GB1572053A - Method for the production of pure magnesium oxide - Google Patents

Description

(54) METHOD FOR THE PRODUCTION OF PURE MAGNESIUM OXIDE (71) We, VEITSCHER MAGNESIT WERKE-ACTIEN-GESELLSCHAFT, a company organised and existing under the laws of Austria, of Schubertring 10-12, Vienna I, Austria, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a method of producing pure magnesium oxide from a starting material containing magnesium car donate magnesium oxide and/or magnesium hydroxide, e.g.. crude magnesite, by reacting the starting material with hydrochloric acid to give a magnesium chloride solution, precipitating and separating impurities from the solution and converting the magnesium chloride by thermal decomposition into magnesium oxide and hydrogen chloride.

In known methods of the above type, the magnesium-containing starting materials, especially natural crude magnesite, are 'dissolved" in a hydrochloric acid of approximately 18-20% concentration. magnesium carbonate and other magnesium-containing compounds being converted to magnesium chloride. Only this magnesium chloride is of value for the thermal decomposition to magnesium oxide carried out in a subsequent process step. Other components of the starting materials are, however. also dissolved bv the hydrochloric acid and these contaminate the magnesium chloride solution and must be separated during the course of the process. For this purpose, the magnesium chloride solution is purified by precipitating and separating impurities from the solution before the thermal decomposition of the magnesium chloride.

In the known methods referred to. the hydrochloric acid and the magnesiumcontaining starting material may be separately introduced into a dissolving vessel and the dissolving and precipitating operations take place in succession. Alternatively, the introduction of the starting material and hydrochloric acid may be effected together by feeding both materials to the dissolving vessel simultaneously. In this way, it is possible to perform the dissolving and precipitating steps in one, since in fact that starting material can also serve as precipitant.

In the known methods referred to, difficulties frequently occur in the purification of the magnesium chloride solution or magnesium chloride liquor. especially when the composition of the starting materials varies, as the filtering of the magnesium chloride solution or liquor for separating the precipitated impurities from the solution is particularly likely to be subjected to drastically varying influences which have an adverse effect upon a smooth production process.It can be assumed, in this connection, that the disturbing fluctuations in production output of the apparatuses used for the purification of the magnesium chloride solution are attributable to fluctuations in the silicate fractions dissolved from the starting materials by the hydrochloric acid and that a varying proportion of colloidally dissolved silica is present in the magnesium chloride solution or liquid to be purified. However, other substances also which, during the dissolving of the starting materials in hydrochloric acid, enter the magnesium chloride solution as a result of an accompanying dissolving of silicate minerals, such as especially chlorite but also sepiolith, serpentine and others, manifest themselves by adverse effects if they vary drastically in quantity during the purification process.

It is now an objective of the present invention to create a method of the type initially mentioned wherein difficulties of the aforementioned kind, such as occur in known processes, are overcome.

According to the present invention there is provided a method of producing pure magnesium oxide from a starting material containing magnesium carbonate, magnesium oxide and/or magnesium hydroxide, by reacting the starting material with hytlroch- loric acid to give a magnesium chloride solution. precipitating and separating impurities from the solution and converting the magnesium chloride by thermal decom- position into magnesium oxide and hyd rogen chloride, characterised in that. the starting material and the hvdrochloric acid are introduced into an agitated magnesium chloride solution having a free lICI content maintained below 1 r/l.

By the method according to this invention, the above-mentioned objective can be sitisfictorily achieved and disturbances in the purification of the magnesium chloride solution or liquor can be prictically eliminated. It appears that the favourable effect of the method according to this invention can be explained by the fact thit, by initiating the reaction between the starting material and the hydrochloric acid in a magnesium chloride solution hiving a free HCl content maintained below 1 log/I . a substantially complete braking down of the magnesium- containing constituents of the stirting materials. especially of the magnesium carbn- ate. takes place but thet the silicate constituents of the starting materials remain largely undissolved.

The method according to this invention cm be carried out either hatch-wise or continuously.

In batch operation, after the reaction of the starting material introduced into the relevant reaction vessel and the added hydrochloric acid has taken place to the intended degree. a portion of the resultant magnesium chloride solution is left in the reaction vessel for carrying out the next batch operation.In continuous operation, the starting materials and the hydrochloric ncid are, with advantage. continuously introduced into the magnesium chloride solution which is agitated and magnesium chloride solution or liquor is removed in corres pondence with the added quantity of start ing materials and hydrochloric acid.

It has been found in the method of this invention to be expedient to introduce the starting materials onlv slowly into the magnesium chloride solution and to keep this solution ill the acid state.

