Brine treatment for trace metal removal

Process for the removal of trace metals from alkali halide brines. The addition of controlled amounts of magnesium ions to brine and subsequent precipitation of magnesium hydroxide removes metal contaminants, and provides a brine suitable for use in the electrolytic production of chlorine and alkali metal hydroxide.

FIELD OF THE INVENTION 
This invention relates generally to a process for the treatment of brine, 
and more particularly concerns the introduction of magnesium ions into 
brine followed by precipitation as magnesium hydroxide to remove metallic 
impurities. 
BACKGROUND OF THE INVENTION 
Alkali metal chlorides in the form of natural or artificial salts and 
brines have widespread use in many industrial processes. In particular, 
the electrochemical production of chlorine, alkali metal hydroxide, and 
related products makes extensive use of alkali metal chloride brine. 
Nearly all commercial salts and brines are contaminated to some degree by 
various impurities, and while the level at which such impurities may be 
tolerated is dependent upon the specific process and method of operation, 
it is desirable in electrochemical uses to obtain a brine of comparatively 
high purity. 
The presence of metallic impurities is of considerable concern in the 
electrolysis of brine to produce chlorine and alkali metal hydroxide, 
since trace metals encourage the evolution of hydrogen in the 
electrochemical cell. Such hydrogen generation is undesirable, as the 
combination of hydrogen, chlorine and oxygen forms an explosive mixture 
over a wide range of proportions. In practice it is preferred to limit the 
amount of hydrogen present in the chlorine to less than one percent. This 
is especially critical where the chlorine product is to be liquefied or 
absorbed, since the proportion of hydrogen in the remaining gas may 
quickly rise into the explosive range. 
The effect of metallic impurities on hydrogen gas evolution in chlor-alkali 
cells has been extensively studied, and it has been determined that heavy 
metals such as vanadium, antimony, molybdenum and arsenic have a 
considerable catalytic effect on hydrogen formation in the cell. Many 
other metals such as aluminum, calcium, and magnesium also promote 
hydrogen evolution, and it has been found that combinations of two or more 
metals often have more effect than do the same metals taken separately, 
e.g. magnesium and iron form a synergistic pair. It has thus been the 
practice to reduce metallic impurities in electrolytic brine to the lowest 
possible level. 
In the conventional purification processes, calcium has been removed from 
brine by the addition of alkali metal carbonate and the resultant 
precipitation of calcium carbonate. Iron and magnesium impurities have 
been removed by precipitation as the hydroxides, usually by the addition 
of alkali metal hydroxide. The sulfate radical is generally removed by the 
addition of a barium salt such as barium carbonate or barium chloride, 
which brings about the precipitation of barium sulfate. In these 
precipitation processes a coagulation, settling or filtering operation is 
used to rid the brine of the precipitated impurities prior to use. 
However, these conventional purification techniques often fail to reduce 
the level of metals such as aluminum and heavy metals such as antimony, 
arsenic, molybdenum, and vanadium to the degree necessary for satisfactory 
use of the brine in electrochemical cells. It would therefore be desirable 
to provide a straightforward, efficient method for the removal of trace 
metal impurities from brine. 
SUMMARY OF THE INVENTION 
It has been discovered that the addition of magnesium (Mg.sup.+2) ions to 
alkali metal chloride brine may be advantageously used to remove metallic 
impurities from the brine. Sufficient brine-soluble magnesium compound is 
introduced into the brine to establish a Mg.sup.+2 concentration of at 
least 5 parts per million (ppm). The alkalinity of the brine is then 
adjusted to a pH of at least 8 to precipitate magnesium hydroxide. The 
Mg(OH).sub.2 precipitate is a large gelatinous floc which settles, 
trapping other undesirable metals present in the brine. Separating the 
precipitate from the brine effectively removes both the magnesium and the 
entrapped metal contaminants. 
The process of the invention provides brine having a trace metal content 
suitable for the most sensitive chlor-alkali cell uses, and has the 
additional advantage of being easily adaptable to the brine purification 
processes generally in use in the industry. The process is unconventional 
in that prior brine purification processes have emphasized the importance 
of removal of magnesium contamination from the brine, rather than 
deliberate addition. However, it has been discovered that unless the 
magnesium content of the brine prior to treatment is higher than about 
five ppm, other metal impurities will not be effectively removed by the 
conventional purification techniques. 
