Filtering of precipitating silica

The addition of an excess of asbestos tailings in the course of the extraction of magnesium from asbestos tailings by digestion with an acid greatly facilitates the elimination of silica and other impurities by filtration of the resulting reaction mixture. The improvement reduces the filtration time and the filtration leaves a cake which is much easier to dispose.

The present invention relates to an improved process for removing silica 
from aqueous solutions of magnesium salts obtained from asbestos tailings 
containing magnesium silicate. 
PRIOR ART 
The asbestos wastes or tailings derived from serpentine after recovery of 
asbestos fibres contain between 20 to 22% by weight of magnesium in the 
form of hydrated magnesium silicate (3MgO.2SiO.sub.2.2H.sub.2 O) along 
with impurities such as calcium oxide, aluminum oxide, iron oxides (either 
ferrous or ferric) and traces of nickel and chrome. Furthermore, since a 
large part of these tailings are in finely divided form they can become a 
most interesting source of magnesium. 
One approach to recover magnesium from asbestos tilings is to attack the 
latter with a mineral acid thereby forming the corresponding water-soluble 
salt of magnesium which must then be separated from the insoluble silica 
as can be seen from the following equation: 
EQU 3MgO.2SiO.sub.2.2H.sub.2 O+3H.sub.2 .fwdarw.3MgX+2SiO.sub.2 
.dwnarw.+5H.sub.2 O 
where X is an anion of a mineral acid such as SO.sub.4.sup.=, 2Cl.sup.-, 
2NO.sub.3.sup.-. 
From the existing literature it can be noted that a fairly large number of 
acids have been used to extract magnesium from asbestos tailings. For 
example references are typical: leaching with sulfuric acid (H.sub.2 
SO.sub.4): H. B. Chalmers, U.S. Pat. No. 2,402,370; leaching with 
bisulfite (HSO.sub.4 --): F. L. Pundsack, U.S. Pat. No. 3,338,667; 
leaching with sulfur dioxide (SO.sub.2 --H.sub.2 O): A. W. Winston et al, 
U.S. Pat. No. 1,865,224; leaching with hydrochloric acid (HCl): J. Marek 
et al, Canadian Pat. No. 1,034,385; leaching with carbonic acid (CO.sub.2 
--H.sub.2 O): M. F. Adams, U.S. Pat. No. 3,320,029 and E. W. Nelson, U.S. 
Pat. No. 4,058,587. 
In all these processes, in order to obtain the magnesium salt it is 
essential to filter off the silica which is liberated by the action of the 
acid on the asbestos tailings from the solution of the magnesium salt. It 
is well known that the filtration of precipitated silica is very difficult 
since such a precipitate is in the form of a gel which tends to plug the 
filter, causes a slow rate of filtration and an important retention of the 
solution of the useful magnesium salt. 
Accordingly, it is very desirable to provide a process for recovering 
water-soluble magnesium salts from asbestos tailings which overcomes the 
drawbacks caused by the necessity of eliminating insoluble metallic salts 
and the presence of silica during the filtration step. 
THE INVENTION 
In accordance with the present invention, there is provided an improvement 
whereby a solution of magnesium salts containing silica and other 
insoluble metal salt impurities can be readily filtered to recover the 
magnesium salt in a time of from 2 to 10 times faster than heretofore 
known. 
In the improvement of the present invention, it has unexpectedly been found 
that the time factor required for filtering the reaction mixture of a 
slurry of asbestos tailings with a mineral acid to form an aqueous 
reaction medium containing water-soluble salts of magnesium, silica gel 
and insoluble metal salts can be substantially reduced by adding to said 
reaction medium a further quantity of asbestos tailings in an amount 
exceeding the stoichiometric amount required for the reaction of the 
initial magnesium oxide contained in the initial asbestos tailings and the 
mineral acid. 
It has therefore been surprisingly found that the addition of an excess of 
asbestos tailings to the reaction mixture greatly facilitates the 
filtration of the precipitated silica in forming a mat on the membrane of 
the filter that is fairly permeable to the solution and much more 
efficient in retaining the silica and other insoluble material present, 
such as iron oxide, without clogging, thus giving a good and constant 
filtration rate of the order of 6.5 gal. per square foot. When an excess 
of asbestos tailings is present at the filtration, the cake which is 
formed on the filter is also easier to handle and to dispose of than the 
gelatinous silica gel cake. 
