Process for manufacture of purified alkaline earth metal carbonate
The invention concerns a process for the manufacture of a purified alkaline
earth metal carbonate, the purified alkaline earth metal carbonate obtainable by
said process, and its use in the manufacture of products and devices in the field
of electronics and glass.
5 High purity alkaline earth metal carbonates, such as SiC03 and BaC03,
are of high importance in the application of the alkaline earth metal carbonates in the electronic and glass industry. Especially, reduction of the total sulfur content in alkaline earth metal carbonate is a concern as the use of such carbonates in the field of glass and electronics, such as batteries, thermistors, capacitors, ceramic 10 electrical components and radio components, require low sulfur contents. Other applications of alkaline earth metal carbonates, for example ceramics, pigments, rubber, paints or welding rods also benefit from low sulfur contents. Sulfur content is usually high in alkaline earth metal carbonates, as the carbonates are typically produced via reduction of the alkaline earth metal sulfate to sulfide and
15 subsequent carbonation to the alkaline earth metal carbonate. The contamination of the alkaline earth metal carbonate product with sulfur containing impurities is inherent in this process. Other process steps or alternative processes can also lead to contamination with sulfur containing impurities. It is further desirable to reduce the amount of other impurities from alkaline earth metal carbonates, such
20 as sodium containing impurities.
JP1031673 describes the addition of a basic compound to the reaction of BaS with C02, heating the resulting BaC03 in an oxygen-free gas stream, and subsequent washing and drying. The resulting product has a sulfur content of 170 ppm.
25 It is an object of the invention to provide an improved process for the
purification of alkaline earth metal carbonate, specifically to reduce the sulfur content of the alkaline earth metal carbonate. Other impurities can also be reduced by the process. The process according to the present invention thus comprises the steps of
30 a) Calcinating said alkaline earth metal carbonate
b) washing the calcinated alkaline earth metal carbonate with an aqueous phase comprising a salt.
The invention further concerns a purified alkaline earth metal carbonate, obtainable the process according to the present invention, and its use in the
35 manufacture of products and devices in the electronics and glass industry.

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It should be noted that "reducing the sulfur content" or "reducing impurities" does not imply that the underlying chemical concept of the present invention is restricted to or solely based on a chemical reduction, meaning a reduction of the oxidation state, of such an impurity. In this context, "reducing" 5 or "reduction" rather means that the purified alkaline earth metal carbonate
contains a lower amount of impurities, in particular sulfuric impurities, than the alkaline earth metal carbonate provided to the process; the content of said impurities is reduced. This does, on the other hand, not exclude that chemical reduction processes can be involved in the impurity content reduction according
10 to the present invention.
In the present invention, alkaline earth metal carbonate intends to denote a compound of the formula MCO3, wherein M is a metal of the second main group of the chemical periodic system, also known as alkaline earth metals. Preferred alkaline earth metal carbonates according to this invention are BaC03 and
15 SrCOj. Mixtures of two or more earth metal carbonates are also included in term "alkaline earth metal carbonate".
In a preferred embodiment of the present invention, the alkaline earth metal carbonate, or a mixture of two or more alkaline earth metal carbonates, being provided to the process step a) according to the invention, is manufactured
20 from MS, which is often manufactured by reduction of MSO4.
In the present invention, the "sulfur content" is expressed in ppm. It is measured by oxidation of all sulfur impurities in an alkaline earth metal carbonate sample by addition of an aqueous bromine solution, addition of HCi and subsequent retrieval and gravimetrical analysis of the insoluble MS04 which
25 is formed. The content MS04 in the sample is expressed as wt. %, and the total sulfur content can be calculated from this amount by multiplying with factor 0.1373. The wt. % may also be expressed as ppm, wherein 1 wt, % is 10,000 ppm. The sulfur content can be suitably determined according to the following procedure:
30 About 25 g MC03 was weighed, exact to 4 digits after the comma, into a
600 mL beaker. 250 ml distilled water, filter paper flakes and a round of filter paper were added. 100 mL of a saturated solution of bromine in water was added, followed by cautious addition of 30 mL of cone. HCI. Distilled water was added to reach a total volume of 400 mL. The beaker was heated, thereby
35 reducing the volume of the solution to 100-150 mL. The mixture was filtered via filter paper, and the residue washed with hot distilled water until the filtrate was

