WO2022123546A1

WO2022123546A1 - Process for preparing magnesium sulphate, magnesium sulphate obtained by said process, and use of said magnesium sulphate as a fertilizer

Info

Publication number: WO2022123546A1
Application number: PCT/IB2021/061649
Authority: WO
Inventors: Alexander Kehrmann
Original assignee: Amateq Holding Gmbh
Priority date: 2020-12-11
Filing date: 2021-12-13
Publication date: 2022-06-16
Also published as: EP4259579A1

Abstract

A process for preparing a magnesium sulphate comprising a step of reacting a calcined magnesite comprising magnesium oxide (MgO) with sulphuric acid (H2SO4) to obtain magnesium sulphate (MgSO4)and water (H2O).Said calcined magnesite is in powder form, wherein said powder comprises powder particles having a specific particle size distribution, wherein said calcined magnesite comprises at least magnesium oxide (MgO) and calcium oxide (CaO). Said sulphuric acid is a mixture of a concentrated sulphuric acid and of a diluted sulphuric acid originated from a titanium dioxide (TiO2) production process. Said mixture of sulphuric acids has a concentration of sulphuric acid comprised from 42%wt to 50%wt, whereby the moles of said sulphuric acid in said mixture are in stoichiometric excess with respect to the moles of magnesium oxide, said stoichiometric excess being based on the actual moles of magnesium oxide and of calcium oxide in said calcined magnesite.

Description

DESCRIPTION of the invention with the title:

“Process for preparing magnesium sulphate, magnesium sulphate obtained by said process, and use of said magnesium sulphate as a fertilized

The present invention relates to a process for preparing a magnesium sulphate, to a magnesium sulphate obtained by said process, and to a use of said magnesium sulphate as a fertilizer.

Titanium dioxide (Ti02) occurs in nature as a mineral and is mainly sourced from ilmenite ore, the most widespread form of titanium dioxide-bearing ore, and from rutile ore, one of the minerals with the highest content (98%) of TIC>2.

The production method of titanium dioxide depends from the feedstock.

Rutile mineral sand is purified with a chloride process.

In the chloride process the rutile mineral sand is treated at high temperatures (1.000°C) with coke (C) and gaseous chlorine (C ) to obtain titanium tetrachloride (TiCk). TiCk is subsequently purified by distillation, and then oxidized in an oxygen flame (or plasma) to give pure titanium dioxide and chlorine (CI2).

Ilmenite ore can be converted into pure titanium dioxide through the chloride process or, as an alternative, through a sulphate process. This latter employs a simpler technology than the chloride route and can use lower grade and cheaper ores.

In the sulphate process the ilmenite mineral is dissolved in sulphuric acid to extract iron (II) sulphate pentahydrate (FeSCkSFkO) and/or iron (II) sulphate heptahydrate (FeSCUJFbO) and to form a mixture of sulphates the most important of which is titanyl sulphate (TiOSCk). The iron (II) sulphate pentahydrate and/or iron (II) sulphate heptahydrate is/are then removed from the liquid reaction mixture, so that such iron salt(s) does/do not spoil the colour of the final product. The titanyl sulphate is then hydrolysed in solution to give insoluble, hydrated titanium dioxide. This solid product is then heated in a calciner to evaporate the hydration water and to decompose the sulphuric acid in the final solid product that is pure titanium dioxide. Even though the sulphate process is technologically simpler than the chloride process, large titanium dioxide producers balance their production between these two processes because both produce TiC>2 in the rutile crystal, but the sulphate route can produce also an anatase crystal form. Anatase is characterized by softer crystals than the rutile crystals, that make the anatase form preferred for a small number of specialist applications (e.g. in the production of self-cleaning glass).

However, the sulphate process produces large amounts of waste materials, in particular of diluted sulphuric acid and of calcium sulphate {chemical gypsum). Consequently, the sulphate process for the production of titanium dioxide involves an expensive pollution control that is necessary for managing, or for disposing of, the waste by-products.

CN 107 963 644 A discloses a method for preparing magnesium salt by using waste acid of titanium white production. In this method, magnesite powder is pulped in order to obtain magnesite powder slurry. CN 107 963 644 A is silent about calcined magnesite, about a particle size distribution of the magnesite powder, about an amount of calcium oxide in said magnesite, about the concentration of sulphuric acid, and about a stoichiometric excess of sulphuric acid.

CN 108 751 236 A discloses a method using titanium dioxide waste acid to prepare magnesium fertilizer. Such method uses a 50-mesh magnesite powder, i.e. a powder with a particle size distribution < 297 pm. CN 108 751 236 A is silent about calcined magnesite, about a magnesite powder having particle size distribution d90 = 85 pm, about an amount of calcium oxide in said magnesite, and about a stoichiometric excess of sulphuric acid.

