CaSO ₄ per 100 g ¾ 0, and its solubility increased to a factor of about two in saline solution. Calcium sulphate is a setting modifier in cements and concretes. It is likely to cause swelling and embrittlement of building materials by delayed formation of ettringite if used in large quantities. However, gypsum is a good raw material for the realization of building materials such as panels or plaster tiles, especially after calcination to transform it into calcium sulphate hemihydrate which has hydraulic setting properties when it is put in. contact with water.

Calcium carbonate and calcium sulphate, although different

chemically for the constitution of their anion, however, have relatively close physical properties (density, dielectric or magnetic susceptibility) which makes it difficult when it is desired to separate at low cost large volumes of such mixtures, especially when calcium carbonate and gypsum both are in the form of small particles or granules. STATE OF THE PRIOR ART

Among the separation techniques of minerals, flotation is one of the techniques used, especially in ore beneficiation. Flotation separates particles or granules from solids, taking advantage of the differences between their surface properties in an aqueous solution.

The principle of flotation (see Encyclopaedia of the Engineer, Edition Lavoisier, Paris, Vol J3350, pages 1 to 2, June 2012) is the following: the solid granules are suspended in a liquid, in general the water, after a possible grinding more or less pushed. This solid-water mixture, also called pulp, is conditioned with a chemical reagent called collector, whose role is to make the surface of the mineral to float, in order to give it a greater affinity for the gas phase than for the liquid phase. . The pulp thus packaged is introduced into reactors equipped with aerated stirrers (flotation cells) or air injectors (flotation column) or

of electrodes (electro-flotation) generating air bubbles and dispersing them. The hydrophobized granules attach to the surface of the bubbles that constitute a transport vector through their upward movement towards the free surface of the pulp. This gives a supernatant foam loaded solid called foam. The size of the bubbles (and in this the interfacial area liquid-air) and the life of the foam is modulated by the optional addition of a foaming agent.

The entrained liquid is drained by gravity into the interior of the foam, which is collected by overflow. Foam and residual pulp are generally subjected to subsequent treatment such as optional washing, decantation and / or filtration, and optional drying, to collect the granules of overflowing solids and the granules of solid underflow, depending on whether the It is desired to use one or other of the solids obtained in wet or dry form. The flotation separation of soluble or partially soluble ores has been intensively studied, particularly for the separation of non-ferrous metal sulfides, metal oxides, phosphate ores, and silvite (KC1).

It is known that sedimentary deposits of phosphate ores such as those of apatite Ca ₅ (PO ₄ ) 3 (F, OH), can be treated by flotation when the gangue consists essentially of siliceous materials, such as phosphate sandstone from Florida. Central. The flotation of the apatite ore then makes it possible to separate the apatite ore and the carbonate with the scum, and underflow into the residual pulp to remove quartz and silica. Sedimentary phosphates with a high carbonate content, for example those in southern Florida and the region

Mediterranean, however, do not lend themselves to flotation (H. Sis, S. Chander / Minerals Engineering Vol.16 (2003) pp 577-585).

The separation of mixtures of carbonate ores and calcium sulphates has been the subject of little development. The presence of calcium ions (Ca ²⁺ ) in large quantities in aqueous solution resulting from the partial solubilization of the gypsum brings a priori negative to the use of ionic surfactants. In fact, alkaline earth ions (Mg ²⁺ , Ca ²⁺ , ...) react with ionic surfactants to form the corresponding magnesium or calcium salts which are insoluble and inhibit their ability to lower the surface tension solution-air , which prevents in particular the formation of stable foam. Thus in the field of detergency, especially for laundry and dishwashers, it is usual to complex (in the use of phosphates, polyphosphonates, citrates) or to precipitate (with zeolites, or builders such as sodium) the Mg ²⁺ or Ca ²⁺ ions of the water used to make it "lighter". This preserves the effectiveness of the surfactants and that they can produce the desired effects.

However, the need to separate the gypsum from the calcium carbonate remains, this to widen the possibilities of valorization of such mixtures, in particular in the fields of the cement works and the civil engineering and to allow a sustainable development of such valorization channels.

SUMMARY OF THE INVENTION

It has been surprisingly found that:

ionic surfactants having one or more carboxylic group, or sulfonate, or sulfate, or phosphonate, or phosphate, or hydroxamate constituted an effective collector for the separation of gypsum granules in a suspension containing gypsum granules and granules of carbonate of calcium in aqueous solution when it comprises at least one sodium salt at a sodium ion concentration of at least about 3 g / L

nonionic surfactants of the polarizable alcohol type and in particular of Guerbet alcohols type did not constitute an effective collector for the separation of gypsum granules in a suspension containing granules of gypsum and granules of carbonate when they were used alone, but could be when they were associated with ionic surfactants mentioned above.

Accordingly, the present invention relates to a method for separating a suspension containing gypsum granules and calcium carbonate granules and an aqueous solution, characterized in that the aqueous solution comprises at least one sodium salt and in that the aqueous solution has a sodium ion concentration of at least about 3 g / L, preferably at least about 10 g / L, preferably at least about 30 g / L, and that said method comprises a flotation separation step using a collector for the gypsum granules, the collector comprising at least one hetero-polar organic surfactant of the formula RX, wherein:

R denotes a hydrocarbon chain comprising from 2 to 50 carbon atoms, preferably from 10 to 30 carbon atoms, preferably from 15 to 25 carbon atoms, and the hydrocarbon chain is chosen from: a linear saturated or unsaturated alkyl chain; a saturated or unsaturated branched alkyl chain,

- X denotes at least one ionizable group chosen from the group consisting of: a carboxylic group, a sulphonate group, a sulphate group, a phosphonate group, or a phosphate group, or a hydroxamate group.

A first advantage of the present invention is the selectivity obtained on the separation of the gypsum granules from a pulp consisting of a mixture of gypsum granules and calcium carbonate.

A second advantage of the present invention is the good selectivity of the gypsum collectors even in the presence of a high ionicity and / or a high calcium content of the solution of the pulp to be treated.