The proportion of free HCl in the magnesium chloride solution into which the starting material and the hydrochloric acid are introduced is preferably maintained below lltl g/t.

To achieve a favourable production output rate. it is of advantage to introduce the magnesium-containing starting material and the hydrochloric acid simultaneously into the magnesium chloride solution, which is agitated to promote the rapid distribution of the hydro-chloric acid throughout the solution and to equalise the concentration of free HCI throughout the solution.

In order as fur as possible to attain complete decomposition of the magnesium containing components of the starting materials, it is advantageius to transfer the magnesium chloride solution, to which starting material and hydrochloric acid his been added in a first vessel, when the reaction is only partially completed, into al least one further vessel, in which the reaction between the starting material and hydrochloric acid is continued under agitation.Provision may with advantage be mude hure for the magnesium chloride solution extracted from the first vessel to be conducted in series through several vessels, in which it is thor oughly stirred, in order further to promote the course of the reaction between the starting material and the hydrochloric acid.

One especially advantageous method of proceeding is achieved if the starting materials and hydrochloric acid are continuously introduced into a tenk, containing a magnc- sium chloride solution and the partially reacted magnesium chloride solution is then drnwn from this tank and caused to flow through a series of vessels or tanks in cascade to complete the reaction, the mixturc being thorilughly agitated in each tank.

The invention will now be explained in more detail with reference to the drawing which shows diagrammitically an installi- tion for carrying out the method according to this invention.

The drawling shows an installation which comprises three tanks 1 2 and 3, in which high-speed agitators 4. 5, 6 are mounted.

These tanks are interconnected by connecting lines 7 and X. which each lead respectively from the intended upper levels 9 and 10 of tanks I and 2 into tanks 2 and 3. In tank 3, a line 12, also leading from the upper filled level 11, is disposed. through which the reacted magnesium chloride solution or liquor flows out for purification and further processing.

When the plant is in operation. the tanks 1. 2 and 3 are each filled to the intended upper levels 9. l(l and 11 with an acid magnesium chloride solution, which is intensively agitated by the agitators 4. 5 and 6.

The starting materials, for example crude magnesite in ground form. which are to be decomposed are here with advantage introduced into the tank l. and an addition of hydrochloric acid is likewise carried out continuously to the magnesium chloride solution in the tank 1, the quantity of added hydrochloric acid being selected to suit the decomposition requirements. As a result of the intensive stirring of the magnesium chloride solution in tank 1, its composition remains practically uniform throughout the entire tank, in spite of the continuing addition of new material.In accordance with the addition of material. magnesium chloride solution flows continuously out through the connecting line 7 to the tank 2, where the reacting of magnesium-containing starting material and hydrochloric acid to magnesium chloride continues its course, to be completed then in the tank 3, into which the magnesium chloride solution flows through the connecting line 8.

The speed of the reaction in the plant can he influenced by appropriate selection of the quantity per unit time of added starting materials and hydrochloric acid and also by the possible provision of heating for the tank 1, it being also of advantage that, in the processing of magnesium carbonate, satisfactory stabilising of the working temperature in the tank 1 can be obtained as a result of the production of CO2 and evaporation of water.

In the following three examples, example 1 relates to a prior art method and examples 2 and 3 correspond to a procedure within the field of this invention.

Example I (Co)nlvn)atilXe) 60 litres of hydrochloric acid containing 206 g/l of HC1 were introduced into a tank equipped with an agitator and were heated to 90by. after which 14.22 kg of crude magnesite was introduced into the tank during the course of l hour using a metering screw.This crude magnesite had a loss on ignition of 47.86 wit.%. an SiO. content of 2.93 cc' by wt., an Fe2O3 content of 1.67coo by wt., and Al2O, content of 0.27 % by wt., a CaO content of 1.13cm bv wt.. and an MnO content of ().l4C bv wt: from this it can be deduced that this crude magnesite contains magnesium corresponding to 45% by wt. MgO. Accordingly. the said quantity of crude magnesite corresponds to the quantitv of the magnesite which can be stoichiometrically converted to magnesium chloride by the aforementioned quantity of hydrochloric acid.

After the introduction of the crude magnesite into the hydrochloric acid the contents of the vessel were further stirred for two hours. in order to bring the decomposition reaction as far as possible to completion. After this. the pH value of the solution was approximatelv l. This solution or liquor was then separated from the insoluble constituents bv filtration. and a sample was taken from the filtrate for the analvtical determination of the content of dissolved silica: from this. an SiO2 content of 0.258 g/l was obtained.