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chloride-free. The filter paper was burned in a tared Pt crucible at 800~900°C in
a furnace until constant weight was reached. The weight difference vs. tared,
empty crucible Js the content of MS04 and is used to calculate the sulfur content
of the weighed MCO3 sample,
5 Other cationic impurities, such as Na+, Li+, Ni~'\ Mn2+, Fe3+, Al'4', etc., can
be suitably determined via ICP-OE-spectrometry after digestion of the sample MC03 with aq. HCi.
Generally, the alkaline earth metal carbonate which is provided to the process in a step a) contains impurities, in particular sulfuric impurities. The
10 total sulfur content of the alkaline earth metal carbonate which is provided to step a), or the content in other impurities, can have a wide range of values. In one aspect of the invention, the alkaline earth metal carbonate, provided in step a) of the process according to the present invention, generally contains a total sulfur content A. A is expressed in ppm. In this aspect, A generally has a
15 value of from 200 to 1700 ppm. Often, A is equal to or higher than 200 ppm. Preferably, A is equal to or higher than 250 ppm. Even more preferably, A is equal to or higher than 270 ppm. Most preferably, A is equal to or higher than 300 ppm. Generally, A is equal to or lower than 1700 ppm. More often, A is equal to or lower than 1650 ppm. Preferably, A is equal to or lower than 1600
20 ppm. More preferably, A is equal to or lower than 1550 ppm. Most preferably, A is equal to or lower than 1500 ppm.
Generally, the particle size of the alkaline earth metal carbonate which is provided to step a) can have a wide range of values. In one embodiment of the present invention, the alkaline earth metal carbonate which is provided to step a)
25 of the process, has a specific particle size. Often, the particles of the alkaline
earth metal carbonate, provided to step a) of the process according to the present invention, have a D90 value of from 12 to 30 um, Often, the D90 value is equal to or higher than 12 um. Preferably, the D90 value is equal to or higher than 13 um. Even more preferably, the D90 value is equal to or higher than 14 um.
30 Most preferably, the D90 value is equal to or higher than 15 um. Generally, the D90 value is equal to or lower than 30 um. More often, the D90 value is equal to or lower than 29 um. Preferably, the D90 value is value is equal to or lower than 28 jam. More preferably, the D90 value is equal to or lower than 27 um. Most preferably, the D90 value is equal to or lower than 26 um. The D90 value
35 determines that 90 % of all particles have a particle size of equal to or lower than the D90 value. Often, the particles of the alkaline earth metal carbonate,

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provided to step a) of the process according to the present invention, have a D50 value of from 2,5 to 9 urn. Often, the D50 value is equal to or higher than 2,5 j.im. Preferably, the D50 value is equal to or higher than 3 um. Even more preferably, the D50 value is equal to or higher than 3,5 um. Most preferably, the 5 D50 value is equal to or higher than 4 j-im. Generally, the D50 value is equal to or lower than 9 um. More often, the D50 value is equal to or lower than 8,5 um. Preferably, the D50 value is equal to or lower than 8 um. More preferably, the D50 value is equal to or lower than 7,5 um. Most preferably, the D50 value is equal to or lower than 7 um. The D50 value determines that 50 % of all particles
10 have a particle size of equal to or lower than the D50. The particle size is suitably determined by laser diffraction.
Generally, the BET value of the alkaline earth metal carbonate which is provided to step a) can have a wide range of values. In one embodiment of the present invention, the alkaline earth metal carbonate which is provided to step a)
15 has a BET value of from 1 to 10 g/rrT. The BET value describes the specific surface area of the particle, and is measured by the physical adsorption of gas molecules on the surface of the particle. The BET is suitably measured by N2 isothermic adsorption according to the method of Brenauer-Emmet-Teller (BET).
20 According to the process of the present invention, an alkaline earth metal
carbonate is provided to the process in a step a) and the alkaline earth metal carbonate is calcinated. "Calcination" is intended to denote a thermal treatment process, which is preferably performed in the presence of a gas comprising oxygen. In another aspect of the invention, the calcination is carried out in an
25 inert atmosphere free of oxygen. Calcination is carried out in suitable reactors, such as furnaces or reactors, sometimes referred to as kilns or calciners, of various designs including shaft furnaces, rotary kilns, multiple health furnaces, and fluidized bed reactors. The calcination may be carried out continuously or batchwise. The calcination temperature is chosen such that the carbonate does
30 not decompose. Generally, the calcination temperature is from 150 degrees
Celsius to 1000 degrees Celsius. Generally, the calcination temperature is equal to or more than 150 degrees Celsius. Preferably, the calcination temperature is equal to or more than 300 degrees Celsius. Most preferably, the calcination temperature is equal to or more than 500 degrees Celsius, Often, the calcination
35 temperature is equal to or lower than 1000 degrees Celsius. Preferably, the calcination temperature is equal to or lower than 950 degrees Celsius. Most