US 9,073,797 B2 discloses a method for the manufacture of a magnesium sulphate product, and a crystalline product, which comprises magnesium sulphate in the form of crystals or granules, as obtainable by carrying out this method. US 9,073,797 B2 discloses that the final crystalline product would generally contain about 21% to 28% w/w soluble magnesium sulphate. US 9,073,797 B2 is silent about calcined magnesite, about a particle size distribution of the magnesite powder, about an amount of calcium oxide in said magnesite, about the concentration of sulphuric acid, and about a stoichiometric excess of sulphuric acid. Also, US 9,073,797 B2 is silent about an amount of soluble magnesium (MgSO4) comprised from 50,00 %wt to 80,00 %wt, and a total magnesium content comprised from 55,00 %wt to 95,00 %wt.

CN 86 105 794 A discloses a method wherein a mild-heated magnesium oxide made from magnesite by calcination is reacted with sulfuric acid of appropriate concentration to get monohydrated magnesium sulphate directly. The particle size of the pulverized light-burned magnesia is preferably 60-100 mesh, i.e. a powder with a particle size distribution < 149 m - 250 pm. The concentration of sulphuric acid in the reaction is preferably 70-80%. CN 86 105 794 A is silent about a magnesite powder having particle size distribution d90 = 85 pm, about an amount of calcium oxide in said magnesite, about a concentration of sulphuric acid comprised from 42%wt to 50%wt, and about a stoichiometric excess of sulphuric acid.

SHI BATA J ET AL (" Solvent Extraction of Scandium from the Waste Solution of TiO2 Production Process", TRANSACTIONS OF THE INDIAN INSTITUTE OF METALS, SPRINGER INDIA, IN, vol. 70, no. 2, 28 November 2016 (2016-11 -28), pages 471-477, XP036141996) is a paper investigating extraction and stripping properties of Sc³⁺ by using a mixed extractant of Versatic Acid 10 (VA10) and Tri-n-butyl phosphate (TBP).

It would be therefore desirable to possess a process that can make use of a part of the by-products of the titanium dioxide production, so as to make the sulphate synthetic route of TiC>2 more profitable from an environmental and economic standpoint.

The Applicant, after a long and intense research and development activity, has developed a process capable of providing an adequate response to the existing limitations, inconveniences and problems. This process revealed to be capable of providing a final product with high reaction yield, and with an improved manageability.

Subject of the present invention is a process for preparing a magnesium sulphate having the features as defined in the enclosed claims.

Subject of the present invention is a magnesium sulphate obtained by said process and having the features as defined in the enclosed claims. Subject of the present invention is a use of said magnesium sulphate as a fertilizer, having the features as defined in the enclosed claims.

The present invention will now be explained in a greater detail with the aid of the accompanying figures, provided as non-limiting examples, wherein:

- Figure 1 is a schematic view of the process of the present invention, according to a possible embodiment, wherein the reference signs are as follows: 1 = calcined magnesite; 2 = concentrated sulphuric acid; 3 = diluted sulphuric acid originated from a TiC>2 production process; 4 = carbon dioxide; 5 = reaction product; 6 = maturation product; 7 = evaporated water; 8 = final product in powder form; 9 = scrubbing solvent; 10 = scrubbing purge; A = step of reacting; B = step of maturing; C = step of drying; D = step of grinding; E = step of scrubbing;

- Figure 2 shows diagrams of laser diffraction particle size analysis of calcined magnesite of two types (named “M1” and "M2”).

Subject of the present invention is a process for preparing a magnesium sulphate comprising a step of reacting a calcined magnesite comprising magnesium oxide (MgO) with sulphuric acid (H2SO4) to obtain said magnesium sulphate (MgSO4) and water (H2O).

The step of reacting is shown as step "A” in figure 1.

In the present description the expression "calcined” means a magnesite material subjected to a calcination process before the step of reacting with H2SO4. This calcination process is performed on a natural magnesite ore for increasing an amount of magnesium oxide (MgO) in the calcined magnesite with respect to the naturally occurring magnesite.

The step of reacting is preferably performed by mixing the raw materials - calcined magnesite comprising MgO and H2SO4 - in a reaction container (e.g. in a continuously stirred reaction container) to give a reaction mixture. Preferably, the step of reacting is performed at the storage temperature of the raw materials (e.g. comprised from 5°C to 30°C, preferably comprised from 15°C to 25°C) and at atmospheric pressure (1 atm) or at a pressure below the atmospheric pressure (< 1 atm). The duration of the step of reacting is preferably comprised from 30 minutes to 2 hours, more preferably comprised from 1 hour to 1 .5 hours.

