Treatment of brine elutriate

The invention relates to the treatment of brine and more particularly the treatment of the brine elutriate drawn off after evaporation of the brine at an elevated temperature to separate some of the sodium chloride thereon. The crude brine normally contains sodium sulphate and potassium chloride, sometimes to relatively high levels, and the invention is directed primarily at the separation out of these salts. To achieve this, the brine elutriate is cooled, in chamber (12) whereby an NaCl--/KCl--/Na.sub.2 SO.sub.4 -- salt mixture is separated off by crystallization. The remaining parent solution can be returned to the evaporation stage (10), without disturbing the equalized balance of the secondary salts in the brine to be processed. The NaCl present in the salt mixture can be recovered by deposition with refrigerated water and separation of the Na.sub.2 SO.sub.4.

BACKGROUND TO THE INVENTION 
The invention relates to the treatment of brine containing sodium sulphate 
(Na.sub.2 SO.sub.4) and potassium chloride (KCl) content in solution. 
Particularly it relates to a treatment of brine elutriates after 
evaporation treatment to extract some of the sodium chloride (NaCl) in the 
brine. Such elutriates have considerable levels of sodium sulphate and 
potassium chloride in solution. 
In the extraction of sodium chloride from natural deposits a method is used 
which consists of dissolving out the salt deposits by the addition of 
water, and of evaporating an almost saturated crude brine thus obtained 
following conventional purification, in order to crystallize out the 
dissolved sodium chloride. The crude brine produced in the dissolving step 
and which is to be processed usually contains further secondary components 
in addition to NaCl, which on the one hand may be undesirable in the 
crystallized product, the NaCl, and on the other hand may lead to 
intrusive incrustations (boiler scale) on the apparatus, heat transmission 
surfaces and pipe ducts in the evaporation process. 
The so-called hardness-causing salts can be removed from the crude brine by 
chemical pretreatment, in a purification stage in which magnesium is 
precipitated as hydroxide and calcium as carbonate. Components of higher 
solubility, such as for example, sulphates and potassium compounds, remain 
in the purified brine solution. 
To prevent the crystallization of the higher solubility components with the 
sodium chloride in the evaporation stage, the stage is controlled such 
that the concentration of the higher solubility components remains below 
their saturation. As a result no crystallization and no mixing with the 
product, the sodium chloride, takes place. To achieve this an evaporation 
ratio is adhered to from which a permanent residual solution or parent 
solution results. The parent solution contains components which are not to 
be crystallized, with a residual content of sodium chloride. Whether the 
evaporation process takes place continuously or in stages, the parent 
solution is drawn off as elutriate. 
For sulphate the permissible threshold concentration in the above treatment 
is dependent on the operation temperature; e.g., 4.5% at 100.degree. C. 
approximately, and 5.2% at 5O.degree. C. approximately, as Na.sub.2 
SO.sub.4 at the same time in addition approximately 26% or 24% NaCl 
respectively will be in solution. 
The parent solution, or brine elutriate, is drained off in more simple 
methods or conveyed to another use. According to known methods the salting 
out effect is used which, by the admixture of easily soluble sulphates 
leads to precpitation of CaSO.sub.4. For this purpose, the parent solution 
which is almost saturated in Na.sub.2 SO.sub.4 can be mixed with the crude 
brine which is to be purified and a salting-out effect, with precipitate 
of CaSO.sub.4, is brought about. 
Insofar as in the above treatment of the elutriate the sulphate content in 
the crude brine is contained to the extent of the calcium sulphate 
solubility, a balanced condition will arise between brine purification, 
evaporation and elutriate, and the available brine can be practically 
fully utilized, in which the brine elutriate is returned and is subjected 
once more to evaporation. However, if the sulphate content in the crude 
brine is higher; i.e., above the calcium sulphate solubility, then no 
equalized balance will arise and a part of the sulphate must be removed 
from the system, either by discarding a part of the elutriate brine, or by 
precipitation of a sulphate content by cooling in the range of the 
Glauber's salt phase; i.e., for example to 0.degree. C., or by chemical 
treatment, for example calcium chloride (CaCl.sub.2)--admixture. The 
balance of the sulphate introduced with the crude brine and elutriate must 
be equalized. 