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preferably, the calcination temperature is equal to or lower than 900 degrees Celsius. Generally, the calcination temperature is held for from 30 minutes to 3 hours. Often, the calcination temperature is held for equal to or more than 30 minutes. Preferably, the calcination temperature is held for equal to or more than 5 45 minutes. Most preferably, the calcination temperature is held for equal to or more than 1 hour. Often, the calcination temperature is held for equal to or less than 3 hours. Preferably, the calcination temperature is held for equal to or less than 2,5 hours. Most preferably, the calcination temperature is held for equal to or less than 2 hours.
10 The calcination process is preferably performed under ambient pressure.
Calcination at elevated pressure or reduced pressure (vacuum) is also possible.
The calcination product has generally a granular character, intending to denote that it forms larger particles in the calcination process compared to the particles provided to the calcination step. Without wishing to be bound by any
15 particular theory, it is believed that sulfuric impurities are converted during calcination into sulfuric impurities which are soluble in the aqueous metal salt phase used in step b), and further migrate to the surface of the calcinated particles during the calcination operation, thereby becoming accessible to a washing operation in step b). It has been surprisingly found that washing the
20 calcinated product with an aqueous phase containing one or more metals salt significantly reduces the total sulfur content of the calcination product. Other impurities can also be reduced. This significant effect provides substantially better results compared to washing uncalcinated alkaline earth metal carbonates. Generally, the particle size of the alkaline earth metal carbonate which is
25 provided to step b), which is the calcination product, can have a wide range of values. In one embodiment of the present invention, the alkaline earth metal carbonate which is provided to step b) of the process, has a specific particle size. Often, the particles of the alkaline earth metal carbonate, provided to step b) of the process according to the present invention, have a D90 value of from
30 700 to 1100 urn. Often, the D90 value is equal to or higher than 725 urn. Preferably, the D90 value is equal to or higher than 750 um. Even more preferably, the D90 value is equal to or higher than 775 um. Most preferably, the D90 value is equal to or higher than 800 urn. Generally, the D90 value is equal to or lower than 1100 pm. More often, the D90 value is equal to or lower
35 than 1075 um. Preferably, the D90 value is equal to or lower than 1050 um. More preferably, the D90 value is equal to or lower than 1025 urn. Most

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preferably, the 1390 value is equal to or lower than 1000 um. The D90 value determines that 90 % of all particles have a particle size of equal to or lower than the D90. Often, the particles of the alkaline earth metal carbonate, provided to step b) of the process according to the present invention, have a D50 value of 5 from 350 to 750 um. Often, the D50 value is equal to or higher than 350 um. Preferably, the D50 value is equal to or higher than 360 um. Even more preferably, the D50 value is equal to or higher than 370 um. Most preferably, the O50 value is equal to or higher than 380 um. Generally, the D50 value is equal to or lower than 750 um. More often, the D50 value is equal to or lower
10 than 740 um. Preferably, the D50 value is equal to or lower than 730 urn. More preferably, the D50 value is equal to or lower than 720 um. Most preferably, the D50 value is equal to or lower than 710 um. The D50 value determines that 50 % of all particles have a particle size of equal to or lower than the D50. The particle size suitably is determined by laser diffraction.
15 Preferably, the calcination is performed on alkaline earth metal carbonate
which has not been dried after an optional previous process step. Often, this previous process step is a precipitation of the alkaline earth carbonate from a MS solution. Independent from the chemical nature of the optional previous process step, in which MCO3 is produced, generally the alkaline earth metal carbonate
20 has a water content of from 10 to 60 wt. % before the calcination step. Often, the water content of the alkaline earth metal carbonate before the calcination step is equal to or higher than 12 wt. %. Preferably, the water content of the alkaline earth metal carbonate before the calcination step is equal to or higher than 10 wt. %. Most preferably, the water content of the alkaline earth metal
25 carbonate before the calcination step is equal to or higher than 15 wt. %. Often, the water content of the alkaline earth metal carbonate before the calcination step is equal to or lower than 60 wt. %. Preferably, the water content of the alkaline earth metal carbonate before the calcination step is equal to or lower than 55 wt. %. More preferably, the water content of the alkaline earth metal
30 carbonate before the calcination step is equal to or lower than 52 wt. %. Most preferably, the water content of the alkaline earth metal carbonate before the calcination step is equal to or lower than 50 wt. %. hi another aspect of the present invention, water can be added to dry alkaline earth metal carbonate before the calcination operation in order to achieve the abovementioned water
35 content. In a further aspect of the invention, a binding agent can be added to the alkaline earth carbonate, for example as described in US5366513, which is

