CA2248474C

CA2248474C - Magnesium compounds from magnesium silicates

Info

Publication number: CA2248474C
Application number: CA002248474A
Authority: CA
Inventors: Jean-Marc Lalancette
Original assignee: Inotel Inc
Current assignee: Inotel Inc
Priority date: 1998-09-28
Filing date: 1998-09-28
Publication date: 2008-11-25
Anticipated expiration: 2018-09-28
Also published as: CA2248474A1

Abstract

A process for the leaching of magnesium compounds from calcined magnesium silicates having initially a phyllosilicate structure; the magnesium silicates show a MgO to SiO2 ratio higher than 1.0 on a weight basis; the leaching is done with carbon dioxide on a slurry of ore in water at substantially ambient temperature and at substantially atmospheric pressure.

Description

TITLE OF THE INVENTION

MAGNESIUM COMPOUNDS FROM MAGNESIUM SILICATES
FIELD OF THE INVENTION

The present invention relates to the production of magnesium compounds, such as magnesium oxide or magnesium carbonate, from naturally occuring magnesium silicates.

BACKGROUND OF THE INVENTION

Silicates of magnesium are minerals of very broad occurrence in nature and appear in various forms such as talc, phlogopite, blotite, chorite, sepiolite and the like.
By the examination of the chemistry of silica and carbon dioxide, it appears obvious that, carbon dioxide in water being much more acidic than silica in the same solvent, the naturally occuring silicates of basic oxides, such as magnesium oxides, should dissolve in water saturated with carbon dioxide with the corresponding release of free silica.
MgO. Si02 + H2O.CO2-~MgCO3 + Si02.H20 In effect, from the data of the chemical litterature, the ionization constants of carbonic acid and silicic acid are the following:
H2CO3: K,=4.31 x10-' HzSiO4: K,=2x10-10 From these data, it can be seen that carbonic acid is roughly one thousand time more acidic than silicic acid and should attack very strongly the structure of naturally occurring magnesium silicates. In practice, this is not observed and minerals such as talc, soapstone and other magnesium silicates can resist weathering (rain containing carbon dioxide) over millions of years. Therefore silicic acid, while being an acid much weaker than carbonic acid, can proved to be inert to carbon dioxide in many instances.
From these considerations, the use of magnesium silicate as a substrate for lixiviation by carbon dioxide has not been very promising. One of the area where the largest amounts of magnesium silicate (3MgO. 2SiO2.
2HZ0) is handled is close by asbestos mining operations. The fibrous structure of asbestos ores represents only a few percents of the ore body which is treated and very large amounts of magnesium silicate wastes is thus rejected as tailings.

OBJECT AND STATEMENT OF THE INVENTION

It is an object of the present invention to produce magnesium compounds of appropriate composition and structure by the digestion of silicates in the presence of carbon dioxide in water at ambient temperature.
This is achieved by providing a process that involves the leaching of magnesium compounds from calcined magnesium silicates having initially a phyllosilicate structure, these magnesium silicates showing a Mg0 to Si02 ratio higher than 1.0 on a weight basis. This leaching is done with carbon dioxide on a slurry of ore in water at substantially ambient temperature and substantially atmospheric pressure.
More specifically, the present invention relates to a process for the leaching of magnesium compounds from calcined magnesium silicates having initially a phyllosilicate structure, the magnesium silicates showing a Mg0 to Si02 ratio higher than 1.0 on a weight basis; the leaching being done with carbon dioxide on a slurry of ore in water at substantially ambient temperature and substantially atmospheric pressure.
The present invention also relates to a process for producing magnesium compounds from calcined magnesium silicates having a phyllosilicate structure and showing a Mg0 to Si02 ratio higher than 1.0 by weight and the presence of free brucite; comprising the step of leaching the silicates with carbon dioxide in water at ambient temperature and at about atmospheric pressure.

-2a-The present invention also relates to a process for producing magnesium compounds comprising the steps of:
grinding mine tailings containing magnesium silicates to form particles having a predetermined mesh size; the magnesium silicates having a phyllosilicate structure and showing a MgO to Si02 ratio higher then 1.0 on a weight basis;
calcining the particles at a temperature between 500 to 700 C to form calcined particles;
leaching the calcined particles in water at substantially ambient temperature while stirring with carbon dioxide at a relative pressure of about one atmosphere;
maintaining the leaching and stirring steps for a given period and filtering to provide magnesium compounds.

DESCRIPTION OF THE INVENTION

Tailings of various asbestos mines in the province of Quebec, in Canada, have been examined in order to evaluate their reactivity towards leaching by carbon dioxide. In order to promote the reaction to optimal yields, the tailings were calcined at low temperature, such as 600 C in order to open-up the structure by the elimination of water.