The pH value of the thus purified acid magnesium chloride brine was then adjusted to pH 7 by addition of magnesium oxide, causing a precipitation of the oxide hydrates of the iron and the aluminium to occur. The resultant magnesium chloride liquor was subjected to a filtration test using a hand filter plate, to enable an estimate to be made of the filtrability of this liquor on vacuum drum filters, such as are commonly used in large-scale execution of the process.

The hand filter plate employed had an area of 113 cm2 and was equipped with a filter cloth of monofilament plastics material having a mesh size of 27 . When a vacuum of 400 mm mercury column (0.53 bar) was used for the filtration, a filtering performance of 27.4 litre/minute referred to 1 m2 of filter area was obtained.

Example II In a manner analogous to Example 1, 14.22 kg of crude magnesite of the aforementioned composition was decomposed with 60 litres of hydrochloric acid having an HCI content of 206 g/l. Without carrying out a separation of the undissolved residue from the acidic brine or liquor, 30 litres of the liquor were removed from the vessel and rejected and after this a further 30 litres of hydrochloric acid were introduced into the vessel and 7.11 kg of crude magnesite was added by the conveying screw and the decomposition was continued under agitation to ensure a substantially uniform concentration of the constituents throughout the liquor.After the decomposition of the added materials was completed, this cycle, extending from the removal of one half of the acidic brine or liquor contained in the vessel, through the addition of new material to the conclusion of decomposition of same, was again repeated four times. After this, the proportion of silica dissolved in the brine remained substantially constant. In the last cycle, after the introduction of 30 litres of hydrochloric acid into the reaction vessel and before the addition of the quantity of crude magnesite corresponding to this cycle, the content of free HCI in the brine or liquor was determined, giving a value of 101.3 g/l.

The undissolved material was separated from the liquor finally obtained, and the dissolved silica content in the liquor was determined, giving a value of 0.123 g/l.

Then, in a manner analogous to example I, a precipitation of the oxide hydrates of the iron and aluminium was carried out by raising the pH value and then a filtration test with the hand filter plate described for Example I was carried out, in which when a vacuum of 400 mm of mercury column (0.53 bar) was used, a filter performance of 58.57 litres/minute referred to 1 m- of filter area was obtained. The purified magnesium chloride solution was then thermally decomposed in known manner to produce pure magnesium oxide.

Example 111 Crude magnesite of the composition given in Example I was decomposed with hydrochloric acid having an HCI content of 2()6 g/l in a 3-part agitator tank cascade, the individual tanks of which each had an effective capacity of 10 litre and embodied agitating means to ensure a substantially uniform distribution or concentration of the constituents in the liquor. Ten litres of hydrochloric acid and 2.37 kg of crude magnesite were supplied continuously to the uppermost vessel of the agitator tank cascade and the acid decomposition liquor having a pH value of 1 was drawn off from the lowest vessel of the cascade.

In order to bring the agitator tank cascade into a state appropritate for continuous operation, first the quantity of magnesium chloride liquor produced during an initial operating period of 10 hours was discarded and then, during the subsequent 6 hours, the 60 litres of magnesium chloride liquor flowing out from the agitator tank cascade were collected and the undissolved material was removed from this in a manner analogous to that described in Example I. The silica content of the liquor was then determined. giving an SiO. content of 0.073 g/l.

The content of free HCI in the first vessel of the cascade when the equilibrium state had become established was then determined, a figure of 61 + 3 g/l being obtained; an analogous determination of the value of free HCI in the second vessel of the cascade gave a value of 22.6 g/l.

The precipitation of hydroxides was then carried out in a manner analogous to that emploved in Example I and following this a filtration test was carried out. In this test, a filtering performance of 46.01 litre/minute for 1 m- of filter area was obtained.

The purified magnesium chloride solution was then thermally decomposed in known manner to produce pure magnesium oxide.

WHAT WE CLAIM IS: 1. A method of producing pure magnesium oxide from a starting material containing magnesium carbonate. magnesium oxide and/or magnesium hydroxide. by reacting the starting material with hydrochloric acid to give a magnesium chloride solution, precipitating and separating impurities from the solution and converting the magnesium chloride by thermal decomposition into magnesium oxide and hydrogen chloride, characterised in that the starting material and the hydrochloric acid are introduced into an agitated magnesium chloride solution having a free HCI content maintained below 110 g/l.

2. A method according to Claim 1 characterised in that the starting material and the hydrochloric acid are simultaneously introduced into the magnesium chloride solution.