A third advantage of the present invention is the simplicity with which the granules of the two calcium minerals can be separated into two enriched enriched phases especially in the fields of cement and civil engineering. A fourth advantage of the present invention in the case where the suspension containing gypsum granules and calcium carbonate granules is obtained by adding milk of lime in a saline solution comprising sulphate ions, then by carbonation of all or part of the lime milk with carbon dioxide, is to make possible the separation of sulphates in the form of granules of gypsum, granules of calcium carbonate, using carbon dioxide at low concentration and thus to reduce the footprint C0 ₂ processes incorporating these steps.

DEFINITIONS

In this memoir we mean:

by "granule": a particle of a solid,

by "gypsum granule": a solid particle generally comprising at least 80%, preferably at least 90% by weight of calcium sulfate dihydrate (CaSO ₄ .2H ₂ O),

- "Calcium carbonate granule": a solid particle generally comprising at least 80%, preferably at least 90% by weight of calcium carbonate (CaCO ₃ ), generally present in the form of calcite,

with "sodium salt": a sodium salt partially soluble in the aqueous solution, such as sodium borate, sodium chloride, sodium sulphate, sodium sulphite, sodium nitrate, sodium nitrite, advantageously the chloride or sodium sulfate, more preferably sodium chloride,

by "collector": a chemical reagent rendering the surface of the granules hydrophobic and thereby increasing the affinity of the granule for the gaseous phase used in flotation,

by "pulp": a suspension of solid granules in an aqueous solution,

- by "foam", also called "foam" or "float" or "overflow": a supernatant foam obtained in overflow from a flotation apparatus,

- "residual pulp", also known as "sterile" or "non-floated" or "underflow": a pulp obtained under-poured from a flotation apparatus after a flotation operation,

- "hydrocarbon chain": an organic chain comprising carbon and hydrogen atoms and optionally one or more other atoms such as oxygen (O), sulfur (S), nitrogen (N), phosphorus (P),

- by "ionizable groupX":

a hydrogenated or alkali metal group such as lithium, sodium, potassium, ..., preferably sodium or potassium, preferentially sodium, capable of losing the hydrogen or the corresponding alkali metal in the form of H +, Li +, Na +, K + ions, etc.

- at least one ionic group selected from the group consisting of: a carboxylic group, a sulphonate group, a sulphate group, a phosphonate group, a phosphate group, a hydroxamate group, at least one carboxylic group (-COOH or - COOM with one of the alkali metals listed in the preceding paragraph), or at least one sulphonate group (-SO ₂ -OH or -SO ₂ -OM with one of the alkali metals listed in the preceding paragraph), or at least one sulphate group ( -O-SO ₂ -OH or -O-SO ₂ -OM with one of the alkali metals listed in the preceding paragraph), or a phosphonate group (-PO- (OH) ₂ or -PO- (OM) ₂ with M a alkali metals listed in the previous paragraph), or a phosphate group (-O-PO- (OH) ₂ or -O-PO- (OM) ₂ with one of the alkali metals listed in the previous paragraph), or a hydroxamate group ( -CO-NH-OH or -CO-NH-OM with one of the alkali metals listed in the previous paragraph), present on the cha hydrocarbon ion, either as the only ionizable group of the hydrocarbon chain, or in the form of several ionizable groups of the same kind (for example in the form of polycarboxylic surfactant, or polysulfonate, or polysulfate, or polyphosphonate, or polyphosphate, or polyhydroxymate) or in the form of several groups of different natures.

In this specification, the choice of one of a group of elements also explicitly describes:

the choice of two or the choice of several elements of the group,

the choice of an element from among a subgroup of elements which is constituted by the group of elements to which one or more elements have been removed.

In the present specification, the description of a range of values for a variable, defined by a low limit, or a high limit, or by a low limit and a high limit, also includes the embodiments where the variable is selected respectively in the value range: excluding the low limit, or excluding the high limit, or excluding the low and high limits.

The term "comprising" includes "consisting essentially of" and also "consisting of".

The use of "a" or "an" in the singular also includes the plural, and vice versa, unless otherwise indicated. If the term "approximately" is used before a quantitative value, it represents a variation of ± 10% of the nominal quantitative value, unless otherwise indicated.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a block diagram of one of the embodiments of the invention using a flotation cell 3.

Fig.2 is a block diagram of one embodiment of the invention using a flotation cell 3, and complementary liquid-solid separation equipment such as decanters 6 and 13 and filters 10 and 17.

FIG. 3 is a block diagram of one of the embodiments of the invention using a flotation cell 3, complementary liquid-solid separation equipment such as decanters 6 and 13 and filters 10 and 17, and contact reactors 23 and 26.

FIG. 4 illustrates the percentage (%) of mass recovery, referred to as 'Recovery (%)', of gypsum granules, calcium carbonate, and apatite after flotation tests, at three concentrations of sodium oleate added to the flotation cell, described in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for separating a suspension containing gypsum granules and calcium carbonate granules and an aqueous solution, characterized in that the aqueous solution comprises at least one sodium salt and in that the aqueous solution at a sodium ion concentration of at least about 3 g / L, preferably at least about 10 g / L, preferably at least about 30 g / L, and that said method comprises a flotation separation step using a collector for gypsum granules, the collector comprising at least one hetero-polar organic surfactant of formula RX, wherein:

- X denotes at least one ionizable group chosen from the group consisting of: a carboxylic group, a sulphonate group, a sulphate group, a phosphonate group, or a phosphate group, or a hydroxamate group. Indeed the inventors have observed that these types of specific hetero-polar organic surfactants had an affinity for granules of gypsum and comparable limestone when these minerals were suspended in water; but that on the other hand they were particularly effective for the selective flotation of gypsum granules when they are present in a suspension containing granules of gypsum and granules of calcium carbonate and an aqueous solution with high ionicity in the presence of sodium ions or sodium and calcium ions. Nonionic surfactants of the polarizable alcohol type and in particular of the Guerbet alcohols type, which are generally not influenced by the presence of sodium ions or of sodium and calcium ions, have not shown any efficacy for the separation of gypsum granules and calcium carbonate when they are used alone, but against these nonionic surfactants improve the selectivity of the separation of the two minerals when they are associated with the ionic surfactants mentioned above.

By selective flotation is meant herein a step of separation by flotation of a pulp, to obtain a foam overflow generally containing at least 60%, preferably 70%,

preferably at least 80%, more preferably at least 85% by weight of the gypsum granules of the initial pulp and for each of these cases in general less than 40%, or less than 30%, or less than 20% by weight granules of calcium carbonate from the initial pulp.

