Recovery of magnesium from magnesium silicates

Substantially pure magnesium hydroxide or carbonate is obtained from naturally-occurring minerals containing at least 10% magnesium silicate by adjusting the pH a magnesium bisulfite solution obtained therefrom to between 8.0 and 8.9 under an oxidizing atmosphere thereby to produce a magnesium sulfate solution from which the magnesium hydroxide or carbonate can be readily obtained.

The present invention relates to a process for recovering substantially 
pure magnesium hydroxide or carbonate from naturally occurring minerals 
containing magnesium silicate. 
PRIOR ART 
The possibility of a reaction of naturally occurring silicates of magnesium 
with sulfur dioxide in the presence of water is known, reference to such a 
reaction being made in early patents, such as U.S. Pat. No. 690,513, Jan. 
7, 1902. This reaction is possible because SO.sub.2 in water is a much 
stronger acid than silicic acid. Depending on the relative amount of 
sulfur dioxide and magnesium silicate, the sulfite or the bisulfite can be 
obtained. With an excess of SO.sub.2, the bisulfite is formed. Thus, with 
chrysotile, the following reaction is observed: 
##EQU1## 
On the other hand, if the magnesium silicate is in excess, then the sulfite 
rather than the bisulfite is the final product: 
EQU 3MgO.2SiO.sub.2.2H.sub.2 O(excess) + 3SO.sub.2 /H.sub.2 O .fwdarw. 
3MgSO.sub.3 + 2SiO.sub.2 .kappa. 
in fact, a solution of bisulfite in water has the tendency to be 
transformed into a solution of sulfite by loss of sulfur dioxide, if the 
solution is aerated, in order to remove the free SO.sub.2. Also the 
treatment of a solution of sulfite by SO.sub.2 will allow the formation of 
the corresponding bisulfite, thus showing clear by the equilibrium which 
exists between these species, as shown by the following equation: 
EQU Mg(HSO.sub.3).sub.2 .revreaction. MgSO.sub.3 + SO.sub.2 + H.sub.2 O 
sulfur dioxide is a most interesting material for the extraction of 
magnesium from mineral sources of this metal because it can be prepared on 
site by the combustion of sulfur, a very abundant and cheap material or 
recovered from the roasting operations of sulfides which are very common 
in the mining industry. The processing of crude petroleum and the 
treatment of flue gas from the combustion of coal are also important 
sources of sulfur or sulfur dioxide. 
The silicates which contain magnesium are numerous and of frequent 
occurrence. The common mineral species containing significant amounts of 
magnesium bounded to silica are listed in the following Table I: 
TABLE I 
______________________________________ 
SILICATES OF MAGNESIUM 
Name Formula % Mg 
______________________________________ 
Serpentine Mg.sub.6 (Si.sub.4 O.sub.10) (OH).sub.8 
26 
Talc Mg.sub.3 (Si.sub.4 O.sub.10) (OH).sub.2 
19 
Phlogopite KMg.sub.3 (AlSi.sub.3 O.sub.10) (OH).sub.2 
17 
Biotite K(Mg,Fe).sub.3 (AlSi.sub.3 O.sub.10) (OH).sub.2 
13 
Chrysolite (olivine) 
(Mg,Fe).sub.2 SiO.sub.4 
23 
Pyrope (garnet) 
Mg.sub.3 Al.sub.2 (SiO.sub.4).sub.3 
15 
Enstatite (pyroxene) 
Mg.sub.2 (Si.sub.2 O.sub.6) 
24 
Diopside CaMg(Si.sub.2).sub.6) 
11 
Chlorite Mg.sub.3 (Si.sub.4 O.sub.10) (OH).sub.2 Mg.sub.3 (OH).sub.6 
32 
Tremolite Ca.sub.2 Mg.sub.5 (Si.sub.8 O.sub.22) (OH).sub.2 
15 
Anthophyllite 
(Mg,Fe).sub.7 (Si.sub.8 O.sub.22) (OH).sub.2 
20 
______________________________________ 
In the case of serpentine, large amounts of the incorporated fibrous 
chrysotile (3MgO.2SiO.sub.2.2H.sub.2 O) are mined for the obtention of the 
fibrous material, asbestos. Also, the amphibole variety of silicates have 
led to the extraction of fibers mainly of crodicolite and amosite (5.5FeO, 
1.5MgO, 8SiO.sub.2.H.sub.2 O) varieties. 