The step of reacting is exothermic. Preferably, depending from the feeding rates of the raw materials, it is possible to perform a temperature control of the reaction mixture, for example for avoiding that the reaction mixture exceeds a threshold temperature (e.g. 90°C, 100°C or 110°C).

In the step of reacting the calcined magnesite is fed in the form of a powder, and the sulphuric acid is fed as an aqueous liquid solution.

A ratio ("RMgA”) between the moles of magnesium and the moles of sulphuric acid in the step of reacting is preferably comprised from 0.9 to 1.3, more preferably comprised from 0.92 to 1.2, even more preferably comprised from 0.95 to 1.1, most preferably comprised from 0.97 to 1.03, for example 1.0.

Said calcined magnesite comprises, in addition to magnesium oxide, also other salts such as calcium oxide (CaO), optionally silicon dioxide (SIO2), optionally iron oxide (Fe2C>3) and/or optionally calcium and magnesium carbonate (CaMg(CC>3)2).

Hence, depending from the composition of the calcined magnesite used in the process, the chemical reactions could lead to reaction products other than magnesium sulphate (MgSO4) and water (H2O) only.

More precisely, the chemical reactions mainly involved in the process of the present invention are the following:

(I) H₂SO₄ + MgO MgSO₄+ H₂O

(I I) 2 H2SO4 + CaMg(CO₃)₂ MgSO₄ + CaSO₄ + 2 CO₂ + H₂O

(III) H2SO4 + CaO CaSO₄'2H₂O + H₂O

Reaction (I) and (II) are both useful for obtaining magnesium sulphates. However, reaction (III) subtracts sulphuric acid from reacting in (I) and (II) and gives water and calcium sulphate di-hydrate (CaSC ^^O) as a by-product.

Also, reaction (II) develops carbon dioxide (CO2). If carbon dioxide is developed during the step of reacting, the present process preferably comprises - during and/or after the step of reacting - a step of scrubbing said CO2, preferably a step of scrubbing a gas flow containing said carbon dioxide with a solvent, more preferably with water or with an aqueous solution. Preferably, said gas flow is obtained by performing the step of reacting at a pressure below the atmospheric pressure (< 1 atm), e.g. by using suction means such as a fan.

The step of scrubbing is shown as step “E” in figure 1 .

The step of scrubbing gives a liquid scrubbing purge containing the scrubbed carbon dioxide.

The step of scrubbing could be performed in a scrubber device positioned fluidically downstream to the step of reacting. The term "fluidically downstream” refers to the stream direction of said gas flow. Preferably, the suction means are fluidically connected to the reaction container (e.g. to the continuously stirred reaction container) so as to move said carbon dioxide from said reaction container to the scrubber device.

Preferably, the step of scrubbing is performed with the scrubbing solvent (preferably water or aqueous solution) at a temperature comprised from 10°C to 30°C, preferably comprised from 22°C to 28°C, more preferably comprised from 24°C to 26°C, at atmospheric pressure (1 atm).

The calcined magnesite used in the present process comprises (percentages by weight (%wt) expressed with respect to the overall weight of said magnesite):

- magnesium oxide (MgO) from 50%wt to 98%wt;

- calcium oxide (CaO) from 0, 1 %wt to 35%wt;

- optionally silicon dioxide (SiO2), iron oxide (Fe2C>3) and/or calcium and magnesium carbonate (CaMg(CO3)2), each independently in an amount from 0.5%wt to 15%wt.

Preferably, said calcined magnesite comprises:

- magnesium oxide (MgO) from 53%wt to 90%wt, more preferably from 55%wt to 70%wt, even more preferably from 60%wt to 65%wt;

- calcium oxide (CaO) from 0.5%wt to 20%wt, more preferably from 0.9%wt to 15%wt, even more preferably from 5.0%wt to 10%wt; - optionally silicon dioxide (Si02), iron oxide (Fe2C>3) and/or calcium and magnesium carbonate (CaMg(CO3)2), each independently in an amount from 0.6%wt to 12%wt, more preferably from 1.0%wt to 10%wt, even more preferably from 1.5%wt to 7%wt.

Said calcined magnesite is in the form of a powder. Said powder comprises powder particles having a particle size distribution d90 = 85 pm.

In the present description the expression "d90” (or its equivalent “Dv(90)”) means a point in the size distribution, up to and including which, 90% of the total volume of powder particles in the sample is contained. More precisely, "d90 = 85 pm” means that 90% of the total volume of powder particles in the sample has a size of 85 pm or smaller. Similar definitions apply also for "d50” or "Dv(50)” and “d10" or “Dv(10)” that refer to points in the size distribution below which 50% and 10%, respectively, of the total volume of powder particles is contained.