Preferably, the hetero-polar organic surfactant RX is chosen from the group consisting of: sodium or potassium oleate, sodium or potassium alkyl sulphonate, sodium or potassium alkyl sulphate, sulphosuccinamate of sodium or potassium, or a mixture thereof, preferably a sodium or potassium oleate. The collector may also comprise a nonionic surfactant such as an iso-alcohol or a Guerbet alcohol.

In the present invention the flotation separation step is performed with a flotation cell well known to those skilled in the mechanical agitation type or pneumatic type such as a flotation column. Such flotation cells are described for example in the encyclopedia "Techniques of the Engineer - Volume J3360 pages 1 to 22, flotation practical aspects - Paris, June 2012, incorporated by reference in the present description.

In such a flotation cell, the pulp, suspension of the granules in the aqueous solution, is brought into contact with air bubbles. In the case of pneumatic flotation using a so-called "counter-current" column the The pulp is injected into the column from the top of the column and collected downward, flowing down the column as the bubbles formed in a bottom bubble generator rise. Gas and aqueous solution meet and the hydrophobic granules stick to the bubbles in an area of the column called "collection area". The bubbles, once reached the surface form a foam, or foam, sufficiently stable so that its amount increases continuously and can be recovered by overflow at the column head. This scum is loaded with granules selectively collected by the bubbles and is one of two flotation products (product "float" or "scum").

The product recovered by pumping system at the bottom of the column constitutes the "non-floated" or "sterile". It corresponds to the aqueous solution freed from particles collected specifically by the bubbles. It may also contain hydrophilic particles that have not adhered to the bubbles.

The flotation cells with mechanical agitation have a similar operation but the pulp is introduced by mechanical suction under the effect of the rotor in the intermediate part of the cell, the scum is collected in the upper part and the aqueous solution and residual "non float Are recovered at the bottom of the flotation cells.

In the present invention, the heteropolar organic surfactant collector is introduced directly into the flotation cell near the bubble generator. This makes it possible to limit the consumption of the collector by the calcium ions in the liquid phase and better adsorption on the surface of the calcium gypsum granules. The collector may also be introduced directly into the flotation column at the pulp-foam interface, for example, so that the collector adsorbs on the mineral surface in the downflow.

In order that the partitioning of gypsum granules and calcium carbonate with such surfactants is effective, it is desirable that the calcium carbonate granules and the gypsum granules have a distribution.

granulo metric such that at least 90% by weight of the particles have a diameter less than 150 μιη, preferably less than 130 μιη, preferably less than 110 μιη and more preferably less than 90 μιη. In general, the granules of calcium carbonate and the granules of gypsum have a granulometric distribution such that at least 10% by weight of the particles have a diameter greater than 0.1 μιη, preferably greater than 1 μιη, preferably greater than at 2 μηι. The amount of gypsum granules or calcium carbonate granules reported per unit volume of suspension (the pulp) is generally at least

10 kg / m 3, advantageously at least 15 kg / m 3. It is generally at most

75 kg / m 3, advantageously at most 50 kg / m 3. The weight ratio between granules of gypsum and calcium carbonate granules of the suspension is generally between 1: 3 and 3: 1, advantageously between 1: 2 and 2: 1.

The surfactants mentioned above can operate over a wide pH range. Acidic pH of the suspension below 5 causes acid attack of the calcium carbonate granules by generating C0 ₂ . This release of C0 ₂ at pH too acidic is detrimental to good selectivity: because then the carbonate granules are covered with microbubbles of C0 ₂ which has the effect of driving the granules of calcium carbonate in an updraft and decrease the selectivity of the separation of the gypsum granules and calcium carbonate granules from the suspension. In general in the present invention the suspension has a pH of at least 5, preferably at least 7, preferably at least 7.5, more preferably at least 8.

On the other hand, at a very high pH above 13, hydroxyl complexes Ca (OH) ⁺ bind to hetero-polar organic surfactants which reduces their effectiveness. In the present invention the suspension has a pH advantageously of at most 13, more preferably at most 11, still more preferably at most 10.

In the method according to the invention the suspension containing granules of gypsum and granules of calcium carbonate and an aqueous solution can be obtained for example by:

a) addition of milk of lime in a saline solution comprising at least 0.5 g of sulphate ions per liter, and

b) carbonation of all or part of the milk of lime with carbon dioxide.

In the method according to the invention the suspension containing granules of gypsum and granules of calcium carbonate present in the aqueous solution can also be obtained by reaction of a lime milk comprising at least one sodium salt, or a mixture of lime milk and a broth of calcium carbonate comprising at least one sodium salt with acid fumes resulting from the combustion of a sulfur-containing organic compound such as a coal, a lignite, a wood, an agricultural residue, an organic residue from the food industry, an organic residue from the paper industry, a sludge from a drinking water or wastewater treatment plant, an organic sludge from a biogas plant. The surfactants mentioned above have also been effective in separating granules of gypsum and granules of calcium carbonates in aqueous solutions with a high content of soluble salts, in particular sodium salts. This is particularly advantageous for example for residual solutions for washing acid gases such as SO _x (SO ₂ , SO ₃ , ....) made with seawater or brines using crushed calcium carbonate and / or CHL, or with concentrated sodium ion permeate from desalination unit of seawater by evaporation, by dialysis, or by reverse osmosis.

During the reaction of calcium ions from calcium carbonate or from lime milk reacting with acid gases such as SO _x or with sulphate ions of seawater, or seawater optionally concentrated, or With acid brine washing equipment using hydrochloric acid (HC1), the calcium ion content is likely to reach significantly higher values than the solubility values of gypsum in water. In the case of high value of concentration of calcium ions, and

possibly with a high sodium ion value, the solubility of the sulphate ions will decrease inversely proportional to the calcium ion concentration, regulated by the solubility product of the gypsum and the corresponding activity coefficients.