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incorporated hereby by reference in its entirety, to improve the granulation effect in the calcination operation.
In one aspect of the present invention, the alkaline earth metal carbonate is cooled after the calcination step. The cooling can be performed either actively, 5 for example by cooling the calcination apparatus, or passively, by removal of the heat source and leaving to coo! the product in the calcination apparatus. Often, the calcination product is cooled to ambient temperature after calcination. Preferably, the calcination product is cooled to a temperature of equal to or higher than 5°C. More preferably, the calcination product is cooled to a
10 temperature of equal to or higher than 10°C, Most preferably, the calcination product is cooled to a temperature of equal to or higher than 3 5°C. Preferably, the calcination product is cooled to a temperature of equal to or lower than 95°C. More preferably, the calcination product is cooled to a temperature of equal to or higher than 80°C. Most preferably, the calcination product is cooled to a
15 temperature of equal to or higher than 65°C.
In the process according to the present invention, the calcinated and optionally cooled alkaline earth metal carbonate is washed in step b) with an aqueous phase containing at least one metal salt. Generally, the at least one metal salt contained in the aqueous phase used in step b) is selected such that the
20 sulfate of the cation contained in the metal salt has a higher solubility in water than the sulfate of the alkaline earth metal cation contained in the one or more alkaline earth metal carbonate. In this context, "higher solubility" intends to denote that the sulfate or sulfates of the cation in the one or more metal salts has a higher solubility in g/lOOg water at 25°C than the sulfate or sulfates of the one
25 or more alkaline earth metal cation contained in the one or more alkaline earth metai carbonate. Preferably, the cation of the metal salt contained in the aqueous phase of step b) is selected from the group of alkaline metals, which is the group consisting of Na-t-, K+, Li+, Cs-i-, Rb+. Na+ and Li+ are preferred cations, Nan-is the most preferred cation. The anion of the metal salt contained in the aqueous
30 phase in step b) is generally selected from the group consisting of carbonate, chloride, bromide, phosphate and citrate, wherein carbonate is preferred. The most preferred metal salt contained in the aqueous phase in step b) is Na2C03. Mixtures of one or more metal salts are also denoted by the term "metal salt". The aqueous phase of step b) has a content of metal salt which is related to
35 the sulfur content of the calcinated alkaline earth metal carbonate. Generally, the aqueous phase of step b) contains at least one stoichiometric equivalent of metal

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salt. By way of illustration, stoichiometric equivalent denotes one mol metal salt cation, if the cation is divalent, or two moles metal salt cation, if the cation is monovalent, per theoretical mol MSO.4 present in the sample as measured by gravimetrical analysis as explained above. Preferably, the metal salt content in 5 the aqueous phase in step b) is equal to or more than 3 stoichiometric
equivalents. Most preferably, the metal salt content in the aqueous phase in step b) is equal to or more than 6 stoichiometric equivalents. Often, the metal salt content in the aqueous phase in step b) is equal to or lower than 30 stoichiometric equivalents. Preferably, the metal salt content in the aqueous
10 phase in step b) is equal to or lower than 25 stoichiometric equivalents. Most preferably, the metal salt content in the aqueous phase in step b) is equal to or lower than 21 stoichiometric equivalents. In a most preferred aspect, the metal salt content in the aqueous phase in step b) is from 10 to 20 stoichiometric equivalents.
15 In a batch process in which step b) is performed on a batch of MC03 by
stirring the MC03 with the aqueous phase, the volume of the aqueous phase in step b) generally is equal to or larger than one volume equivalent of the bulk volume of the batch MC03. The volume of the aqueous phase is mainly determined by the stirrability of the batch one the aqueous phase has been added.
20 The volume of aqueous phase preferably is equal to or larger than 1,5 volume equivalents of the bulk volume of the batch MC03. The volume of aqueous phase often is equal to or smaller than 8 volume equivalents of the bulk volume of the batch MC03. Preferably, the volume of aqueous phase often is equal to or smaller than 5 volume equivalents of the bulk volume of the batch MC03. More
25 preferably, the volume of aqueous phase often is equal to or smaller than 3 volume equivalents of the bulk volume of the batch MC03.
The temperature of the aqueous phase in step b) generally is from 0°C to X°C, wherein X denotes the boiling point of the aqueous wash solution containing alkaline metal salt. Often, the temperature of the aqueous phase
30 and/or the temperature maintained after addition of the phase in step b) is equal to or higher than 5°C. More preferably, the temperature of the aqueous phase and/or the temperature maintained after addition of the aqueous phase in step b) is equal to or higher than 10°C. Most preferably, the temperature of the aqueous phase and/or the temperature maintained after addition of the aqueous phase in
35 step b) is equal to or higher than 15°C. Generally, the temperature of the
aqueous phase and/or the temperature maintained after addition of the aqueous