Without going into lengthy considerations, it is believed that the 
particular structure of the tailings contributes to a large extent to the 
improved filtrability of the reaction mixture after an addition of excess 
material. 
A preferred but not exclusive embodiment of this invention calls for the 
addition of the excess of tailings, or part of the excess, when the 
initial digestion of the tailings by the acid has been completed. Such an 
approach preserves the structure of the material in excess and gives an 
increased assistance for the filtration. 
The amount of tailings added in excess can vary from 5% of the initial 
charge to about 200% of the reacted material with the acid available. 
Since the commercial value of the asbestos tailings is minimal, this 
addition does not affect the cost of the operation in a significant way. 
But the improvement of the rate of filtration by a factor of two to ten 
and the resulting ease of the operation do improve the profitability of 
the extractive operation by allowing a much faster rate of production. 
The nature of the acid used for the leaching is indifferent to the observed 
effects of excess of tailings on filtration since the improved 
filtrability is related to the presence of an excess of waste in the 
presence of the precipitated silica rather than the presence of a given 
anion, sulfate, chloride, etc., associated with the magnesium in solution. 
The evaluation of the assistance of an excess of asbestos tailings in the 
course of the filtration of the precipitated silica has been done under 
two sets of conditions: filtration by gravity and filtration under a 
differential of pressure. In each case, a noted improvement of the rate of 
filtration has been noted. Particularly with filtration under a 
differential of pressure, the effect has been important, as shown by the 
following examples.

EXAMPLES 1-4 
Filtration by gravity of the reaction mixture when the excess of waste was 
added at the onset of the reaction. 
A slurry of 100 ml of HCl 37%, 73 g of asbestos tailings (-200 mesh) and 51 
ml of water was heated at 100.degree. C. for twelve hours, under reflux 
and mechanical stirring. The reaction mixture was then filtered by gravity 
in a constant level filter with a membrane made of paper (Whatman 2 V) 
diam. 10 cm, the fluid head being kept constant at 4 cm. The volume of 
filtrate was noted after 30 minutes. 
The example was then repeated using increasing excess of asbestos tailings. 
The percentage of suspended solid was kept constant at 30% in these 
Examples by adjustment of the added volume of water. The speeds of 
filtration are reported in Table I. 
TABLE I 
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Filtration by gravity with excess at the onset of the reaction 
Excess of Volume of 
Residues Acid tailing over 
Water 
Filtrate 
Ex. 
Weight 
Mole 
Mole 
Volume 
stoichiometry: 
added 
after 30 min. 
No. 
(g) (Mg) 
(HCl) 
(ml) % residue over acid 
(ml) 
(ml) 
__________________________________________________________________________ 
1 73 0.63 
1.20 
100 5 51 21.4 
2 109.5 
0.95 
1.20 
100 58 137 26.7 
3 146 1.26 
1.20 
100 110 222 56.2 
4 219 1.89 
1.20 
100 215 392 71.6 
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EXAMPLES 5-7 
Filtration by gravity of the reaction mixture when the excess of waste was 
added at the end of the reaction. 
A suspension of 100 g of asbestos tailings (mesh -200) in 144 ml of HCl and 
62 ml of H.sub.2 O was heated at 95.degree.-100.degree. C., under reflux, 
with good stirring, for twelve hours. The initial slurry was 30% solid 
(weight over weight) and contained 105% of the theoretical amount required 
to react with the magnesium present. 
After this contact, a weighted sample of asbestos tailings was added in one 
portion and the contact was maintained at 100.degree. C. for another 
period of two hours. With the addition of waste, water was also added in 
order to maintain the slurry at a 30% (w/w) in solid in suspension, thus 
giving comparable suspensions in terms of solids to be filtrated. The 
filtration was done as in Example 1. 
The complete example was repeated with different excess of residues and the 
results are reported in Table II. 