The surfactants listed in the present invention have again surprisingly been shown to be a good collector of gypsum granules for the separation of a suspension containing gypsum granules and calcium carbonate in such brines. Accordingly, the present invention also relates to a method for separating gypsum granules and calcium carbonate granules in an aqueous solution, wherein the aqueous solution further comprises at least one calcium salt, and that the aqueous solution has a calcium ion concentration of at least about 0.5 g / L, preferably at least about 5 g / L, preferably at least about 10 g / L.

By calcium salt is meant a calcium salt partially soluble in the aqueous solution, such as calcium hydroxide, calcium chloride, calcium nitrate, calcium nitrite, preferably calcium chloride.

The calcium salt (s) can be co-present with the sodium salt (s). Separations of granules of gypsum and calcium carbonate in mixed saline solutions of sodium and calcium salts having a concentration of soluble salts up to about 4 mol of sodium and / or calcium per liter give still excellent results of separation of the two types of granules according to the present invention.

In a particularly advantageous embodiment of the present invention, the gas used for flotation is a gas whose content of carbon dioxide (CO ₂ ) is less than 10%, preferably less than 1%, and more preferably less than 10%. 0.5% by volume on dry gas.

Flotation carried out with a gas composed of enriched air at 15-25% vol. in C0 ₂ gives at the level of the solid float correct gypsum separation yields: of the order of 70%, but a lower selectivity at the level of the carbonates. With a gas having a C0 ₂ concentration of between 15 and 25% vol., The carbonates separation efficiencies in the float are of the order of 35%. On the other hand, the use of a gas such as air or an inert gas such as nitrogen, whose CO ₂ contents are limited, gives improved separation and selectivity results according to the invention. Thus, with the same ionic and nonionic surface-active agents and similar operating conditions, a flotation gas composed of air or nitrogen makes it possible to obtain recovery efficiencies in the foam (the float) of 71 to 88.degree. % yield of sulfate and 6 to 21% of carbonate yield corresponding to higher selectivity coefficients (selectivity between 12 and 95).

Thus, in the case where the suspension containing gypsum granules and calcium carbonate granules is obtained either by adding lime milk in a saline solution comprising at least 0.5 g of sulphate ions per liter, then by carbonation of all or part of the milk of lime with carbon dioxide, either by reaction of a milk of lime comprising at least one sodium salt, or of a mixture of lime milk and a broth of calcium carbonate comprising at least one sodium salt with carbon dioxide or acid fumes from the combustion of an organic sulfur compound, the carbonation step is preferably separated and prior to the flotation step. On the other hand, the flotation step is preferably carried out with a gas whose carbon dioxide content is limited according to the limits indicated above.

The feed rate of the pulp in a flotation cell may range from 0 to a limit speed beyond which the bubbles are entrained with the under-poured calcium carbonate granules. In the present invention, feeding of the flotation cell that flow rates suitable to match surface speeds empty cask _J of at least 0.1, preferably at least 0.2, preferably at least 0.5 cm / s. Speed The empty surface of the pulp feed rate corresponds to the volume flow rate per unit of time divided by the area of the maximum useful horizontal section of the flotation cell. Advantageously, the feed rates of the flotation cell are chosen so that the surface velocities in empty casing J _a are at most 5.0, more advantageously at most 3.0, and even more

advantageously at most 1.7 cm / s. Values between 0.5 and 1.7 cm / s are particularly suitable.

The average superficial gas velocity in the portion of the flotation cell where the gas rises is generally at least 0.1, preferably at least 0.2, preferably at least 0.5 cm / s. The average superficial gas velocity in the part of the flotation cell where the gas rises is advantageously at most 5.0, more preferably at most 3.0, still more advantageously at most 1.7 cm / s. Most preferably the average superficial gas velocity in the portion of the flotation cell where the gas rises is at least 0.5 and at most 1.5 cm / s. The average superficial gas velocity in the portion of the flotation cell where the gas rises is defined as the volume flow rate of gas divided by the area of the average horizontal section in the portion of the flotation cell where gas and pulp are in contact.

In a particular embodiment of the present invention, the flotation separation step is carried out with a mechanical agitation or pneumatic type flotation cell such as a flotation column, and the average surface gas velocity in the flotation cell is at least 0.1, preferably at least 0.2, preferably at least 0.5 cm / s and advantageously at most 5.0, more preferably at most 3.0, even more advantageously at most 1.7 cm / s, and most preferably at least less than 0.5 and not more than 1.5 cm / s.

The dispersion of the gas in the flotation cell is chosen from the devices known in the art in order to generate in the flotation cell in general gas bubbles with a volume average diameter in general of at least 0.4 mm, advantageously from less 0.6 mm, more preferably at most 0.65 mm. The average volume bubble diameter is generally at most 2.5 mm, advantageously at most 1.2 mm, more preferably at most 1.0 mm. Medium volume bubble diameters of at least 0.65 mm and at most 0.95 mm are particularly suitable.

In the case where obtaining granules of gypsum and granules of calcium carbonate in an aqueous solution is obtained by: a) adding lime milk in a saline solution comprising at least 0.5 g of sulfate ions per liter, and b) carbonation of all or part of the milk of lime with carbon dioxide, it is particularly advantageous to inject sufficient carbon dioxide into the saline solution comprising the lime milk so that the content of granules of slaked lime (Ca (OH) ₂ ) relative to the weight of the granules of gypsum and calcium carbonate of the pulp is less than 5%, preferably less than 3%, more preferably less than 1% by weight. In general, to reach such a content of granules of slaked lime (Ca (OH) ₂ ) relative to the weight of granules of gypsum and of calcium carbonate granules of the pulp, the carbonation with carbon dioxide is carried out at a pH of about 7, preferably about 6.5, preferably about 6.0, more

preferably about 5.5. At the end of the carbonation, when the acidic carbon dioxide is no longer introduced into the mixture of granules, the pH of the mixture rises rapidly from about 0.1 to 2.5 pH units to reach pH values of between about 7.0 and about 9.5. This rise in pH is due to the presence of granules of calcium carbonate which is a mild alkali, and the presence of residual lime that is alkaline. The more the lime is completely carbonated, the more the pH of the broth at the end of the carbonation approaches the neutrality.