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phase in step b) is equal to or lower than X°C. More preferably, the temperature of the aqueous phase and/or the temperature maintained after addition of the aqueous phase in step b) is equal to or lower than (X-5)°C. Most preferably, the temperature of the aqueous phase and/or the temperature maintained after 5 addition of the aqueous phase in step b) is equal to or lower than (X-10)°C.
In one embodiment of the invention, step b) is carried out by addition of the aqueous phase to the calcinated alkaline earth metal carbonate, or, alternatively, by addition of the calcinated alkaline earth metal carbonate to the aqueous phase. Stirring is applied during and/or after addition, The stirring time 10 often is from 30 seconds to 6 hours. Preferably, the stirring time is equal to or longer than 1 minute. More preferably, the stirring time is equal to or longer than 5 minutes. Most preferably, the stirring time is equal to or longer than 10 minutes. Preferably, the stirring time is equal, to or shorter than 5 hours. More preferably, the stirring time is equal to or shorter than 3 hours. Most preferably,
15 the stirring time is equal to or shorter than 1 hour. In another aspect of the
present invention, step b) can be carried out by rinsing the calcinated alkaline earth metal carbonate, for example by placing the alkaline earth metal carbonate on a filter or in another solid/liquid separation apparatus, and adding the aqueous phase to the alkaline earth metal carbonate, separating the spent washing solution
20 from the alkaline earth metal carbonate after passing through the bed of alkaline earth metal carbonate. In yet another aspect of the invention, the suspension of the calcinated alkaline earth metal carbonate in the aqueous phase is stirred, then filtered and the bed of alkaline earth metal carbonate rinsed. One or more of step b) are optionally performed subsequently, advantageously separating the spent
25 aqueous phase from the alkaline earth metal carbonate between steps.
Optionally, the alkaline earth metal carbonate is washed with water between repeated steps b). When step b) is repeated one or more times, the alkaline earth metal carbonate optionally is submitted to a drying step, such as applying heat, vacuum of an air stream, between repeated steps b), although this is not
30 preferred. Step b) or multiple steps b) can be carried out batch-wise or continuously.
After completed washing of the alkaline earth metal carbonate in step b), the spent aqueous phase is suitably separated from the earth alkaline metal. Suitable methods for separating the spent aqueous phase include, for example,
35 filtering, spinning or decanting.

-10-In one preferred embodiment of the invention, the alkaline earth metal carbonate is washed with an aqueous washing agent after step b). Preferably, the aqueous washing agent is water. The term "water" intends to denote water of any quality (e.g. deionized water, purified water, distilled water, double-distilled 5 water, filtered water, water from industrial processes or also municipal water, tap water, hard water, soft water). In another aspect of the invention, the alkaline earth metal carbonate of step b) is washed after step b) with an aqueous washing agent containing a salt. Often, the temperature of the water or aqueous phase and/or the temperature maintained after addition of the water or aqueous washing
10 agent is equal to or higher than 5°C. More preferably, the temperature of the aqueous washing agent or the temperature maintained after addition of the aqueous washing agent is equal to or higher than 10°C. Most preferably, the temperature of the aqueous washing agent and/or the temperature maintained after addition of the aqueous washing agent is equal to or higher than 15°C.
15 Generally, the temperature of the aqueous washing agent and/or the temperature maintained after addition of the aqueous washing agent is equal to or lower than Y°C, wherein Y is the boiling temperature of the aqueous washing agent, e.g. Y=100 when water is used as aqueous washing agent. More preferably, the temperature of the aqueous washing agent and/or the temperature maintained
20 after addition of the aqueous washing agent is equal to or lower than (Y-2)°C. Most preferably, the temperature o^ the wales- or aqueous phase and/or the temperature maintained after addition of the aqueous washing agent is equal to or lower than (Y-10)°C. It is preferred that the washing step using the aqueous washing agent is performed in the same reactor as step b) or the liquid/solid
25 separation apparatus used to separate the spent aqueous phase containing a metal salt from the alkaline earth metal carbonate after step b). Optionally, the alkaline earth metal carbonate can also be transferred to a suitable reactor for washing with an aqueous washing agent after step b). The aqueous washing agent can be separated from the alkaline earth metal carbonate after completed washing by
30 suitable techniques selected from the group consisting of filtration, spinning and decanting. One or more steps b), separating the aqueous phase of step b) from the alkaline earth metal carbonate and washing the alkaline earth metal carbonate with an aqueous washing agent can be combined in any suitable order. If one or more steps b), separating the aqueous phase of step b) from the alkaline earth
35 metal carbonate and washing the alkaline earth metal carbonate with an aqueous washing agent are combined, the alkaline earth metal carbonate is preferably not