The particle size distribution (d90, d50 and/or d10) in the present invention is preferably determined with a laser diffraction particle size analysis.

Preferably, said powder of calcined magnesite consists of powder particles having a particle size distribution d90 = 85 pm.

More preferably, said powder comprises or, alternatively, consists of powder particles having a particle size distribution d90 = 85 pm, d50 = 33 pm, d10 = 4.6 pm, more preferably a particle size distribution d90 = 78 pm, d50 = 25 pm, d10 = 4.3 pm; even more preferably a particle size distribution d90 = 70 pm, d50 = 22 pm, d10 = 4.0 pm.

As an example, said powder comprises powder particles having a particle size distribution d90 = 69.1 pm, d50 = 21.6 pm, d10 = 3.96 pm, or a particle size distribution according to the curve M1 in figure 2.

Said sulphuric acid is a mixture of a concentrated sulphuric acid and of a diluted sulphuric acid originated from a titanium dioxide (TIO2) production process. In the present description "concentrated sulphuric acid” means H2SO4 in a concentration comprised from 90%wt to 99%wt, preferably comprised from 95%wt to 98.5%wt, more preferably comprised from 97%wt to 98%wt, e.g. 98%wt.

In the present description "diluted sulphuric acid” means H2SO4 in a concentration comprised from 10%wt to 40%wt, preferably comprised from 20%wt to 35%wt, more preferably comprised from 27%wt to 33%wt. Said resulting mixture of sulphuric acids has a concentration of sulphuric acid comprised from 42%wt to 50%wt, preferably comprised from 44%wt to 48%wt, more preferably comprised from 45%wt to 47%wt, most preferably comprised from 45.5%wt to 46.5%wt e.g. of 46%. Such concentration is obtained by combining the concentrated sulphuric acid and the diluted sulphuric acid in a suitable ratio.

The concentrated sulphuric acid and the diluted sulphuric acid originated from the titanium dioxide (TiO2) production process may be pre-mixed with each other before the step of reacting (i.e. before being contacted with the magnesite and the MgO therein contained), or these sulphuric acids may be mixed with each other at the step of reacting. In this latter embodiment, the raw materials magnesite, concentrated sulphuric acid and diluted sulphuric acid may be fed independently to the reaction container (e.g. to the continuously stirred reaction container) to give the reaction mixture.

In the present description the expression "originated from a titanium dioxide (TiO2) production process” means a hydrolysed solution of titanyl sulphate (TiOSO4) from which an insoluble, hydrated titanium dioxide has been separated. In other terms, the diluted sulphuric acid originated from the titanium dioxide production process is a filtered hydrolysed solution of titanyl sulphate. The "production process” referred to in such expression is therefore implicitly a sulphate process for the production of titanium dioxide.

The diluted sulphuric acid originated from the titanium dioxide (TiO2) production process preferably comprises sulphuric acid in an amount comprised from 20%wt to 35%wt, ferrous sulphate (FeSO4) in an amount comprised from 1%wt to 10%wt and ferric sulphate (Fe2[SC>4]3) in an amount comprised from

1%wt to 3%wt in aqueous solution. Before being mixed with said concentrated sulphuric acid, said diluted sulphuric acid originated from the titanium dioxide production process is preferably subjected to a step of separating at least in part one or more rare earth elements from said diluted sulphuric acid. Preferably said rare earth element(s) is/are selected from the group consisting of cerium, dysprosium, erbium, europium, gadolinium, holmium, lanthanum, lutetium, neodymium, praseodymium, promethium, samarium, scandium, terbium, thulium, ytterbium, yttrium and their mixtures, more preferably scandium and/or yttrium, even more preferably scandium.

In other words, the diluted sulphuric acid originated from the titanium dioxide production process (i.e. the filtered hydrolysed solution of titanyl sulphate) firstly undergoes a step of separating of at least one rare earth element, and then is used for mixing with the concentrated sulphuric acid.

Preferably, the step of separating at least in part one or more rare earth elements comprises an ion exchange separation or a solvent extraction.

Said ion exchange separation is preferably a selective ion exchange separation on a gel sulfonated polystyrene cation exchange resin at a pH value comprised from 1.0 to 3.0, preferably comprised from 1.5 to 2.0.

Preferably, said gel sulfonated polystyrene cation exchange resin is in the form of particles or spherical beads. More preferably, the mean particle size distribution of said particles or spherical beads is comprised from 100 pm to 1500 pm, preferably comprised from 300 pm to 1200 pm.