This carbonation can be carried out with a carbon dioxide originating from fumes produced by the combustion of a sulfur-containing organic compound such as a coal, a lignite, a wood, an agricultural residue, an organic residue of the food industry, an organic residue of the paper industry, a sludge from a drinking water or wastewater treatment plant, an organic sludge from a biogas plant. In general, such smoke contains a concentration of carbon dioxide by volume on dry gas of at least 8%, or at least 10%. It generally comprises a concentration of carbon dioxide by volume on dry gas of not more than 20%, or not more than 18%.

In the case where obtaining granules of gypsum and granules of calcium carbonate in an aqueous solution is obtained by: a) adding lime milk in a saline solution comprising at least 0.5 g of sulfate ions per liter, and b) carbonation of all or part of the milk of lime with carbon dioxide, or in the case where the mixture of gypsum granule and calcium carbonate is obtained by washing sulphurous fumes with a lime milk, it is particularly advantageous to carry out carbonation and flotation in this order and not in the opposite or concomitant order. It has been found that the The flotation and selectivity of the flotation of gypsum granules and calcium carbonate with the surfactants described herein were much better, all parameters being constant, once the carbonation was completed. Concurrent flotation with carbonation, namely flotation with a combustion gas or smoke containing carbon dioxide C0 ₂ according to the method of the present invention also gives much lower efficiency and flotation selectivity compared to those obtained when the carbonation of the possible lime granules is carried out before the flotation.

The present invention thus makes it possible to separate a mixture of granules of gypsum and granules of calcium carbonate present in an aqueous solution in two enriched phases respectively:

- in gypsum, overflow of the flotation cell

- Calcium carbonate, underflow of the flotation cell.

These two phases enriched respectively in granules of gypsum and calcium carbonate allow an easier valuation particularly in the fields of cement and civil engineering.

Thus, the present invention also relates to the use of granules of calcium carbonate or gypsum granules derived from the method of the present invention: in cement works, or in civil engineering, or in road engineering, or for the manufacture of materials construction, or the manufacture of panels or plasterboard, or the manufacture of road aggregates, or the manufacture of backfill material to fill underground cavities, or the desulfurization of fumes.

The following examples serve to illustrate the invention. They are not limiting.

Fig. 1 is a block diagram of one of the embodiments of the invention using a flotation cell 3. A suspension 1 containing gypsum granules and calcium carbonate granules and a sodium aqueous solution is introduced into the flotation cell 3. The hetero-polar organic surfactant collector 2 is also introduced into the flotation cell 3. A phase 4 enriched in gypsum granules is collected overflow of the flotation cell 3. A phase 5 enriched in granules of Calcium carbonate is collected underflow from the flotation cell 3.

Fig. 2 is a schematic diagram of one embodiment of the invention using a flotation cell 3, and separation equipment complementary liquid-solid such as decanters 6 and 13 and filters 10 and 17. A suspension 1 containing gypsum granules and granules of calcium carbonate and a sodium aqueous solution is introduced into the flotation cell 3. The organic surfactant collector hetero-polar 2 is also introduced into the flotation cell 3. A phase 4 enriched in gypsum granules is collected overflow of the flotation cell 3, and is introduced optionally diluted with water or mother liquors in a decanter and optionally introduced with a flocculant and / or an anti-foaming agent in a decanter 6. The settling water or the settling mother liquors 7 are collected in overflow from the decanter 6. A broth 8 enriched with granules of gypsum is then introduced. in a filter 10, and a washing water or washing mother liquor 9 is optionally used in the filter 10 to wash the granules of the broth 8 optionally washed. A filtered cake 12 enriched with gypsum granules is collected from the filter 10, as well as the filtration and optionally washing waters 11 which are collected from the filter. Phase 5 enriched in granules of calcium carbonate is collected underflow of the flotation cell 3. The phase 5 is introduced, optionally with a flocculent and / or an anti-foaming agent, into the decanter 13. The sodium water or settling mother liquors 14 are collected overflow from the settler 13. A broth 15 enriched in granules of calcium carbonate is then introduced into a filter 17, and a washing water or washing mother liquor 16 is (are ) optionally used at the filter 17 to wash the granules of the broth 15. A filtered cake 19 enriched with granules of calcium carbonate is collected from the filter 10, and the filtration water and optionally washing 18 which are collected from the filter 17. Optionally (not shown in FIG. 2), the filtration waters and optionally the washing waters 11 are recycled upstream of the decanter 6 to repulse the phase 4 (scum) before introduction into the decanting chamber. r 6, and thus organize a countercurrent to optimize the water balance of the method according to the invention. It is the same (not shown in Figure 2), for the filtration water and optionally washing water 18 which can optionally be recycled upstream of the decanter 13 to plump phase 5 (underflow of the flotation cell 3 ) before introduction into the decanter 13, and thus organize a counter current to optimize the water balance of the method according to the invention.

Fig. 3 is a block diagram of one embodiment of the invention using a flotation cell 3, complementary liquid-solid separation equipment such as decanters 6 and 13 and filters 10 and 17, and contact reactors 23 and 26. A saline solution 21 comprising at least 3 g of sodium ions per liter and 0.5 g of sulfate ions per liter is introduced into a contact reactor 23 with a milk of lime 22, forming in the contact reactor granules of gypsum . The broth 24 obtained is then introduced into a contact reactor 26 with a gas comprising carbon dioxide (CO ₂ ) in order to carbonate the optional residual calcium hydroxide present in the broth 24 collected from the contact reactor 23 to form granules of calcium carbonate. A suspension 1 containing gypsum granules and calcium carbonate granules and an aqueous solution is then introduced into a flotation cell 3 for processing according to one embodiment of the method of the present invention described in FIG. 2.

The following examples illustrate the invention without being limiting.

EXAMPLES

Example 1 (not in accordance with the invention).

In a rectangular laboratory flotation cell, 185 mL of useful volume, and length x width x height dimensions equal to 65 x 65 x 14 mm, various tests were carried out with gypsum granules (origin: Central Atlas, Idharen , Morocco) granules of calcium carbonate (origin: Provence, France) apatite granules (origin: Fort Dauphin, province Tuléar Madagascar) introduced alone in demineralized water) at various levels of hetero-polar organic surfactant which the pH was adjusted by adding 1% NaOH or HCl solutions. The agitation of the suspension in the cell was provided by a rotor developing a speed of 600 to 3000 rpm. The tests were carried out with 3 g of the pure minerals mentioned above, with a granulometric fraction of between 20 and 100 microns, with 175 ml of water.