TABLE II 
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Filtration by gravity with excess added at the end of the reaction 
Excess of 
Initial weight Added weight 
tailing over 
Water added 
Volume of 
of residues 
Acid of residues 
stoichiometry: 
with excess 
filtrate af- 
Ex Weight 
Mole 
Mole 
Volume 
Weight 
Mole 
total residues over acid 
residues 
ter 30 min. 
No (g) (Mg) 
(HCl) 
(ml) (g) (mg) 
(%) (ml) (ml) 
__________________________________________________________________________ 
5 100 0.86 
1.81 
144 58 0.50 
50 136 28.1 
6 100 0.86 
1.81 
144 111 0.96 
101 259 30.8 
7 100 0.86 
1.81 
144 216 1.87 
201 504 59.4 
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EXAMPLES 8-14 
Filtration under vacuum 
In this series of examples, a given weight of residues (mesh -200) was 
digested with hydrochloric acid and water at 100.degree. C. for twelve 
hours, the initial slurry being fixed at 30% solid (w/w). In some 
examples, a measured excess of wastes was added at the end of the twelve 
hour reflux and heated at 100.degree. C. for a further two hours. In other 
cases, the excess was present at the beginning of the reaction. Also, in 
some cases, an excess of acid rather than an excess of residues was used. 
The filtration was done by vacuum (5 cm Hg) using a Buchner funnel (diam. 
10 cm) and a Whatman 2 V filter paper. The liquid was collected in a 
graduated flask and measured against time. Results are reported in Table 
III. 
TABLE III 
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Filtration under vacuum 
Excess of 
Initial weight Added weight of 
tailing over 
Time required 
of residues 
Acid residues after 12 hrs. 
stoichiometry: 
to filtrate 
Ex. 
Weight 
Mole 
Mole 
Volume 
Weight 
Mole 
Total residues over acid 
100 ml 
No. 
(g) (Mg) 
(HCl) 
(ml) (g) (Mg) 
(%) (sec) 
__________________________________________________________________________ 
8 100 0.86 
1.84 
144 0 0 -7 1280 
(excess acid) 
9 100 0.86 
1.72 
135 0 0 0 1174 
10 100 0.86 
1.72 
135 5 0.04 
5 510 
11 200 1.73 
1.65 
137 0 0 100 91 
12 120 1.04 
1.72 
135 0 0 20 196 
13 100 0.86 
1.65 
137 100 0.86 
100 45 
14 100 0.86 
1.65 
137 10 0.09 
15 224 
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EXAMPLES 15-18 
Digestion by HNO.sub.3 and H.sub.2 SO.sub.4 
In order to evaluate the effect of the acid on the speed of filtration, the 
Examples No. 8 and No. 14 have been repeated as Examples 15 and 16 using 
the same technique as described in Example 8 except that the acid reagent 
was sulfuric acid in Examples 15 and 17 or nitric acid in Examples 16 and 
18. From the results reported in Table IV, it can be noted that these 
substitutions gave essentially equivalent speeds of filtration. 
TABLE IV 
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Digestion with H.sub.2 SO.sub.4 or HNO.sub.3 
Time required to 
Ex. Acid used filtrate 100 ml 
No. Acid Mole Volume (sec.) 
______________________________________ 
15 H.sub.2 SO.sub.4 
0.92 145 1298 
16 HNO.sub.3 
1.84 144 1276 
17 H.sub.2 SO.sub.4 
0.83 137 215 
18 HNO.sub.3 
1.65 137 231 
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EXAMPLES 19-20 
Effect of dilution 
Examples 8 and 12 were repeated as described except that the volume of the 
initial solution was increased, thus giving a slurry of 22% in the case of 
Example 19 and 27% in the case of Example 20. The time of filtration is 
reported in Table V. 
TABLE V 
______________________________________ 
Effect of dilution 
Water Slurry Time required 
Ex. Residues Acid added % solid 
to filtrate 
No. (g) (ml) (ml) w/w (%) 
100 ml (sec.) 
______________________________________ 
19 100 145 300 22 918 
20 120 135 310 27 137 
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