More preferably, said gel sulfonated polystyrene cation exchange resin is cross-linked with styrene divinyl benzene (DVB). These resins are known in the art with the tradename "Purolite C-100” (Purolite Corporation) in various percentages of DVB linkages.

An amount of said DVB is preferably at least equal to or higher than 8% by weight (%wt), preferably comprised from 8%wt to 20%wt, more preferably comprised from 8%wt to 16%wt, with respect to the overall weight of said gel sulfonated polystyrene cation exchange resin. Preferably, said gel sulfonated polystyrene cation exchange resin cross-linked with styrene divinyl benzene (DVB), in NHv or in Na⁺ form, is loaded into a vertical separation column, and said diluted sulphuric acid originated from the titanium dioxide production process is passed through said column from the bottom upward with a predefined flow-rate. For example, for a vertical column having 1 litre capacity, the predefined flowrate could be comprised from 2 litre/hour to 8 litre/hour, e.g. 5 litre/hour.

Said solvent extraction separation of said at least one rare earth element is preferably a separation step with an organic phase or extraction solvent, preferably 2-(ethylhexyl)phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) or with di-(2-ethylhexyl)phosphoric acid (HDEHP).

The general formulae of HEH[EHP] (CAS N. 14802-03-0) and of HDEHP (CAS N. 298-07-7) are the following:

HDEHP

More preferably, said solvent extraction separation is a stepwise separation comprising at least a first separation step with HEH[EHP] as a first extraction solvent, and a subsequent second separation step performed on the aqueous phase obtained from the first separation step with HDEHP as a second extraction solvent. Consequently, according to this embodiment, the stepwise separation bases on the different extraction abilities and a cation-exchange mechanism of HEH[EHP] and of HD EHP with respect to the rare earth element(s).

Preferably, each separation step could comprise more than one stage (e.g. from 2 to 6 stages, preferably from 2 to 4 stages, e.g. 3 stages), whereby the aqueous phase is contacted with fresh extraction solvent in each stage.

A ratio VA/VO between the volume of the aqueous phase (VA) and the volume of the organic phase (Vo) in the solvent extraction separation is preferably comprised from 5:1 to 25:1, more preferably comprised from 10:1 to 20:1.

For HEH[EHP], the ratio VA/VO in the solvent extraction separation is preferably comprised from 5:1 to 15:1, more preferably comprised from 8:1 to 12:1, e.g. of 10:1.

For HDEHP, the ratio VA/VO in the solvent extraction separation is preferably comprised from 15:1 to 25:1, more preferably comprised from 17:1 to 23: 1, e.g. of 20:1.

Preferably, HEH[EHP] and/or HDEHP (preferably both) are in a non-saponified form. In the present description the expression "non- saponified” means that these phosphoric esters are not hydrolysed under basic conditions to form an alcohol and the salt of their acid.

After the solvent extraction separation is concluded, the present process preferably comprises a step of stripping the organic phase or extraction solvent loaded with the rare earth element(s) with a solution of hydrochloric acid (HCI). Preferably, the step of stripping is performed in one or more stages (e.g. from 1 to 5 stages, preferably from 1 to 3 stages).

A ratio VA/VO between the volume of the aqueous phase (VA) and the volume of the organic phase (Vo) in the step of stripping is preferably comprised from 20:1 to 1:25, more preferably comprised from 18:1 to 1 :22.

For HEH[EHP], the ratio VA/VO in the step of stripping is preferably comprised from 20:1 to 10:1, more preferably comprised from 18:1 to 14: 1, e.g. of 16:1. For HDEHP, the ratio VA/VO in the step of stripping is preferably comprised from 1:15 to 1:25, more preferably comprised from 1 :18 to 1 :22, e.g. of 1 :20.

The moles of said sulphuric acid in said mixture are in a stoichiometric excess with respect to the moles of magnesium oxide. More precisely, said stoichiometric excess is based on the actual moles of magnesium oxide, of calcium oxide and of the optional calcium and magnesium carbonate in said calcined magnesite. In fact, the actual moles of magnesium oxide, of calcium oxide and of the optional calcium and magnesium carbonate in said calcined magnesite are relevant for establishing to what extent the moles of sulphuric acid in the mixture of sulphuric acids are used in reaction (I) and in optional reaction (II) - both leading to magnesium sulphate - and to what extent reaction (III) subtracts H2SO4 from the other desired reactions (I) and optionally (II) to obtain magnesium sulphate.

Preferably, the present process comprises a step of determining said actual moles of magnesium oxide, of calcium oxide and of the optional calcium and magnesium carbonate in said calcined magnesite before the step of reacting said calcined magnesite with sulphuric acid (H2SO4).