In most if not all of the mineral operations where the above silicates are 
involved, large amounts of rocks rich in magnesium have to be crushed in 
order to obtain the desired product, such as asbestos fiber, talc or 
simply aggregate for concrete. For example, in the case of chrysotile 
asbestos, the annual production of this material in Canada is of the order 
of 1.5 million tons per year. Since the fiber represents about 5% of the 
weight of the rocks involved in the extraction, the total mass of mineral 
which is grounded in the course of the extraction is in the range of 30 
million tons per year. The granulometry of the waste rocks or tailings, 
varies greatly but a significant portion, between one and five percent, is 
already very finely grounded. Even if only one tenth of one percent is in 
the appropriate form for digestion by SO.sub.2 for the extraction of 
magnesium the amount of silicate thus made available, 30,000 tons, appears 
very important. At the present time, there is no use for such tailings. In 
fact, specially with the finely divided material, there is an 
environmental problem created by dust and an expenditure to carry these 
tailings on top of huge piles of wastes. Consequently, the cost of finely 
divided serpentine at the outlet of an asbestos mine is minimal, if not 
negative. 
The chemistry of the sulfite or bisulfite of magnesium shows that this 
material can be transformed into several magnesium derivatives having 
useful properties. By thermal decomposition, the sulfite can lead to the 
formation of magnesium oxide with evolution of SO.sub.2. In the course of 
this process however, some sulfite is transformed into sulfate: 
EQU MgSO.sub.3 .fwdarw. MgO + SO.sub.2 .uparw. 
by exchange with calcium chloride, the sulfite can give magnesium chloride: 
EQU MgSO.sub.3 + CaCl.sub.2 .fwdarw. CaSO.sub.3 .dwnarw. + MgCl.sub.2 
The magnesium hydroxide can be formed by treatment of the sulfite with a 
strong base, such as NaOH. Further heat treatment of Mg(OH).sub.2 leads to 
magnesium oxide: 
EQU MgSO.sub.3 + 2NaOH .fwdarw. Mg(OH).sub.2 .dwnarw. + Na.sub.2 SO.sub.3 
EQU mg(OH).sub.2 .fwdarw. MgO + H.sub.2 O 
the carbonate can be obtained through a similar exchange reaction: 
EQU MgSO.sub.3 + Na.sub.2 CO.sub.3 .fwdarw. MgCO.sub.3 .dwnarw. + Na.sub.2 
SO.sub.3 
all these reactions are well known and do not represent a novelty in the 
area of extraction and transformation of magnesium salts but show the 
interest of magnesium sulfite or even sulfate as a starting material for 
the formation of magnesium salts. 
Many uses for the magnesium salts, MgO, Mg(OH).sub.2, MgCl.sub.2, 
MgCO.sub.3, MgSO.sub.4, require a product of very high degree of purity. 
For example a very important use of magnesium oxide is in the manufacture 
of refractory material. The presence of a few percent of impurities in the 
oxide modifies the resistance of MgO to high temperature in a very adverse 
way and renders the material useless as refractory. The usual impurities 
are calcium oxide, aluminum oxide and iron oxides (either ferrous or 
ferric). In TRAITE DE CHIMIE MINERALE, Paul BAUD, TOMEI, p. 115, 1951 it 
is shown that the presence of small amounts of contaminants decreases the 
melting temperature of the magnesium oxide by several hundred degrees. 
Another area where the purity of the magnesium salt is critical is the 
formation of the metal by the electrolysis of magnesium chloride. 
These two examples from areas representing major uses for magnesium 
illustrate the need for very pure magnesium salts. 
The naturally occurring magnesium silicates are very seldom in the pure 
state when found in nature. The secondary metals can be either part of the 
stoichiometry of the silicate of magnesium or simply mixed with the 
silicate. For example, chrysotile wastes 3MgO.2SiO.sub.2.2H.sub.2 O, are 
contaminated by 6% by weight of iron, plus traces of nickel and chrome. 
Also, up to one percent of calcium can be found in many samples. In the 
case of chrysotile, the iron is mixed with crystal, in a very intimate 
manner, but not chemically combined to the main structure of the silicate. 
In other instances, for example with amosite, another variety of asbestos, 
(5.5FeO, 1.5MgO, 8SiO.sub.2.H.sub.2 O) the iron is part of the crystal 
structure. In practice, many of the silicates containing magnesium are 
metamorphic rocks, which implies that there is always a certain amount of 
substitution of magnesium by another metal such as iron, calcium or 
aluminum. A good illustration of this situation is given by the formula of 
biotite K(Mg,Fe) (AlSi.sub.3 O.sub.10) (OH).sub.2 or phlogopite KMg.sub.3 
(AlSi.sub.3 O.sub.10) (OH).sub.2. 