3. A method according to Claim 1 or 2 characterised in that the starting material and the hydrochloric acid are introduced into a first vessel, containing magnesium chloride solution and the thus produced solution, after the reaction has partly taken place, is transferred into at least one further vessel, in which the reaction between the starting material and the hydrochloric acid continues further.

4. A method according to Claim 3 characterised in that the magnesium chloride solution extracted from the first vessel is conducted in series through several vessels, in which it is thoroughly agitated, in order to continue the reaction between the starting material and the hydrochloric acid.

5. A method according to any one of the preceding claims characterised in that the starting material and the hydrochloric acid are introduced continuously into the magnesium chloride solution and magnesium chloride in solution is continuously removed in correspondence with the added quantity of starting material and hydrochloric acid.

6. A method according to Claims 4 or 5 characterised in that the starting material and the hydrochloric acid are continuously supplied to a tank, containing magnesium chloride solution, of an agitator tank cascade.

7. A method according to Claim 1 substantially as herein described.

8. Magnesium oxide produced according to the method of any one of the preceding claims.

9. A method of producing magnesium chloride in solution for use in the production of pure magnesium oxide substantially as herein described with reference to Example 2 or 3.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. was obtained. The purified magnesium chloride solution was then thermally decomposed in known manner to produce pure magnesium oxide. Example 111 Crude magnesite of the composition given in Example I was decomposed with hydrochloric acid having an HCI content of 2()6 g/l in a 3-part agitator tank cascade, the individual tanks of which each had an effective capacity of 10 litre and embodied agitating means to ensure a substantially uniform distribution or concentration of the constituents in the liquor. Ten litres of hydrochloric acid and 2.37 kg of crude magnesite were supplied continuously to the uppermost vessel of the agitator tank cascade and the acid decomposition liquor having a pH value of 1 was drawn off from the lowest vessel of the cascade. In order to bring the agitator tank cascade into a state appropritate for continuous operation, first the quantity of magnesium chloride liquor produced during an initial operating period of 10 hours was discarded and then, during the subsequent 6 hours, the 60 litres of magnesium chloride liquor flowing out from the agitator tank cascade were collected and the undissolved material was removed from this in a manner analogous to that described in Example I. The silica content of the liquor was then determined. giving an SiO. content of 0.073 g/l. The content of free HCI in the first vessel of the cascade when the equilibrium state had become established was then determined, a figure of 61 + 3 g/l being obtained; an analogous determination of the value of free HCI in the second vessel of the cascade gave a value of 22.6 g/l. The precipitation of hydroxides was then carried out in a manner analogous to that emploved in Example I and following this a filtration test was carried out. In this test, a filtering performance of 46.01 litre/minute for 1 m- of filter area was obtained. The purified magnesium chloride solution was then thermally decomposed in known manner to produce pure magnesium oxide. WHAT WE CLAIM IS:

1. A method of producing pure magnesium oxide from a starting material containing magnesium carbonate. magnesium oxide and/or magnesium hydroxide. by reacting the starting material with hydrochloric acid to give a magnesium chloride solution, precipitating and separating impurities from the solution and converting the magnesium chloride by thermal decomposition into magnesium oxide and hydrogen chloride, characterised in that the starting material and the hydrochloric acid are introduced into an agitated magnesium chloride solution having a free HCI content maintained below 110 g/l.

2. A method according to Claim 1 characterised in that the starting material and the hydrochloric acid are simultaneously introduced into the magnesium chloride solution.

3. A method according to Claim 1 or 2 characterised in that the starting material and the hydrochloric acid are introduced into a first vessel, containing magnesium chloride solution and the thus produced solution, after the reaction has partly taken place, is transferred into at least one further vessel, in which the reaction between the starting material and the hydrochloric acid continues further.

4. A method according to Claim 3 characterised in that the magnesium chloride solution extracted from the first vessel is conducted in series through several vessels, in which it is thoroughly agitated, in order to continue the reaction between the starting material and the hydrochloric acid.

5. A method according to any one of the preceding claims characterised in that the starting material and the hydrochloric acid are introduced continuously into the magnesium chloride solution and magnesium chloride in solution is continuously removed in correspondence with the added quantity of starting material and hydrochloric acid.

6. A method according to Claims 4 or 5 characterised in that the starting material and the hydrochloric acid are continuously supplied to a tank, containing magnesium chloride solution, of an agitator tank cascade.

7. A method according to Claim 1 substantially as herein described.

8. Magnesium oxide produced according to the method of any one of the preceding claims.

9. A method of producing magnesium chloride in solution for use in the production of pure magnesium oxide substantially as herein described with reference to Example 2 or 3.