If the crude brine also contains a proportion of potassium chloride (KCl), 
then this component will accumulate in the above methods and finally will 
crystallize out with the NaCl and may be considered as an undesired 
constituent part. To prevent this, a part of the elutriate is discarded in 
a quantity such that the balance of introduced and elutriated potassium is 
equalized. 
In order to avoid the above-mentioned loss of NaCl in the elutriate brine 
which is to be discarded, known methods consist of precipitating out the 
sodium sulphate for the most part through the addition of calcium chloride 
(CaCl.sub.2) and through evaporation at a high temperature (e.g. 
120.degree. C.), and again crystallizing out NaCl until almost the 
saturation of KCl is reached. Additionally, as is known from the potassium 
industry, by temperature lowering KCl can be crystallized out and 
thereafter once more NaCl can be crystallized out by further evaporation 
in the higher temperature range. Whatever the case may be, in the end a 
residual quantity, which is again enriched with sodium sulphate, is either 
discarded or, if required, may be returned into the brine purification 
process. The disadvantage of this method lies in that an additional 
chemical treatment is necessary, requiring calcium and sodium carbonate 
and corresponding apparatus. 
SUMMARY OF THE INVENTION 
The invention is directed at a treatment of brine elutriate of the type 
described, in which the excess secondary salts, namely sulphates and KCl, 
are removed from the elutriate, so that the separated solution can be 
returned for renewed evaporation without disturbing the balance of the 
secondary salts. In this way, a maximum proportion of the NaCl present in 
the crude brine can be extracted as a product through evaporation. 
Advantageously, the treatment is to be carried out in a simple manner and 
without the addition of supplementary chemicals. According to the 
invention the drawn off elutriate with NaCl, KCl-- and Na.sub.2 SO.sub.4 
-- content, is cooled for the simultaneous crystallization of the three 
salts and the precipitated NaCl/KCl/Na.sub.2 SO.sub.4 salt mixture is 
separated from the parent solution. Advantageously the cooling of the 
elutriate takes place by flash evaporation. 
The invention also provides a process for treating crude brine containing 
sodium sulphate and potassium chloride in solution in addition to sodium 
chloride, which method comprises extracting a portion of the sodium 
chloride from the brine in an evaporation stage, and treating the 
elutriate comprising the residual solution in accordance with the above 
method. 
The parent solution, which is separated off after cooling, can be used as 
an addition to the crude brine which is to be processed. To utilize the 
remaining salt content the salt mixture, can be deposited with 
refrigerated water in the range of the Glauber's salt phase for separation 
of the decahydrate-residue and of the potassium chloride from the parent 
solution. 
With a simultaneous excess of sulphate and potassium, and corresponding 
proportions of both components, a simultaneous crystallization occurs in 
the method of the invention as the elutriate brine is cooled after drawing 
off from the evaporator. If an elutriate brine is cooled, for example, 
from 118.degree. C. to 45.degree. C. by flash evaporation, the following 
conditions arise: 
______________________________________ 
Elutriate Expansion Vapour 
Residual 
g/kg 118.degree. 
resp. salt precipitate 
solution 45.degree. 
______________________________________ 
Na.sub.2 SO.sub.4 46.0 
14 32 
KCl 120 25 95 
NaCl 181 27 154 
H.sub.2 O 653 
103 550 
______________________________________ 
Through corresponding adjustment of the concentration of secondary salts in 
the pure brine and expedient selection of the expansion gradient it is 
therefore possible to separate off the secondary salts which are 
introduced with the crude brine and to arrive at an equalized balance of 
the secondary salts in the brine which is to be processed. Thus, the 
advantages of the return of the parent solution into the brine 
purification process can be utilized, without a portion of the NaCl having 
to be discarded. 
The salt residue from the flash evaporation contains, according to the 
above example, in addition to the secondary salts which are to be 
separated, a proportion of NaCl, which is to be considered as a loss if 
the residue is discarded. By comparison, however, the parent solution to 
be discarded would contain per kg secondary salt at the same time 2.6 kg 
NaCl, whilst with the salt residue only approximately 0.7 kg NaCl would be 
given up. If in addition the shift in the KCl/Na.sub.2 SO.sub.4 proportion 
of the resulting pure brine is taken into account, which occurs between 
the process with discarding of the elutriate brine and that with 
discarding of the expansion salt, then there even results a ratio in the 
NaCl losses of approximately 1:7.8 in favour of the second proposed 
method. 