When sulfur dioxide is used in combination with water, to leach magnesium 
from a silicate, all the other metals present, either combined or mixed, 
such as calcium and iron are dissolved at the same time. And the crude 
sulfite solution, thus obtained, contains such impurities as calcium and 
iron which render the crude sulfite improper to be used as a source of 
pure magnesium compound. 
The problem of obtaining a pure magnesium compound from SO.sub.2 leaching 
is well illustrated by the patent of Trubey, et al, U.S. Pat. No. 
3,085,858, Apr. 16, 1963. In the case of this patent, the leaching was 
done by SO.sub.2 on dolomite, a natural mixture of calcium oxide and 
magnesium oxide rather than on a silicate. This patent reports that even 
with this simple source of magnesium, 3% of CaO was contaminating the 
magnesium oxide after processing, in spite of careful precautions not to 
leach calcium in the course of the digestion of the mineral. The approach 
of Trubey which controls the presence of calcium by using a stream of 
CO.sub.2 cannot be applied to eliminate iron, heavy metals and aluminum 
from a solution of crude sulfite of magnesium as obtained from naturally 
occurring silicates. 
SUMMARY OF THE INVENTION 
The object of the present invention is related to the treatment of the 
crude solution of the SO.sub.2 -leached magnesium obtained from a silicate 
and contaminated by all the minor components of a natural silicate namely 
sodium, potassium, calcium, iron, aluminum traces of heavy metals such as 
chrome and nickel, in order to remove these impurities and to obtain a 
magnesium compound of very high purity. 
In accordance with the present invention, there is now provided an improved 
process for recovering substantially pure magnesium hydroxide or magnesium 
carbonate from naturally-occurring minerals containing magnesium silicate. 
The improved process of the present invention comprises digesting an 
aqueous suspension of a naturally-occurring mineral containing magnesium 
silicate with sulfur dioxide to produce a crude solution of magnesium 
bisulfite, adjusting the pH of said magnesium bisulfite solution to 
between 8.0 and 8.9 under an oxidizing atmosphere thereby to precipitate 
the impurities contained therein and causing the magnesium bisulfite to be 
converted to magnesium sulfate and precipitating the magnesium as 
magnesium hydroxide or carbonate by adjusting the pH of the magnesium 
bisulfite solution to between 9.5 and 10.5 with an alkali metal hydroxide 
or by treatment with an alkali metal carbonate, and recovering the 
substantially pure magnesium hydroxide or carbonate thus produced. 
A main feature of the present invention is that the metallic contaminants 
found in naturally-occurring minerals containing magnesium silicate can be 
selectively precipitated from a bisulfite solution obtained by leaching 
the naturally-occurring minerals by the adjustment of the pH in the range 
of 8.0 to 8.9 under an oxidizing atmosphere in opposition to prior art 
procedure where each contaminant has to be eliminated separately and where 
mixtures of magnesium sulfite and sulfate were found in the end product. 
Another advantage of the present invention is that beside obtaining the 
desired salt of magnesium a good proportion of the starting sulfur dioxide 
can be recovered in the form of ammonium or alkali metal sulfate, the 
latter having well known commercial utilities. 
A further advantage of the present invention is that since the process 
oxidizes the bisulfite formed to sulfate, the pollution problems generated 
by the handling of sulfites and recovery of sulfur dioxide are eliminated. 
It is generally known that release of sulfur dioxide is highly undesirable 
in industrial processes because of the adverse effect of sulfur dioxide on 
the environment. 
The naturally-occurring mineral containing magnesium silicate which can be 
used in accordance is preferably one which contains at least 10% 
magnesium, and where the specific impurities such as calcium is not more 
than 20%, iron 15%, aluminum 10%, agglomerated chrome, nickel and cobalt 
2%, while the total agglomerated alkali (Na and K) do not exceed 15%. 
Another feature of the starting mineral is that its mesh size should be 
smaller than 60 with a mesh size of at least 200 to 325 being preferred. 
It is possible to use a mineral containing less than 10% magnesium or 
having a mesh size greater than 60, but it should be appreciated that 
yield and purity of the magnesium compound obtained being of commercial 
importance, the exercise of the improved process of the present invention 
will be carried out under the preferred conditions which give the superior 
results. 