A hetero-polar organic surfactant: sodium oleate was added to the suspension to be floated in 3 different amounts to obtain concentrations of ¹ × 10 ^-5 , ⁵ × 10 ^-5 , and 10 × 10 ^-5 (10 ^-4 ) mol / L in the suspension. to float.

The flotation cell was operated at a speed of 1750 rpm to inject air into the flotation cell for a period of 5 minutes.

The formed foam was collected and the mass quantity of the gypsum granules, or calcium carbonate, or apatite was evaluated by comparing the yields of the floated mineral weights relative to the total mineral weights introduced in each series. test. For this, the floated and non-floated products were dried in an oven at 80 ° C and were then weighed to determine their weights respectively designated Mf and Mnf. The yield of Rf float mass recovery was calculated according to the formula Rf = 100 x Mf / (Mf + Mnf).

Fig. 4 reports the% mass recovery, noted as 'Recovery ()', of gypsum granules, calcium carbonate, and apatite at the end of each flotation test for three concentrations of sodium oleate added to the cell. flotation before the actual flotation operation.

It is found that the three minerals have similar buoyancy in the concentration range of 10 ^-5 to 10 ^-4 mol / L in surfactant. More

particularly the flotation behavior of gypsum and calcium carbonate are very close over the surfactant concentration range tested with very close recovery yields of gypsum and calcium carbonate for each of the surfactant concentrations, and leading to the conclusion that poor oleate selectivity for these two minerals when mixed in water.

Example 2 (in accordance with the invention) the tests of Examples 2 to 5 were carried out in two different pilot flotation columns with however a similar operating scheme and procedures. The tests were carried out using a brine whose sodium content (in the form of dissolved sodium chloride) was at least 3 g / l and up to 23 g / l (1 mol / l). The calcium content (in the form of dissolved gypsum and soluble calcium chloride) was at least 0.5 g / L and up to 40 g / L (1 mol / L).

The first column used measures 3.5 meters in height and has a diameter of 75 mm ensuring a feed rate of 0.01 to 0.5 m / h.

Various surfactant collectors were tested. Concentrations of the collectors and additives ranged from 0 to 100 ppm.

The second column used is an industrial pilot of 10 m height and 300 mm diameter, ensuring a feed rate of up to 5 m / h. This second flotation column was only used with sodium oleate at concentrations ranging from 0.5 to 10 ppm. The results of tests on this second column were comparable to the results obtained on the first column under the equivalent operating conditions (amount of flotation agent, surface velocities of flotation air and pulp feed rate. The different points of introduction of the collector into the columns were tested starting from the introduction directly in the bubble generator and two different levels of the column.

Superficial gas and feed rates were used instead of flow rates to control the operation of the column. The limits of these parameters were chosen as follows:

superficial velocity of upwardly directed gas: the minimum superficial velocity must ensure the formation of bubbles which will have an upward movement in the column. The maximum value must not cause the formation of very large bubbles in order to avoid the brine entrainment by the mechanical effect.

- superficial feed rate, directed downwards: it can vary from 0 to a limit speed beyond which the bubbles are carried with the waste rock (underflow).

The feed rates of the pulp in this example were varied to achieve downwardly directed pulp surface velocities ranging from 0.5 cm to 1.7 cm / sec.

The air supply flow rates were adjusted to obtain superficial velocities J _g rising gas between 0.5 to 1.9 cm / s.

For each test condition, the yields by weight of the solids (granules) collected in each of the phases were reported: either the ratio, in percentage, between what was recovered in the foam ("foam" or

"Scum" or "floated") and all sampled at the different outputs of the column: foam + sterile, all in a similar amount of time. The water yields have the same definition as that of the solids, namely: ratio between the weight of the water in the foam and the sum of the weights of the water in the foam plus that of the waste rock. The weight of the water of the floated phase is obtained by difference of the weight of the floated phase to which is subtracted the weight of the granules of gypsum and calcium carbonate and the weight of the dissolved salts.

The fluxes of material were thus calculated by timing of each intake and weighing of the collected samples, component flows by component, also, thanks to the chemical analyzes.

The sulphate analysis for determining the separation efficiency of the gypsum between the floated phase and the unfloated phase is carried out by acid dissolution of the solids and then gravimetry of the sulphate by precipitation of BaSO ₄ with BaCl ₂ . The carbonate analysis for the determination of the separation efficiency of calcium carbonate between the floated phase and the unfloated phase was carried out on the filtered solids (floated and sterile), and dried in study at 80 ° C and then ground. The carbonate analysis was carried out by gravimetry of C0 ₂ contained in the solids by a standard measurement of the ACA (Absorption Carbonate Analysis) type. The solids are attacked with concentrated hydrochloric acid (9N). Acid attack until the methyl orange (helianthine) turn of the carbonated compounds causes the release of C0 ₂ , the latter is absorbed in a saturated solution of barium (Ba (OH) ₂ previously filtered at room temperature The absorbed C0 ₂ precipitates in the clear solution of barite in the form of insoluble barium carbonate (BaCO ₃ ) The BaC0 ₃ precipitate is then quantified by gravimetry.

The following calculation examples are given for sulphates.

The mass of dry matter in floats and not floated is calculated according to:

(1 -humidity) x (cake mass)

MS = - - x 100

(cake mass) + (filtrate mass)

The calculation of the flow of S0 ₄ ^" in the same products is carried out from the determination of the sulphate content y ^p

y sa, ^{X Ms}

Bone

taking

where t _p ri _se = sampling time

and p = froth (foam) or sterile (underflow).

Calculation of the recovery of S0 ₄ ^" in the foam is calculated considering the balance of the separation operation:

so ^{2 ~}

Rdtso; ² = - ⁴ ", x 100

⁴ Φ ™ ⁴ foam + Φ ™ ⁴ ste, rn, e

The yield therefore indicates the portion recovered in the foam even when we talk about the yield of carbonate. The yield of the sulphates in the waste rock is calculated according to the formula below but taking the flow of the sulphates in the sterile <bso 4 ^~ sterile. Carbonate yields are calculated in the same way.