We have found also that the salt mixture can be processed to a large extent 
from the flash evaporation described above. For example, the mixture may 
be mixed with cold water (e.g. at 5.degree. C.), whereby the entire NaCl 
and in addition selectively the whole, or a part, of the KCl is dissolved, 
whilst only a very small part; e.g. 2% of Na.sub.2 SO.sub.4 goes into 
solution. By evaporation at an increased temperature; (e.g. 120.degree. 
C.) NaCl can be crystallized out again from this solution, and on reaching 
the saturation range of KCl, this can be brought again to precipitation by 
cooling, or respectively by renewed flash evaporation. Alternatively, 
insofar as the KCl is only dissolved to a restricted extent, the acquired 
solution may also be returned to the brine purification process, in order 
to convey it to the evaporation process together with the pure brine 
current. The quantity of elutriate and the original temperature in the 
flash evaporation are then to be adjusted to the easily altered ratios.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Crude brine generated by dissolving the natural deposits with water emerges 
from a bore hole 1. In addition to the approximate 25% NaCl which is to be 
recovered, the brine also contains parts of potassium chloride (KCl), 
sodium sulphate (Na.sub.2 SO.sub.4), magnesium sulphate (MgSO.sub.4) and 
calcium sulphate (CaSO.sub.4). The crude brine arrives via a duct 2 to the 
crude brine purification section 3, where it is mixed via a duct 4 with 
calcium hydroxide (Ca(OH).sub.2) and via a duct 5 with soda (Na.sub.2 
SO.sub.3). Magnesium hydroxide (Mg(OH).sub.2) is precipitated out of the 
solution at 6, calcium sulphate (CaSO.sub.4) at 7, and calcium carbonate 
(CaCO.sub.3) at 8. 
The brine, thus purified, at 9 contains KCl and Na.sub.2 SO.sub.4 in 
addition to the NaCl. The purified brine is subjected to evaporation 
treatment at 10, at which the concentration of the components of higher 
solubility remains below their saturation. Water (H.sub.2 O) is extracted 
as steam from this solution and a major part of the NaCl is extracted as a 
crystallized product. The residue is withdrawn after the evaporation as 
brine elutriate. 
The brine elutriate contains the residual part of the NaCl and the 
proportion of KCl and Na.sub.2 SO.sub.4 which have remained in the 
solution during the evaporation. The elutriate brine is withdrawn at an 
elevated temperature which corresponds to the processing temperature of 
the evaporation. It is then cooled in a cooler, preferably by flash 
evaporation in chamber 12, to 45.degree.-55.degree. C., whereby so much 
NaCl/KCl/Na.sub.2 SO.sub.4 -mixture is separated off by crystallization 
that the parent solution, which is separated off at a separator 13 can be 
returned via ducts 17 and 18 as admixture the crude brine which is to be 
processed in purifier 3, whilst maintaining an equalized balance of the 
secondary salts. 
The salt mixture, crystallized out by cooling at 12 and accumulating on 
separation at 13, contains the residue of the NaCl content and also 
residues of the KCl-- and of the Na.sub.2 SO.sub.4 content. In order to 
also recover the NaCl residue, the salt mixture is carried through a duct 
14 to a deposition section 16, where through the addition of refrigerated 
water 15 the salts, particularly the NaCl, are again dissolved. A portion 
of the Na.sub.2 SO.sub.4 remains as residue in the range of the Glauber's 
salt phase and is separated off at the separator 19. The parent solution 
separated here with the dissolved NaCl-- and KCl-- component can be 
returned via ducts 20 and 18 back to the brine which is to be evaporated 
and the residual NaCl can be recovered as a product at the evaporation 
stage 10. 
In order to recover or separate the residual KCl content or the residual 
NaCl content in the parent solution separated at 19, the parent solution 
present at 19 may, if desired, be carried via a duct 22 to an evaporation 
section 23, where at an elevated temperature the residual NaCl is 
crystallized out. Subsequently the solution is passed on for cooling in a 
flashing chamber 24, where the residual KCl is crystallized out. The 
residual solution remaining thereafter can be returned via ducts 25 and 18 
as admixture to the brine which is to be processed.