DETAILED DESCRIPTION OF THE INVENTION 
In the practice of the process an alkali metal chloride brine, usually an 
aqueous solution of sodium chloride, is treated by the addition of soluble 
magnesium to obtain a level of at least about 5 ppm Mg.sup.+2 in the 
brine. The magnesium is normally introduced in the form of magnesium 
chloride to avoid contamination of the brine with additional ionic 
species, although any brine-soluble magnesium salt is effective in 
achieving the object of the treatment. The magnesium salt can be in either 
solid or solution form. Levels of Mg.sup.+2 less than about five ppm, 
which occur often in the brines used for chlor-alkali production, have 
been found to be ineffective in removing many of the other undesirable 
metal impurities. Much higher levels of Mg.sup.+2 may be helpful in 
removing some of the contaminants, however there is little incentive to 
exceed Mg.sup.+2 concentrations of about 200 ppm. A range of about 5 to 
about 30 ppm Mg.sup.+2 in the brine is preferred for most effective 
removal of other metal impurities. 
The pH of the brine is then adjusted to the range of about 8 to about 12 in 
order to initiate the precipitation of Mg(OH).sub.2 floc. In some 
instances the brine may be alkaline at the time the magnesium is 
introduced, and in other instances the pH is adjusted subsequent to 
magnesium addition. Both approaches are equally effective, and the choice 
is normally a matter of operating convenience. 
The pH adjustment is normally effected with sodium hydroxide, although any 
alkaline material which does not introduce undesirable salts into the 
brine is satisfactory. In the conventional brine purification process for 
chlor-alkali cells, sodium hydroxide and sodium carbonate are added to the 
brine to remove undesirable impurities, and it is convenient to use this 
alkaline treatment to adjust the pH of the magnesium-treated brine. The 
addition of Mg.sup.+2 to the brine at the same stage of purification as 
the NaOH/Na.sub.2 CO.sub.3 treatment has the additional advantage of 
allowing the use of existing clarification and filtration apparatus to 
remove the Mg(OH).sub.2 floc. Separation of the Mg(OH).sub.2 from the 
brine may otherwise be effected by conventional techniques. 
A typical brine feed for chlor-alkali electrolytic cells may contain heavy 
metal impurities such as arsenic, chromium, copper, iron, molybdenum, 
antimony, vanadium, tantalum, and titanium as well as other metallic 
impurities such as aluminum, calcium, magnesium, and strontium. Of all of 
these materials, only calcium will generally be present in more than trace 
amounts, i.e. in a concentration greater than about five ppm. The 
concentration of all of the metals except calcium and occasionally 
magnesium will normally be below one ppm. However, even at these low 
levels many of the metallic impurities can promote the undesirable 
formation of hydrogen in the cell. Thus the trace metal content of the 
feed brine must be reduced to the lowest possible level. 
The conventional purification process involves the addition of sodium 
hydroxide, sodium carbonate, and barium or calcium chloride to the brine 
in a slight stoichiometric excess in order to precipitate the impurities. 
This process is satisfactory for the removal of calcium and certain of the 
trace metal contaminants such as copper, magnesium, and iron. However, if 
the brine does not contain sufficient magnesium many impurities fail to 
co-precipitate or absorb on the floc during the conventional purification. 
The improved process of the invention is successful in greatly reducing 
the level of such impurities, particularly aluminum, antimony, arsenic, 
molybdenum, strontium, tantalum and vanadium, to acceptable 
concentrations. 
The invention further illustrated in the following specific examples.

EXAMPLE 1 
A brine solution having a concentration of 310 grams per liter (gpl) was 
prepared using reagent grade NaCl. Portions of this stock solution were 
then fortified with a soluble metal salt to a concentration of 0.005 grams 
per liter (5 ppm) of the metal to be tested. The metal-containing brine 
samples were heated to about 65.degree. C, and the various purification 
methods were simulated by the addition of either 0.15 gpl NaOH, 0.2 gpl 
Mg.sup.+2 plus 0.15 gpl NaOH (over the stoichiometric equivalent of 
Mg.sup.+2), or 0.25 gpl Na.sub.2 CO.sub.3. After addition of the 
precipitating agent the solution was gently agitated for 15 minutes, then 
was allowed to stand for 6 hours at 65.degree. C to insure complete 
precipitation. The solution was then filtered through a fine (4-5.5 
micron) glass filter. The filtrate was acidified to a pH of 2 to insure 
the dissolution of any solids, then was analyzed by atomic absorption 
spectroscopy using standard analytical methods for each metal. The results 
are set forth in Table I. 
TABLE I 
______________________________________ 
Percent Removed by: 
Metal NaOH Na.sub.2 CO.sub.3 
MgCl.sub.2 + NaOH 
______________________________________ 
Al 28 60 90 
Sb 0 0 50 
As.sup.+3 
10 0 26 
As.sup.+5 
0 0 75 
Mo 40 38 52 
Sr 0 20 50 
Ta 58 58 84 
V.sup.+4 
4 14 25 
V.sup.+5 
6 0 26 
______________________________________ 
In each test, all (&gt;99%) of the Mg.sup.+2 added in the MgCl.sub.2 /NaOH 
treatment was removed from the brine by precipitation. These tests 
demonstrate the improved removal of trace metal impurities from brine by 
the addition of Mg.sup.+2 prior to precipitation, as compared to the 
conventional precipitation methods. 