The adjustment of the pH to from 8.0 to 8.9 is carried out with ammonium or 
alkali metal hydroxide such as sodium or potassium hydroxide. The 
preferred pH adjustment is about 8.5 and is carried out under an oxidizing 
atmosphere which is obtained by bubbling air or oxygen in the solution and 
in the presence of a strong base such as an alkali metal hydroxide whereby 
magnesium hydroxide is precipitated or an alkali metal carbonate whereby 
magnesium carbonate is precipitated. The cobalt or nickel present in the 
starting mineral will accelerate the oxidation of the bisulfite to 
sulfate. 
In the final precipitation step the pH of the adjustment to from 9.5 to 
10.5 with 9.8 to 10.2 being preferred is also carried with a strong base 
such as an alkali metal hydroxide whereby magnesium hydroxide is 
precipitated or an alkali metal carbonate whereby magnesium carbonate is 
precipitated.

DETAILED DESCRIPTION 
Step 1 -- DIGESTION OF THE MAGNESIUM-BEARING MINERAL BY SULFUR DIOXIDE IN 
THE PRESENCE OF WATER 
The naturally-occurring mineral containing magnesium silicate is a divided 
material with a mesh size smaller than 60 mesh. With a product larger than 
mesh 60, the reaction proceeds but in order to have a faster and more 
complete reaction, it is preferable to have a material finer than mesh 
200. The volume of water present is also related to the ease of reaction. 
A ratio of mineral weight to volume of water that allows for the 
solubility of magnesium bisulfite is important. The reaction of digestion 
can be performed at atmospheric pressure or at a higher pressure. At 
atmospheric pressure, it is advisable to avoid too high temperatures, 
above 50.degree. C., in order not to decrease the solubility of SO.sub.2 
in water. At 25.degree. C., with a water ratio of 20 g of mineral (for 
example, serpentine) per liter of water the reaction is complete after, 
about, three hours. It is indicated, when the reaction nears completion, 
to stop the addition of SO.sub.2. In that manner, all the remaining 
SO.sub.2 present in solution reacts with the magnesium-bearing mineral to 
give a solution of bisulfite free of SO.sub.2. This simplifies the 
environmental considerations related with the presence of SO.sub.2. 
When the digestion is completed, the reaction mixture is filtered to remove 
the silica and any unreacted solid material present. In that manner, a 
solution of bisulfite, sulfite and sulfate of magnesium and other metals 
present, such as calcium and iron is obtained. The digestion is done under 
very acidic conditions, the pH of the solution being kept in the range of 
0.10 to 2.0 at this stage. 
Step 2 -- SELECTIVE PRECIPITATION OF METALLIC IMPURITIES FROM THE CRUDE 
SOLUTION 
The precipitation of the impurities from the bisulfite solution is done by 
transforming the sulfite or bisulfite into sulfate in an oxidizing 
atmosphere while increasing the pH of the solution in the range of 8.5 by 
addition of a strong base. The advantage of transforming the bisulfite 
into sulfate is to fall upon a stable and uniform type of anion at this 
stage. The solution of bisulfite is easily oxidized by air and this 
oxidation is much accelerated by the presence of traces of heavy metals, 
such as cobalt. Since the presence of some sulfate is unavoidable, it is 
simpler to proceed to full oxidation at this stage and thus avoid having a 
mixture of salts as the end products. In practice, it has been found that 
aeration of the solution of crude bisulfite for about one hour at room 
temperature allows the transformation of bisulfite to sulfate to be 
complete. 
While this aeration is proceeding, the pH of the solution is raised to a 
value of 8.5 by addition of a basic reagent. Ammonia or an alkali metal 
hydroxide for example sodium hydroxide have proved to be appropriate base 
for this pH adjustment. Iron, calcium, aluminum and other heavy metals are 
thus precipitated at that pH. By filtration, they are eliminated and the 
clear solution which contains magnesium sulfate is obtained. 
Step 3 -- RECOVERY OF MAGNESIUM SALTS 
The precipitation of magnesium hydroxide is obtained by further increase of 
the pH to 9.5-10.5 by a strong base such as an alkali metal or ammonium 
hydroxide. By filtration, very pure magnesium hydroxide is recovered, 
indicating a purity of over 99%. If magnesium carbonate is the desired 
product, this second raise of pH is replaced by addition of sodium 
carbonate. 
Step 4 -- RECOVERY OF SULFATES 
When the magnesium salts have been recovered, there remains a solution of 
the sulfate of the base which has been used (sodium or ammonium) and the 
sulfate of the small amount of alkali metals present in the starting 
silicate. By evaporation of the filtrated, ammonium sulfate (or sodium 
sulfate) can be obtained. 