The selectivity S of the flotation separation process is calculated from the chemical analyzes carried out and the contents of carbonates and sulphates in the foams and waste products according to the following formula: sterile foams

yso ₄ · yœ ₃

sterile foams

yso ₄ · yœ ₃

The results of Example 2 on column 1 75 mm in diameter are shown in Table 1, separation tests carried out with a surfactant of sodium oleate type at 20 ppm concentration.

In Table 1 are shown:

the water yield of the floated phase

- the percentage by weight of the two floated phases (foam) and not floated (under poured) collected and standardized by the sum of the two phases collected,

the carbonate and sulphate content by chemical analysis of each of the two floated and non-floated phases,

- Calculation of the recovery budget of the gypsum and carbonate granules calculated from the chemical analyzes.

The results of Table 1 show that irrespective of the pH (between 7 and 11) and the flow rate of the pulp, the flotation allows a selective separation of gypsum granules and calcium carbonate as demonstrated by the increase in the content of the pulp. S0 ₄ and a decrease in the content of C0 ₃ in the float product (scum). Conversely, the unflattered product (under pour) is enriched in CO ₃ with a very significant decrease in the sulphate content. Test 2.3 reproduced in test 2.5 gives an idea of the repeatability of the tests.

Examples 3 and 4 (in accordance with the invention)

In the same column 1, with the same pulp and the same aqueous solution of Example 2, the following surfactants ("collectors") were tested:

- sodium oleate (C ₁₇ H ₃₃ COO ^" + Na ⁺ )

sodium laurate (C ₁₂ H ₂₃ COO + Na ⁺ )

alkyl sulphate (R-S0 ₄ ^2- ): Flotinor S 072 (Clariant)

- alkyl sulphate (R-S0 ₄ ^" ): pure dodecyl sulphate

alkyl sulfonate (R-SO ₃ ): Aero 830 Promoter (Cytec)

sulfosuccinamate: Aero 845N (Cytec) composed of Tetrasodium N- (1,2-dicarboxyethyl) n-octadecyl sulfosuccinamate, having three carboxylic groups and a sulfonate group.

modified sulfosuccinamate: Procol CA540 (Allied Colloid Ltd) composed of N- (3-carboxylato-1-oxo-3-sulfonatopropyl) -N-octadecyl-DL-tetrasodium aspartate non-ionic surfactants of Guerbet alcohol type with 12 or 16 carbon atoms: Isofol 12, denoted ISF12 and Isofol 16, denoted ISF 16 (Sasol Olefins & Surfactants).

Table 2 (Example 3) groups together anionic surfactant collector tests used alone, of the carboxylic type (oleate and sodium laurate), of the alkyl sulphate type (dodecyl sulphates), of the mixed carboxylic and sulphonate (sulpho succinamate) type.

Table 3 (Example 4) groups together anionic surfactant type collector tests used in combination with anionic surfactants or with nonionic surfactants, always in a 1: 1 mass ratio for each collector.

In Tables 2 and 3 are shown:

the feed rates of the pulp expressed in empty drum speed Ja,

the flotation gas feed rates expressed in gas velocity in empty barrels Jg,

- the nature and concentration of the surfactant collector (s) added to the feed pulp,

- the point of introduction of the collector in the flotation column (pump: at the pump for recirculating the pulp in the column and in which the flotation gas is introduced, high CL: high column: between the foam interface -pulp and the feeding point of the pulp),

- the percentage of recovery of calcium carbonate and gypsum in the foam,

the selectivity S of the gypsum collected relative to the calcium carbonate, the mean diameter of the bubbles during the test.

The results in Table 2 show that:

- Without a surfactant collector, the high ionic strength of the solution already allows flotation a calcite / gypsum separation with non-solid yields.

negligible,

Among the collectors used, sodium oleate at concentrations of 0.8 to 11.8 ppm resulted in the best performance in terms of sulphate yields (from 76 to 94%) and selectivity of the separation (12.7%). 95.2). These variations in the concentration combined with the flow rates of gas and pulp make it possible to obtain either a very high sulphate removal rate (from 90 to 94%) for superficial gas velocities ranging from 0.54 to 0, 8 cm / s with flow constant feed rate (1.13 cm / s), a very high selectivity with an increase in the surface speed of the tested pulp up to 1.26 cm / s,

the other anionic collectors, although showing satisfactory performance, result for the sulphosuccinamate (Procol CA540) at a high sulphate yield of 90% but a lower selectivity: 4.7,

sodium dodecyl sulphate shows a slightly lower separation yield of the gypsum, 72-76%, and a selectivity lower than that obtained with sodium oleate,

- the consumption of these collectors, ranging from 5.8 to 11.8 ppm for

dodecyl sulphate, from 22 to 60 ppm for sulphosuccinamate is also greater than with sodium oleate whose optimal results are obtained with consumptions of between 0.8 and 3.0 ppm.

The addition of a non-ionic reagent (ISF12, number of carbon atoms = 12) in the column with sodium laurate promotes an increase in the yield of the sulphates without appreciably modifying the buoyancy of the carbonates by ensuring a selectivity which goes from 32.5 for laurate alone at 53.5 with the mixture.

The same nonionic reagent was tested with sodium oleate always in a ratio of 1: 1 for a total collector consumption ranging from 2.8 to 30 ppm. The results of separation show a good selectivity but the sulphate yields are low compared to the oleate used alone.

An increase in the chain length of the nonionic additive of 12 to 16 carbon atoms decreases the selectivity for a total consumption of 11 ppm. However a decrease in the consumption of the mixture of 11 to 3 ppm leads to the production of a float product consisting almost of sulphates. The yield of the carbonates is only 2.8% for a yield of sulphates of 55%.

Example 5 (non-compliant)

Various nonionic reagents such as iso-alcohols and Guerbet alcohols were tested, as well as a cationic collector, Cataflot from CECA.

(primary amine chain length C16-C18 in the form of a chloride salt forming a positive ion in aqueous solution) under conditions similar to Examples 3 and 4.