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dried between the steps, although a partial or complete drying can be performed if desired. It has surprisingly been shown that the amount of cationic impurities, e.g. from the metal salt contained in the aqueous phase in step b), in particular in the case of the use of soda ash in the aqueous washing solution in step b), is not 5 significantly higher, and may even be reduced, especially if water is used in a washing step after step b). In the case when Na2CO;i is contained in the aqueous phase in step b), the final sodium content of the alkaline earth metal carbonate is often equal to or less than 150 ppm. Preferably, the final sodium content of the alkaline earth metal carbonate is equal to or less than 120 ppm. Most preferably,
10 the final sodium content of the alkaline earth metal carbonate is equal to or less than 100 ppm.
In one preferred embodiment of the invention, the alkaline earth metal carbonate is dried after step b) after the separation of the aqueous phase from the alkaline earth metal carbonate or after the separation of the aqueous washing
15 agent of a washing step from the alkaline earth metal carbonate subsequent to step b). Drying of the purified alkaline earth metal carbonate can be performed by methods known to the person skilled in the art, for example, by selecting one 03- more techniques from the group consisting of heating, applying vacuum, or blowing a gas through or on the alkaline earth metal carbonate. A preferred
20 drying procedure comprises heating the purified alkaline earth metal carbonate in a rotary kiln. Often, the drying temperature, independent of the apparatus, is from 40°C to 300°C. Generally, the drying temperature is equal to or higher than 45°C. More preferably, the drying temperature is equal to or higher than 60°C. Most preferably, the drying temperature is equal to or higher than 80°C.
25 Generally, the drying temperature is equal to or lower than 800°C. Preferably, the drying temperature is equal to or lower than 500°C. More preferably, the drying temperature is equal to or lower than 230°C. Most preferably, the drying temperature is equal to or lower than 200°C, The residual moisture content can be adjusted to a desirable level by choosing the appropriate drying conditions,
30 such as temperature and time. Generally the alkaline earth metal carbonate after drying has a water content of from 10 to 1000 ppm. Preferably, the water content of the alkaline earth metal carbonate after drying is equal to or higher than 10 ppm. Even more preferably, the water content of the alkaline earth metal carbonate after drying is equal to or higher than 15 ppm. Most preferably, the
35 water content of the alkaline earth metal carbonate after drying is equal to or higher than 20 ppm. Preferably, the water content of the alkaline earth metal

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carbonate after drying is equal to or lower than 1000 ppm. Even more
preferably, the water content of the alkaline earth metal carbonate after drying is
equal to or lower than 950 ppm. Most preferably, the water content of the
alkaline earth metal carbonate after drying is equal to or lower than 900 ppm.
5 In one embodiment of the invention, the alkaline earth metal carbonate
obtained by the drying step can be further granulated thermally in a separate granulation process by thermal treatment at temperatures of from 60°C to 850°C.
In another embodiment of the present invention, granulation auxiliaries, such as NaOIl or sodium silicate, are added to the alkaline earth metal carbonate 10 during drying and/or subsequent separate granulation.
The invention further concerns a purified alkaline earth metal carbonate, obtainable by the following process:
a) Calcinating an alkaline earth metal carbonate
b) washing the calcinated alkaline earth metal carbonate with an aqueous phase 15 comprising a salt.
Other aspects of the process have been described above. The purified alkaline earth metal carbonate, which is obtainable by the process according to the present invention, displays a low content of impurities, in particular in total sulfur. Low sulfur content in alkaline earth metal carbonate
20 is desirable, especially in the glass and electronics field, where alkaline earth metal carbonates are used. It has so far not been possible to reduce the total sulfur content to very low amounts as has been shown surprisingly according to the present invention, especially if the alkaline earth metal carbonate is manufactured from MS or in other processes involving sulfur containing
25 intermediates or starling material. In one aspect, the alkaline earth metal carbonate obtainable by the process according to the present invention is manufactured from MS by reaction with C02 or an alkaline earth metal carbonate. The MS often is manufactured by reduction of MS04. Generally, the total sulfur content of the purified alkaline earth metal carbonate is from 0.1 ppm
30 to 200 ppm. Often, the final total sulfur content of the purified alkaline earth metal carbonate is equal to or less than 200 ppm. Preferably, the final total sulfur content is equal to or less than 150 ppm. More preferably, the final total sulfur content is equal to or less than 100 ppm. Most preferably, the total sulfur content is equal to or less than 80 ppm. Often, the final total sulfur content of the
35 purified alkaline earth metal carbonate is equal to or higher than 0.1 ppm.
Preferably, the final total sulfur content is equal to or higher than 1 ppm. More