The present process preferably comprises a step of determining actual moles of unreacted magnesium oxide, of unreacted calcium oxide and/or of unreacted calcium and magnesium carbonate eventually contained in the magnesium sulphate obtained after the step of reacting or in the final product.

Even more preferably, said step(s) of determining - performed before the step of reacting said calcined magnesite with sulphuric acid (H2SO4), after the step of reacting and/or in the final product - is/are performed with a spectroscopic analysis. Preferably, the step(s) of determining are performed in situ on the magnesite raw material, on the reaction product and/or in the final product.

Said spectroscopic analysis is preferably an X-ray spectroscopic analysis, more preferably an X-ray fluorescence (XRF) analysis.

After the step of reacting, the present process preferably comprises a step of maturing the reaction product, wherein the reaction product ends the exothermic reaction to give magnesium sulphate and terminates the optional development of carbon dioxide to give a maturation product. The step of maturing is shown as step “B” in figure 1 .

The step of maturing is preferably performed at room temperature at atmospheric pressure (1 atm), for a maturing time comprised from 10 minutes to 30 minutes.

After the step of reacting or after the step of maturing, the present process preferably comprises a step of drying the reaction product or the maturation product.

The step of drying is shown as step “C” in figure 1 .

The reaction product or the maturation product are a sponge-like mass that - during the step of drying - loses its water content (evaporated water) to give a dried product. A dried product preferably has an amount of residual water comprised from 0.05% to 10% by weight with respect to the weight of the said product, preferably comprised from 0.1 %wt to 5%wt, more comprised from 0.5%wt to 2%wt.

The step of drying is preferably performed at a temperature comprised from 40°C to 80°C, preferably comprised from 45°C to 75°C, more preferably comprised from 50°C to 70°C, even more preferably comprised from 55°C to 65°C.

The duration of the step of drying is preferably comprised from 30 minutes to 8 hours, preferably from 60 minutes to 6 hours.

After the step of drying, the present process preferably comprises a step of grinding the dried product to give a final product in powder form.

The step of grinding is shown as step “D” in figure 1.

Preferably, the final product in powder form has powder particles with a mean particle size distribution below 500 pm, preferably below 250 pm, more preferably below 100 pm, even more preferably below 50 pm.

The step of grinding preferably comprises a first step of grinding the dried product to give granules having a mean particle size distribution comprised from 0.5 mm to 5 mm, preferably comprised from 0.75 mm to 2 mm, and a subsequent second step of grinding said granules to give the final product in powder form having powder particles with a mean particle size distribution below 500 pm, preferably below 250 pm, more preferably below 100 pm, even more preferably below 50 pm.

The first step of grinding is preferably performed with a crusher bucket.

The second step of grinding is preferably performed with a hammermill.

Preferably, the final product in powder form has powder particles with a particle size distribution d90 = 45 pm, d50 = 25 pm, preferably d90 = 40 pm, d50 = 20 pm, more preferably d90 = 35 pm, d50 = 15 pm.

Subject of the present invention is also a magnesium sulphate obtained by said process.

Preferably, said magnesium sulphate has a composition according to Table A.

Table A

More preferably, said magnesium sulphate has a composition according to Table B.

Table B

Even more preferably, said magnesium sulphate has a content of metals according to Table C. Table C

Most preferably, said magnesium sulphate has a composition according to Table A and to Table B.

Subject of the present invention is also a use of said magnesium sulphate as a fertilizer.

The magnesium sulphate obtained with the process of the present invention is beneficial and particularly suitable for being used as a fertilizer due to its high magnesium sulphate and soluble magnesium contents.

Preferably, the present magnesium sulphate is particularly suitable as a fertilizer in oil palm plantations.

Some non-limiting examples of the present invention will be reported below.

EXPERIMENTAL PART

The purpose of the following tests is to determine a satisfactory recipe for the production of MgSC>4 starting from the mixture of sulphuric acids and calcined magnesite.

For this determination, parameters such as soluble MgO, acid concentration and product manageability have been considered.

Following Table 1 resumes a part of the tests that have been performed to that purpose.

For the laboratory tests the following procedure has been implemented.

The recipe quantities (expressed in grams) of diluted sulfuric acid, of concentrated sulfuric acid (98%) and of solids indicated in Table 1 were added to a plastic reactor having a volume of 50 I, stirred at 200 rpm, and allowed to react for 10 minutes. Each reaction product was recovered entirely, spread on an aluminium tray and left to dry in an oven at 60°C for 24 hours, evaluating the quantity of product obtained and the loss at 60°C. It was then ground first manually and then for 60 seconds in a type 2 laboratory pulverizing mill (“Pulverisette”).