It has been observed that, with tailings having a ratio of magnesium oxide to silica close to one or less than one, in such mining sites as JM Asbestos (0.97), King Beaver (0.95), British Canadian #1 (0.99), Bolduc (0.97) or Bell (0.99), the leaching of the material under standard conditions (ore ground to -200 mesh, calcined at 600 C, water saturated with CO2 at one atmosphere, stirring at 20 C for 24 hrs) led to very poor yields of leaching, about two percent of the available magnesium oxide in the starting silicate being extracted. However, with other mineral sources where the ratio of magnesium oxide to silica was above 1.0 in such mining sites as National (1.15), Carey (1.12) or Boston (1.10), it was possible to extract about 50%
of the available magnesium oxide from the starting ore.
Following these observations, the mineralogy of the various ores were examined in order to provide an understanding as to why a slight variation of the magnesium oxide to silica ratio had such a striking effect on the availability of magnesium oxide during the leaching with carbon dioxide.
Asbestos tailings are phyllosilicates with a structure of an indefinitely extended silicon-oxygen sheet, the so-called "siloxane sheet array". The hydroxyl groups are coordinated by magnesium ions above and below siloxane sheets. If their giant molecules are seen as stack of such alternates layers of Si-O planes and Mg(OH)2 planes, it will be readily understood that the access to the Mg(OH)2 can be very limited if there is an excess of Si-O planes. But, on the other hand, if there is an excess of Mg(OH)2, even if this excess is slight, there might be a rather improved access to the Mg(OH)2 by an opening of the lamellar structure. This explanation is corroborated by the fact that with materials where the leaching by carbon dioxide is efficient, it is noted that the ore contains some brucite, the tetrahedral arrangement of Mg3(OH)6.
Therefore, without going into limitative theoritical considerations, it is noted that the main condition required for a magnesium silicate to be leachable by carbon dioxide is to have a silicate structure which is opened up by an excess of MgO over Si02, on a percent of weight basis.
This is confirmed by the fact that, with the asbestos fiber which has a MgO
to Si02 ratio of 1.0, the MgO can not be leached by carbon dioxide to a significant amount. This opening of the phyllosilicate structure by the presence of brucite in excess over silica is a basic requirement for leaching.
Additional opening is obtained by relatively low temperature calcination of the starting ore so as to disrupt the phyllosilicate planar structure by removal of water. This calcination must be sufficient to eliminate water from the hydroxyl planes but must not exceed 700 C since, above that temperature, there is a sintering of the material which takes place and the gains obtained by removing water are lost due to the closure of the structure.
Hence, the implementation of the present invention calls for a source of phyllosilicate of magnesium, relatively rich in MgO, with a ratio of MgO to Si0z above 1.0 on a weigh basis. This material, if obtained from tailings of asbestos mines, is already finely divided but can be submitted to further grinding so as to obtain a granulometry of -200 mesh and then calcinated at 600 C to constant weight. A slurry of such material is made with water, the solid content being in the 1-10% range. This slurry is saturated with carbon dioxide, a slight pressure being maintained over the surface of liquid so as to insure a one atmosphere pressure (absolute) of CO2 in the system, or thereabout. A slow stirring insures the homogeneous consistency of the slurry.
The leaching process can thus be described by the following equation:
3MgO.2SiO2.2H20 600 c 3MgO.2SiO2 + 2H20 3MgO.2Si02 + 3CO2 + 1.5H20 20 c 1.5Mg (HCO3)2 + 1.5MgO.2SiO2 After about twelve hours, the unreacted magnesium silicate is filtered off and the solution of magnesium bicarbonate is heated to 100 C
inducing the formation of hydromagnesia (basic magnesium carbonate).

4Mg (HCO3)2 loo c 3Mg C03.Mg(OH)2.3H20 + 5CO2 This basic magnesium carbonate is the commercial form of magnesium carbonate and can be sold as such or transformed into magnesium oxide by calcination at 600 C.
3MgCO3.Mg (OH)2.3H20 6oo c 4MgO + 3CO2 + 4H20 Carbon dioxide liberated on the course of the precipitation of hydromagnesia or during the calcination of hydromagnesia can be recycled in order to maintain a closed loop as far as the circuit of CO2 is concerned. The purity of the magnesium oxide thus obtained is better than 99% MgO, the main impurity being CaO.
While proceeding with the leaching of the starting ore, a circuit of the slurry circulating over a magnet can remove a large portion of the magnetic fraction which is rich in iron oxides, with some nickel.