These surfactants gave poor results on the same granule mixtures and failed to separate the gypsum granules from the calcium carbonate granules. Table 1. Example 2 - Examples of Flotation Tests at Various Pulp Feed Rates:

Table 2. Example 3 - Flotation tests at various surfactant collectors, pulp and air supply rates, collector concentration in relation to the pulp, and various points of introduction of the collector (pump: at the of the recirculating pump of the pulp in the column in which the flotation gas is introduced, high CL: between foam-pulp interface and the feed point of the pulp)

Concentration Yield

collector recovery

diameter

Ja Jg (ppm) in select foam

Reactive bubbles

(cm / s) (cm / s) and place ()

(mm) introduction

C03 S04

manifold

Oleate 1,13 1,05 11,8 / pump 21 77 12,7 0,84

Oleate 1,13 1,14 2,8 / pump 11 78 28,9 0,86

Oleate 1,13 1,20 5,9 / pump 10 71 23,0 0,96

2.8 2.8 /

Oleate 1,13 1,07 13 81 30,2 0,77 pump

5.9 + 5.9 /

Oleate 1,13 1,00 19 94 64,6 0,70 pump

Oleate 1,13 0,80 3,0 / pump 6 76 48,4 0,78

Oleate 1,13 0,89 3,0 / high CL 16 94 82,5 0,80

Oleate 1,13 0,89 3,0 / high CL 14 93 76,3 0,81

Oleate 1,13 0,54 1,4 / high CL 13,90 62,7 0,56

Oleate 1,22 0,75 1,7 / high CL 16 87 33,5 /

Oleate 1,13 0.67 0.9 / high CL 8 84 57.5 0.60

Oleate 1.26 0.85 0.8 / tall CL 6 86 95.2 /

Oleate 0.92 0.87 1.1 / high CL 8 88 83.9 /

Sodium laurate 1,13 0,89 2,8 / pump 12 82 32,5 0,81

Dodecyl sulphate

1.13 0.82 5.6 / pump 15 76 18.3 0.66 sodium

Dodecyl sulphate

1.13 0.94 11.8 / pump 20 72 10.6 0.76 sodium

sulfosuccinamate

1.13 0.65 7.0 / pump 22 83 17.7 0.57 CA540

sulfosuccinamate

1.13 0.65 14.0 / pump 46 84 6.0 0.55 CA540

sulfosuccinamate

1.13 0.65 39.0 / pump 66 90 4.7 0.55 CA540 Table 3. Example 4 - Flotation tests at various air supply rates and with mixtures of surfactant collectors:

In the event that the disclosure of patents, patent applications and publications which are incorporated herein by reference would cause a conflict in the understanding of a term making it unclear, the present specification prevails.

Claims

A method of separating a suspension containing gypsum granules and calcium carbonate granules and an aqueous solution, characterized in that the aqueous solution comprises at least one sodium salt and that the aqueous solution has a concentration of sodium ion of at least about 3 g / L, preferably at least about 10 g / L, preferably at least about 30 g / L and that said method comprises a flotation separation step using a collector for granules of gypsum, the collector comprising at least one hetero-polar organic surfactant of formula RX, wherein:

2. Method according to claim 1 characterized in that the hetero-polar organic surfactant of formula RX is chosen from the group consisting of: a sodium or potassium oleate, a sodium or potassium alkyl sulphonate, an alkyl sulphate sodium or potassium, a sodium or potassium sulfosuccinamate, or a mixture thereof.

3. Method according to claims 1 or 2 characterized in that the hetero-polar organic surfactant of formula RX is a sodium or potassium oleate.

4. Method according to any one of the preceding claims, characterized in that the collector also comprises a nonionic surfactant such as an iso-alcohol or a Guerbet alcohol.

5. Method according to any one of the preceding claims, characterized in that the granules of calcium carbonate and the granules of gypsum have a granulometric distribution such that at least 90% by weight of the particles have a diameter of less than 150 μιη, preferably less than 130 μιη, preferably less than 110 μιη and more preferably less than 90 μηι.

6. Method according to any one of the preceding claims characterized in that the amount of granules of gypsum or granules of calcium carbonate reported per unit volume of suspension is at least 10 kg / m 3, preferably from less than 15 kg / m 3.

7. Method according to any one of the preceding claims characterized in that the weight ratio between granules of gypsum and calcium carbonate granules of the suspension is between 1: 3 and 3: 1,

advantageously between 1: 2 and 2: 1.

8. Method according to any one of the preceding claims characterized in that the suspension has a pH of at least 5, preferably at least 7, preferably at least 7.5, more preferably at least 8.

9. Method according to any one of the preceding claims characterized in that the suspension has a pH of at most 13, more preferably at most 11, more preferably at most 10.

10. Method according to any one of the preceding claims characterized in that the aqueous solution further comprises at least one calcium salt, and in that the aqueous solution has a calcium ion concentration of at least about 0.5 g / L, preferably at least about 5 g / L,

preferably at least about 10 g / L.

11. Method according to any one of the preceding claims, characterized in that the flotation separation step is carried out with a flotation cell of mechanical stirring type or of pneumatic type such as a flotation column, and in that the speed superficial average gas in the flotation cell is at least 0.1, preferably at least 0.2, preferably at least 0.5 cm / s.

12. Method according to claim 11, characterized in that the average surface velocity of gas in the flotation cell is at most 5.0, more preferably at most 3.0, still more advantageously at most 1.7 cm / s, and most preferably is at least 0.5 and not more than 1.5 cm / s.

13. Method according to any one of the preceding claims, characterized in that the suspension containing granules of gypsum and granules of calcium carbonate an aqueous solution is obtained by:

b) carbonation of all or part of the milk of lime with carbon dioxide.

14. Method according to any one of the preceding claims, characterized in that the suspension containing gypsum granules and calcium carbonate granules and an aqueous solution is obtained

by reacting a milk of lime comprising at least one sodium salt, or a mixture of lime milk and a calcium carbonate broth comprising at least one sodium salt, with acid fumes resulting from the combustion of a sulfur-containing organic compound such as a coal, a lignite, a wood, an agricultural residue, an organic residue from the food industry, an organic residue from the paper industry, a sludge from a drinking water or wastewater treatment plant, an organic sludge from a biogas plant.

15. Use of granules of calcium carbonate or gypsum granules derived from a method according to any one of the preceding claims in cement works, or in civil engineering, or in road engineering, or for the manufacture of construction materials, or the manufacture of panels or plasterboard, or the manufacture of road aggregates, or the manufacture of backfill material to fill underground cavities.