For each sample 10 g of substance were weighed and placed in 90 g of water (10% mixture) for one hour with constant stirring. Then the pH value of the solution and the soluble magnesium content were measured by reading with cationic ion chromatography on the filtered solutions.

Two types of magnesite were used:

- magnesite M1 : has a loss on drying at 800°C of 15,6%, a composition according to following Table 2 and a diagram of laser diffraction particle size distribution according to the curve M1 in figure 2 and following Table 3.

Table 2

Table 3

- magnesite M2: has a loss on drying at 800°C of 5.5%, a composition according to following Table 4 and a diagram of laser diffraction particle size distribution according to the curve M2 in figure 2 and following

Table 5. Table 4

Table 5

With tests from 1 to 4 an influence of the overall acid concentration in the mixture of sulphuric acids and the relationship between soluble MgO and acid concentration were analysed. From these tests, it can be observed that the amount of soluble MgO increases with the acid concentration. For test 1 with a ratio between the moles of magnesium and the moles of sulphuric acid ("RMgA”) equal to 1.4 and from test 4 (RMgA = 1,0) the same amount of soluble MgO is obtained. This circumstance advises against working with an excess of MgO with respect to H2SO4.

Tests from 5 to 7 correspond to tests from 2 to 4 considering the loss on drying of the magnesites and that the carbonates eventually contained in M1 and M2 at 800°C have completely converted into MgO and CO2. Test 5 with RMgA = 0.8 provides a non-homogeneous and sponge-like mass impregnated with acid and with an excessively low pH (« 1,5). Consequently, the most suitable RMgA is 1.

Test 8 uses magnesite M1, that has a lower MgO content and a higher CaO content. In spite of the lower MgO content of this magnesite, the amount of soluble MgO is 21%, i.e. corresponds to a target concentration.

Tests from 9 to 14 employ pure solid raw materials and pure sulphuric acid so as to understand the upper limits of soluble MgO that can be achieved. In tests 11 and 12 the following mixture of solids was used: MgO 70%wt + CaO 30%wt. Test 11 uses a stoichiometric amount of H2SO4 with respect to MgO, while test 12 an excess amount of H2SO4 based on the actual moles of CaO in the mixture of solids. The higher amount of soluble MgO in test 12 versus test 11 confirms that CaO subtracts H2SO4 from reaction (I).

In tests 13 and 14 the concentration of the mixture of acids was reduced to 30%, with pure MgO and with RMgA = 1,0 (test 13) or RMgA = 0,8 (test 14). Test 13 allows to obtain a solid product but the soluble MgO is 27% (i.e. lower than soluble MgO in tests 9 and 10). For test 14 no usable solid was obtained because the solid is immersed in sulphuric acid.

Tests from 15 to 17 were performed with magnesite M1 considering a stoichiometric excess of sulphuric acid based on the actual moles of CaO. These tests confirm the trends that have been previously identified due to a similar concentration of soluble MgO in test 15 and in test 16. The product of test 17 was not usable.

Test 18 uses the diluted sulphuric acid only (30%wt), but the product so obtained solidifies only on its surface and remains wet inside. This test confirms that a mixture of sulphuric acids (comprising also concentrated sulphuric acid) is necessary for obtaining a final product with acceptable physical parameters.

At last, tests 19 and 20 use an acid concentration of 40%, but the concentration of soluble MgO is below the target concentration of 21%. The acid concentration must consequently be higher than 40%.

CONCLUSIONS:

In view of the above, satisfactory conditions for the preparation of magnesium sulphate are the following:

- an acid concentration (%) of the mixture of sulphuric acids comprised from 42%wt to 50%wt, because concentrations higher than 50%wt do not provide benefits in terms of soluble MgO, and concentrations lower than 42% (i.e. in presence of pure filtered hydrolysed solution of titanyl sulphate) provide reaction products that do not transform into solid products or that are not usable; - stoichiometric excess of H2SO4 based on the actual moles of magnesium oxide, of calcium oxide and of the optional calcium and magnesium carbonate, because carbonates influence the reaction stoichiometry of reaction (I) and (II) and CaO - that reacts according to reaction (III) - takes away sulphuric acid faster than desired reactions (I) and (II) that produce magnesium sulphate; - the raw material magnesite M1 is preferable with respect to magnesite M2 in spite of being a magnesite poorer in magnesium oxide (M1 = 62% vs M2 = 86%), because the higher particle size distribution of magnesite M2 has the drawback of reducing the concentration of soluble MgO.

The products so obtained are also better manageable for the step of grinding because can be easily crumbled with a low energy consumption of the equipment employed for this purpose.