Example The ores were obtained from tailings at mining sites and were ground to -200 mesh by ball milling. Calcination was done in an electrical furnace at various temperatures, from 400 to 800 C, 600 C giving optimal yields of leaching. The leaching operation was performed on 50g samples in 1500m1 of tap water at 20 C. Stirring was maintained with a magnetic stirrer and carbon dioxide was admitted to the system at a relative pressure of 70cm of water, giving an absolute pressure close to one atmosphere.
Yields were determined against various durations of contact and temperatures of calcination. Contact of about 12 hours gave near maximum yields. The magnet used to achieve stirring collected the magnetic fraction in ores which had a Ni/Fe ratio in the range of 0.02.
After the contact with C02, the slurry was filtered and the filtrate heated at 1 00 C for half an hour. The resulting precipitated hydromagnesia was filtered and dried at 110 C overnight. Results thus obtained are presented in Table 1.
The calcination of the hydromagnesia, at 600 C gave the corresponding magnesium oxide with a purity above 99%.

Table I
Lixiviation of silicates Sources MgO Si02 Ratio Yield of (%) (%) MgO/SiOZ extraction. #
J.M. Asbestos 38.1 39.2 0.97 2.1 King Beaver 36.7 38.6 0.95 2.7 Br. Can.# 1 39.2 39.4 0.99 2.6 Bolduc Can. 36.3 37.5 0.97 1.9 Bell 38.2 38.6 0.99 2.3 National 38.9 33.9 1.15 51.5 Carey 41.0 36.5 1.12 49.2 Boston 38.5 34.9 1.00 50.1 Fiber #7 38.5 38.5 1.00 1.8 $ The yield is the percentage of the leached magnesium calculated as MgO
related to the available MgO in the starting material.

Claims

1. A process for the leaching of magnesium compounds from calcined magnesium silicates having initially a phyllosilicate structure, said magnesium silicates showing a MgO to SiO2 ratio higher than 1.0 on a weight basis; said leaching being done with carbon dioxide on a slurry of ore in water at substantially ambient temperature and substantially atmospheric pressure.

2. A process as defined in claim 1, wherein the slurry contains from 1 to 10% of mineral at 20 to 25°C with a relative pressure of CO2 between 5 to 100 cm of H2O, calcination being done at 500 to 700°C.

3. A process as defined in claim 1 or 2, wherein calcination is carried out at 600°C.

4. A process as defined in claim 1, wherein the duration of leaching is from 4 to 20 hrs with CO2 recycling.

5. A process as defined in claim 1 or 4, wherein the duration of leaching is 12 hours.

6. A process as defined in claim 1, wherein hydromagnesia is recovered.

7. A process as defined in claim 1, wherein magnesium oxide is recovered.

8. A process as defined in claim 1, wherein a magnetic fraction of said ore is collected.

9. A process as defined in any one of claims 1 to 8, said leaching being done on a slurry of ore showing initially the presence of free brucite.

10. A process as defined in any one of claims 1 to 9, wherein the slurry of ore consists of asbestos tailings.

11. A process for producing magnesium compounds from calcined magnesium silicates having a phyllosilicate structure and showing a MgO to SiO2 ratio higher than 1.0 by weight and the presence of free brucite; comprising the step of leaching said silicates with carbon dioxide in water at ambient temperature and at about atmospheric pressure.

12. A process for producing magnesium compounds comprising the steps of:
grinding mine tailings containing magnesium silicates to form particles having a predetermined mesh size; said magnesium silicates having a phyllosilicate structure and showing a MgO to SiO2 ratio higher then 1.0 on a weight basis;
calcining said particles at a temperature between 500 to 700°C to form calcined particles;
leaching said calcined particles in water at substantially ambient temperature while stirring with carbon dioxide at a relative pressure of about one atmosphere;
maintaining said leaching and stirring steps for a given period and filtering to provide magnesium compounds.

13. A process as defined in claim 12, wherein said magnesium compounds include magnesium oxide, magnesium carbonate or mixtures thereof.

14. A process as defined in claim 12, wherein said stirring is carried out with a magnetic stirrer to collect magnetic fraction of the particles.

15. A process as defined in claim 12, wherein said calcining step is carried out at about 600°C.

16. A process as defined in claim 12, wherein the filtrate obtained by said filtering step is heated to precipitate hydromagnesia; said hydromagnesia being filtered and dried at about 110°C for a given time period.

17. A process as defined in any one of claims 12 to 16 wherein said mine tailings are asbestos tailings.

18. A process as defined in any one of claims 12 to 17 wherein said mine tailings are ground to approximately -200 mesh.