The sequential operations are represented by the following Flowsheet I. 
##STR1## 
The present invention will be more fully understood by referring to the 
following Examples which are given to illustrate the invention rather than 
limit its scope. 
EXAMPLE 1 
In a 2 liter flask, a 40 g sample of chrysotile mesh + 200-325 is suspended 
in 1,500 ml of water. The atmosphere over the suspension is kept saturated 
with SO.sub.2 and the liquid phase is strongly agitated at room 
temperature. After a contact of 3.5 hours, the suspension is filtered over 
asbestos. 
The filtrate is then treated with sodium hydroxide (44 ml NaOH 10%) in 
order to bring the pH to 8.6. The solution is aerated as it is neutralized 
by a stream of air (150 ml/min), for 1.5 hour, in order to oxidize the 
bifulsite to sulfate. 
After filtration, the pH of the sulfate solution of magnesium is raised to 
9.9 by addition of 204.5 ml NaOH 10%. Magnesium hydroxide (15.6 g) is thus 
precipitated with a yield of 68% related to the amount of magnesium 
available in the starting material. The analysis of the precipitate gives 
99.1% Mg(OH).sub.2. 
By evaporation, 71.5 g of Na.sub.2 SO.sub.4.7H.sub.2 O is recovered, a 
yield of 82%. 
A similar procedure is used for the other examples. 
EXAMPLES 2-11 
By proceeding in the same manner and using the conditions set forth in 
Table II, the results shown in Table II are obtained. It is noted that 
Example 8 is given to illustrate the decreased yield obtained with the 
process of the present invention when using a mineral wherein the 
magnesium content is less than 10% by weight. 
3 TABLE II 
STEP 1 SO.sub.2 STEP 2 STEP 3 YIELDS STARTING MATERIAL Duration pH 
Oxidant Duration pH Compo- Mg(OH).sub.2 Sulfate Ex. Starting Weight 
Composition (%)Mesh pH digestion Base pption Atmos- oxidation pption 
sition recovery recovery No. silicate (gm) Mg Fe Ca Al size digestion 
(hrs) used Fe,Ca,Al phere (hrs) Mg(OH).sub.2 * Mg(OH).sub.2 * (%)** 
(%)**.sup.# 
1 Serpentine 40 22 6.5 1.1 0.1 +200-325 1.1 3.5 NaOH 8.6 air 1.5 9.9 
99.1 68 82 (chrysotile) 2 Serpentine 45 22 6.5 1.1 0.1 +325 1.0 3.0 
NaOH 8.8 air 1.2 10.1 99.0 76 81 (antigorite) 3 Serpentine 42 22 6.5 
1.1 0.1 + 60-200 0.8 3.3 NH.sub.4 OH 8.5 air 1.4 10.0 99.1 78 76 
(chrysotile- antigorite) 4 Serpentine 20 23 5.9 0.8 trace + 60-200 1.0 
3.9 NH.sub.4 
OH 8.6 O.sub.2 0.5 10.0 99.3 81 74 (chrysotile- antigorite) 5 Talc 40 
19 0.2 0.1 -- +325 0.9 3.5 NaOH 8.4 air 1.2 9.8 99.6 86 81 6 Talc 40 19 
0.2 0.1 -- +325 0.8 3.1 NH.sub.4 OH 8.5 O.sub.2 1.1 9.9 99.5 88 77 7 
Phlogopite 45 16 1.1 0.7 6.6 + 60-200 0.9 4.2 NaOH 8.4 air 1.5 10.1 99.0 
69 82 8 Amosite 30 3.4 29 0.8 0.1 + 60-200 0.8 4.0 NaOH 8.6 air2.0 10.0 
97.2 32 80 9 Diopside 45 10 1.8 18 0.2 +200-325 0.9 3.0 NaOH 8.4 air 2.0 
9.9 99.0 63 82 10 Biotite 40 11 19 1.0 6.1 + 60-200 1.0 4.2 NH.sub.4 OH 
8.6 air 1.5 10.0 99.0 64 71 11 Chlorite 25 28 5.1 1.2 0.3 +200-325 0.9 
3.5 NH.sub.4 
*Percent of Mg(OH).sub.2 in the final product. 
**Percent yield of available salt in the starting product. 
.sup. # Sulfate of Na or NH.sub.4 depending of base used. 
EXAMPLE 12 
By proceeding in the same manner as in Ex. 1-11 and substituting the 
appropriate amount of sodium carbonate for the sodium hydroxide in Step 2 
there is obtained magnesium carbonate in substantially quantitative yields 
.