Claims

1. A process for preparing a magnesium sulphate (MgSO4) comprising a step of reacting a calcined magnesite comprising magnesium oxide (MgO) with sulphuric acid (H2SO4) to obtain magnesium sulphate and water (H2O); wherein said calcined magnesite is in the form of a powder, wherein said powder comprises powder particles having a particle size distribution d90 = 85 pm, and wherein said calcined magnesite comprises (percentages by weight (%wt) expressed with respect to the overall weight of said magnesite):

- magnesium oxide (MgO) from 50%wt to 98%wt;

- calcium oxide (CaO) from 0, 1 %wt to 35%wt;

- optionally silicon dioxide (SiCk), iron oxide (Fe20s) and/or calcium and magnesium carbonate (CaMg(CO3)2), each independently in an amount from 0.5%wt to 15%wt; wherein said sulphuric acid is a mixture of a concentrated sulphuric acid and of a diluted sulphuric acid originated from a titanium dioxide (TiO2) production process, said diluted sulphuric acid originated from a titanium dioxide (TiO2) production process being a filtered hydrolysed solution of titanyl sulphate, wherein said mixture of sulphuric acids has a concentration of sulphuric acid comprised from 42%wt to 50%wt, and wherein the moles of said sulphuric acid in said mixture are in a stoichiometric excess with respect to the moles of magnesium oxide, said stoichiometric excess being based on the actual moles of magnesium oxide, of calcium oxide and of the optional calcium and magnesium carbonate in said calcined magnesite.

2. The process according to claim 1, wherein said powder comprises powder particles having a particle size distribution d90 = 85 pm, d50 = 33 pm, d10 = 4,6 pm; preferably a particle size distribution d90 = 78 pm, d50 = 25 pm, d10 = 4.3 pm; more preferably a particle size distribution d90 = 70 pm, d50 = 22 pm, d10 = 4.0 pm; even more preferably a particle size distribution d90 = 69.1 pm, d50 = 21.6 pm, d10 = 3.96 pm.

3. The process according to any of the previous claims, wherein said calcined magnesite comprises (percentages by weight (%wt) expressed with respect to the overall weight of said magnesite):

- magnesium oxide (MgO) from 53%wt to 90%wt, preferably from 55%wt to 70%wt, more preferably from 60%wt to 65%wt;

- calcium oxide (CaO) from 0.5%wt to 20%wt, preferably from 0.9%wt to 15%wt, more preferably from 5.0%wt to 10%wt;

- optionally silicon dioxide (SIO2), iron oxide (Fe2C>3) and/or calcium and magnesium carbonate (CaMg(CO3)2), each independently in an amount from 0.6%wt to 12%wt, preferably from 1.0%wt to 10%wt, more preferably from 1.5%wt to 7%wt.

4. The process according to any of the previous claims, wherein - before being mixed with said concentrated sulphuric acid - said diluted sulphuric acid originated from the titanium dioxide (TIO2) production process is subjected to a step of separating at least in part one or more rare earth elements, said step of separating comprising an ion exchange separation or a solvent extraction; said at least one rare earth element being preferably scandium, yttrium or their mixtures, more preferably scandium.

5. The process according to claim 4, wherein said ion exchange separation is a selective ion exchange separation on a gel sulfonated polystyrene cation exchange resin at a pH value comprised from 1.0 to 3.0, preferably comprised from 1.5 to 2.0, wherein said sulfonated polystyrene cation exchange resin is crosslinked with styrene divinyl benzene (DVB), and wherein an amount of said DVB is at least equal to or higher than 8% by weight (%wt) with respect to the overall weight of said sulfonated polystyrene cation exchange resin.

6. The process according to claim 4, wherein said solvent extraction is a stepwise separation comprising a first separation step with 2-(ethylhexyl)phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) as a first extraction solvent, and a subsequent second separation step with di-(2-ethylhexyl)phosphoric acid (HDEHP) as a second extraction solvent.

7. The process according to any of the previous claims, comprising a step of determining said actual moles of magnesium oxide, of calcium oxide and of the optional calcium and magnesium carbonate in said calcined magnesite before the step of reacting said calcined magnesite with sulphuric acid (H2SO4); and optionally a step of determining actual moles of unreacted magnesium oxide, of unreacted calcium oxide and/or of unreacted calcium and magnesium carbonate eventually contained in the magnesium sulphate obtained after the step of reacting; wherein said step(s) of determining is/are performed with a spectroscopic analysis.

8. The process according to claim 7, wherein said spectroscopic analysis is an X-ray spectroscopic analysis, preferably an X-ray fluorescence (XRF) analysis.

9. A magnesium sulphate obtained by the process according to any of claims from 1 to 8; wherein said magnesium sulphate has the following composition:

10. Use of the magnesium sulphate according to claim 9 as a fertilizer.