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preferably, the final total sulfur content is equal to or higher than 2 ppm. Most preferably, the total sulfur content is equal to or higher than 3 ppm.
Generally, the particle size of the purified alkaline earth metal carbonate can have a wide range of values. In one embodiment of the present invention, 5 the purified alkaline earth metal carbonate has a specific particle size.
Often, the purified alkaline earth metal carbonate has a D90 value of from 700 to 11.00 urn. Often, the D90 value is equal to or higher than 725 um. Preferably, the D90 value is equal to or higher than 750 um. Even more preferably, the D90 value is equal to or higher than 775 um, Most preferably,
10 the D90 value is equal to or higher than 800 um. Generally, the D90 value is
equal to or lower than 1100 um. More often, the D90 value is equal to or lower than 1075 u m. Preferably, the D90 value is equal to or lower than 1050 um. More preferably, the D90 value is equal to or lower than 1025 um. Most preferably, the D90 value is equal to or lower than 1000 um. The D90 value
15 determines that 90 % of all particles have a particle size of equal to or lower than the D90. Often, the purified alkaline earth metal carbonate has a D50 value of from 350 to 750 um. Often, the D50 value is equal to or higher than 350 um. Preferably, the D50 value is equal to or higher than 360 ,um. Even more preferably, the D50 value is equal to or higher than 370 um. Most preferably,
20 the D50 value is equal to or higher than 380 um. Generally, the D50 value is equal to or lower than 750 um. More often, the D50 value is equal to or lower than 740 um. Preferably, the D50 value is equal to or lower than 730 um. More preferably, the D50 value is equal to or lower than 720 um. Most preferably, the D50 value is equal to or lower than 710 um. The D50 value determines that
25 50 % of all particles have a particle size of equal to or lower than the D50. The particle size suitably is determined by laser diffraction. Most preferred is a purified alkaline earth metal carbonate which has a D90 of from, 800 to 1000 um and a D50 of from 500 to 700 um.
Generally, the BET value of the purified alkaline earth metal carbonate can
30 have a wide range of values. In one embodiment of the present invention, the purified alkaline earth metal carbonate has a BET value of <1 g/m2. The BET value describes the specific surface area of the particle, and is measured by the physical adsorption of gas molecules on the surface of the particle. The BET is suitably measured by N2 isothermic adsorption according to the method of
35 Brenauer-Emmet--Tellcr (BET).

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The invention further concerns the use of a purified alkaline earth metal carbonate, obtainable by the process according to the invention, as additive in the glass oi* ceramics production and electronics industry. Glass which is manufactured with purified alkaline earth metal carbonate, obtainable by the 5 process according to the invention, as additive, is particularly suited for
television production, OLED (organic light-emitting diode) and LCD (liquid crystal displays) devices. The principles of glass manufacturing are described, for example, in US5395806 or Beerkens, R. G. C, Nijnatten, P. A. and Le Bourhis, E. 201 1. Glass, 2. Production. Ullmann's Encyclopedia of Industrial
10 Chemistry. Often, the one or more MC03 obtainable by the process according to the invention is added to the glass melt or mixed with the compounds to be melted in the glass production step. For example, Ceramics which are manufactured with purified alkaline earth metal carbonate, obtainable by the process according to the invention, as additive, are particularly suited for
15 manufacturing electronic parts, for example capacitors or thermistors.
US2009213527, for example, describes a ceramic chip capacitor which is manucfatured using BaC03 as additive. Generally, in the ceramics production, the one or more purified alkaline earth metal carbonate, obtainable by the process according to the invention, is mixed with other ceramics precursors and
20 heated to form the ceramic material. The purified alkaline earth metal carbonate, obtainable by the process according to the invention, can also be used in the manufacture of Iuminophores for LED (Light Emitting Diode) production. DE102010030473 and EP2129740B1, for example, describe the use of BaC03 and SrC03 as additives in the LED luminophore production. Often, the one or
25 more MC03 obtainable by the process according to the invention is mixed and heated to form the luminophore material or precursor. All foregoing publications are hereby incorporated by reference. Example : Calcination of alkaline earth metal carbonate :
30 C02 was passed through a solution of MS, obtained by reduction of the
corresponding sulfate. The resulting MC03 was filtered, washed with water and the washing water was separated from MC03. The recovered MC03 was heated in a rotary kiln in an atmosphere containing oxygen to about 800°C. The calcination product was cooled to ambient temperature.
35 Washing of the calcinated alkaline earth metal carbonate :