EXAMPLE 2 
A commercial NaOH/Na.sub.2 CO.sub.3 brine purification treatment was 
conducted on a laboratory scale, and compared to the treatment process of 
the invention. Two liter batches of saturated NaCl brine containing 2.5 
ppm Mg and 1.0 ppm aluminum were treated by both methods, held at 
66.degree. C for 20 minutes, then filtered through a 5 micron vinyl 
filter. The aluminum content of the filtrate was then determined. Results 
are set forth in Table II. 
TABLE II 
______________________________________ 
Al in filt- 
Reduction 
Treatment Mg(ppm) rate (ppm) in Al 
______________________________________ 
0.1 gpl NaOH 2.5 1.0 0% 
0.7 gpl Na.sub.2 CO3 
0.1 gpl MgCl.sub.2 . 6H.sub.2 O 
15 0.14 86% 
0.5 gpl NaOH 
0.6 gpl Na.sub.2 CO.sub.3 
0.5 gpl MgCl.sub.2 . 6H.sub.2 O 
62 0.26 74% 
0.5 gpl NaOH 
0.6 gpl Na.sub.2 CO.sub.3 
1.0 gpl MgCl.sub.2 . 6H.sub.2 O 
122 0.16 84% 
0.5 gpl NaOH 
0.6 gpl Na.sub.2 CO.sub.3 
______________________________________ 
Tests similar to those summarized in Table II were conducted, eliminating 
the Na.sub.2 CO.sub.3 treatment and in some cases the filtration. 
Unfiltered batches were allowed to settle and the supernatant was analyzed 
for Al content. Results are shown in Table III. 
TABLE III 
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Al in Product 
Al 
Treatment Mg(ppm) (ppm) Reduction 
______________________________________ 
0.1 gpl MgCl.sub.2 . 6H.sub.2 O 
15 0.3 70% 
0.5 gpl NaOH 
1 hr. settling 
Not filtered 
0.55 gpl MgCl.sub.2 . 6H.sub.2 O 
68 0.08 92% 
0.9 gpl NaOH 
3 hr. settling 
Filtered (.45 micron 
filter) 
1.2 gpl MgCl.sub.2 . 6H.sub.2 O 
146 0.06 94% 
0.9 gpl NaOH 
3 hr. settling 
Not filtered 
1.0 gpl MgCl.sub.2 . 6H.sub.2 O 
122 0.04 96% 
0.5 gpl NaOH 
3 hr. settling 
Filtered (.45 micron 
filter) 
______________________________________ 
EXAMPLE 3 
The purification process of the invention was carried out on the brine feed 
of a full-scale electrolytic chlor-alkali cellroom of the mercury type. 
Using the conventional NaOH/Na.sub.2 CO.sub.3 purification treatment, the 
normal NaCl brine feed to this cellroom contained about 0.2 ppm Mg and 0.1 
ppm Al. This feed brine allowed the cellroom to operate normally and 
produce chlorine gas with an acceptable hydrogen content of about 0.6 
percent. It was the normal practice to mix a small stream of waste brine 
into the main stream of brine before purification. During a period of 
several weeks, the aluminum level in this waste stream increased, while 
other parameters remained constant. The normal brine treatment was not 
satisfactory to remove the aluminum, and its concentration built up to 
about 1.2 ppm in the treated feed brine. Concurrently, the hydrogen in the 
cell gas increased to the undesirable level of 2.4 percent even after a 
current reduction to 75% of full capacity. At this point an aqueous 
solution of MgCl.sub.2 was continuously metered into the brine at a rate 
calculated to provide from 5-25 ppm Mg.sup.+2 (or an average of about 10 
ppm Mg.sup.+2) in the brine. The MgCl.sub.2 solution was injected after 
addition of NaOH and prior to the addition of Na.sub.2 CO.sub.3 to the 
brine. After mixing, the brine was allowed to settle for 8-10 hours and 
was then filtered through a down flow sand and gravel system. 
Within 24 hours after the Mg.sup.+2 treatment was begun, the Al level in 
the feed brine had been reduced by 90%, and the cellroom could again be 
operated at full capacity with normal hydrogen evolution. Within 72 hours 
the Al concentration had decreased to less than 0.02 ppm. 
While the invention has been described with particular reference to 
specific embodiments, it is evident that alternatives, modifications, and 
variations will be apparent to those skilled in the art in light of the 
foregoing description. Accordingly, it is intended to embrace all such 
alternatives, modifications, and variations as fall within the spirit and 
scope of the appended claims.