- 15-
A 100 g sample of product was inserted into 500 ml of a solution of soda containing 10-20 stoichiometric equivalents calculated on the sulfur content of the calcinated MC03 in a beaker glass. The suspension was stirred at 25°C and ~ 250 rpm for 15 minutes and then filtrated by suction filter, The residue was 5 washed three times with the threefold volume (compared to the MC03 volume)
of hot pure water and dried at 130 °C in a drying cabinet.
Washing of the calcinated alkaline earth metal carbonate at different temperatures:
10 A 100 g sample of product was inserted into 500 mL of a solution of soda
containing 10-20 stoichiometric equivalents calculated on the sulfur content of the calcinated MC03 in a beaker glass. The suspension was stirred at indicated temperature and ~ 250 rpm for 15 minutes and then filtered by suction filter. The residue was washed three times with the threefold volume (compared to the
15 MC03 volume) of hot pure water and dried at 160 °C in a drying cabinet.
Initial Sulfur content calcinated BaC03: 275 ppm
Final .Sulfur content calcinated BaC03, after washing at 20° and drying: 140 ppm
Final Sulfur content calcinated BaC03, after washing at 90° and drying: 74 20 ppm

- 16-CLAIMS
1. Process for the manufacture of a purified alkaline earth carbonate,
comprising the steps of
a) calcinating an alkaline earth metal carbonate
5 b) washing the calcinated alkaline earth metal carbonate with an aqueous phase comprising a salt.
2. The process of claim 1, wherein the alkaline earth metal carbonate is
BaC()3 and/or SrC03.
3. The process according to claim 1 or 2, wherein the calcination
10 procedure of step a) is performed at a temperature of from 150 to 1000°C.
4. The process according to anyone of claims 1 to 4, wherein the salt
comprised in the aqueous phase of step b) is a carbonate, chloride, bromide,
phosphate or citrate, preferably a carbonate,
5. The process according to claim 4, wherein the salt is an alkaline metal
15 carbonate, preferably sodium carbonate.
6. The process according to claim 1 to 6, wherein the content of salt in
the aqueous phase of step b) corresponds to from 1 to 200 stoichiometric
equivalents, preferably from 10-20 stoichiometric equivalents, calculated on the
basis of total sulfur content of the calcination product of step a).
20 7. The process according to any of claims i to 6, wherein step b) is
repeated.
8. The process according to anyone of claim 1 to 7, wherein the alkaline earth metal carbonate provided to the calcination procedure of step a) has a total sulfur content of from 200 to 1700 ppm.
25 9. The process according to anyone of claim 1 to 8, wherein the alkaline
earth metal carbonate after step b) has a total sulfur content of from 0.1 to 200 ppm.

- 17-
10. The process according to anyone of claim 1 to 9, wherein the alkaline earth metal carbonate provided to the calcination procedure of step a) has a particle size D90 value of from 12 to 30 urn and a has a particle size D50 value of from 2,5 to 9 um.
5 11. The process according to anyone of claim 1 to 9, wherein the alkaline
earth metal carbonate of step a) has a particle size D90 value of from 700 to 1100 pm and a particle size D50 value of from 350 to 750 um after calcination.
12. Alkaline earth metal carbonate, obtainable by a process comprising the
steps
10 a) calcinating an alkaline earth metal carbonate
b) washing the calcinated alkaline earth metal carbonate with an aqueous phase comprising a salt.
13. Alkaline earth metal carbonate according to claim 12, wherein the
alkaline earth metal carbonate has a total sulfur content of from 0.1 to 200 ppm.
15 14. Alkaline earth metal carbonate according to claim 12 or 13, wherein /•>
the alkaline earth metal carbonate has a particle size D90 value of from 700 to
1100 um and a particle size D50 value of from 350 to 750 um.
15. The use of an alkaline earth metal salt according to any one of claims 12 to 1.4, in the manufacture of glass, ceramics, products and/or devices in 20 the field of electronics, in particular organic light-emitting diodes (OLED), light emitting diodes (LED), liquid crystal displays (LCD), ceramic capacitors or television screens.