Method for desalinating and demineralizing solutions containing acids and/or metal salts

A process for the desalination and demineralization of solutions conmining acids and/or metal salts comprising a number of steps after introducing a solution to be treated. Initially, chemical agents are added to precipitate predetermined salts. Afterward clarification occurs. An inhibitor is then introduced for inhibiting further precipitation of the salts. The process proceeds by concentrating the salts in 5%-80% of tile flow of the solution to be treated into a supersaturated concentrate to form a concentrated fraction, the remaining fraction being a demineralized permeate. There is then removal of the effect of the precipitation inhibitor allowing precipitation of crystallizable supersaturated salts in the concentrate.

FIELD OF THE INVENTION 
The present invention relates to the qualitative and quantitative reduction 
of the content of precipitable ions in aqueous or other solutions and to 
the use of these solutions. 
The invention more particularly applies to the desalination and to the 
demineralization of water, in particular of potable water or of process 
water, for the purpose of satisfying potability restrictions or 
restrictions related to the use of water and to the recycling of waste 
water in industry. 
BACKGROUND OF THE INVENTION 
Various desalination or demineralization or crystallization processes are 
currently known. The main processes will be restated hereinbelow with the 
disadvantages which are associated with them: 
the precipitation of salts of very slight solubility which are separated 
from the treated water by settling or filtration. Thus, French Patent 
FR-A-2,339,575 discloses a process and a plant for the treatment of water 
by crystalline precipitation and settling which consists in producing a 
homogeneous dispersion of water to be treated, of reagents necessary for 
the crystalline precipitation and of sludges separated from the water to 
be treated and in settling this dispersion, the crystalline precipitation 
reaction being completed during the settling. The disadvantage of this 
known technique lies in the use of reagents which can be very expensive 
for the purpose of reducing their contents of salts of very slight 
solubility below their solubility; 
percolation through a bed of ion-exchange resins which makes it possible to 
replace all or part of the ions by other ions. This solution exhibits the 
disadvantage of high consumption of chemical reagents and that of the 
production of saline, indeed highly saline, regeneration eluates; 
filtration through reverse osmosis or nanofiltration membranes which are 
selective with respect to certain salts and which concentrate them in a 
fraction of the treated flow. The disadvantage of this technique lies in 
the production of a filtration concentrate which represents a significant 
fraction of the flow of the water to be treated (of the order of 10 to 
70%, depending on the initial salinity and the degree of conversion 
adopted); 
evaporation, which produces demineralized water by condensation of the 
vapor produced and which concentrates the salinity into a brine. This 
solution leads to very high energy consumption and the production of a 
saline brine which has to be either discharged or crystallized, the latter 
stage resulting in a very high investment cost; 
electrodialysis, involving ion-exchange membranes which make it possible to 
extract the inorganic salts from the solutions and to concentrate them 
into a brine, the purified water flow being separated from the brine flow 
by the said ion-exchange membranes. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention provides for the introduction of a process for 
desalination or demineralization with the possibility of crystallization 
which does not exhibit the disadvantages of the currently known solutions 
briefly restated hereinabove. 
The desalination and demineralization process according to the present 
invention mainly relates to solutes, the solutions of which can be 
supersaturated by addition of chemical reagents. 
The solutes can be, inter alia, inorganic or organic acids and their salts. 
Consequently, the subject-matter of the present invention is a process for 
the desalination, demineralization and optionally crystallization of 
solutions containing acids and/or metal salts, characterized in that it 
comprises the following successive stages: 
a) conditioning the solutions, clarified before-hand, using a chemical 
reagent which is an inhibitor of the precipitation of salts, optionally 
followed by a filtration; 
b) concentrating the salts into a supersaturated brine representing a 
fraction of the flow of between 5 and 80% of the flow of the solution to 
be treated; 
c) in this brine, reducing, indeed removing, the effect of the 
precipitation inhibitor and precipitating the crystallizable 
supersaturated salts in the concentrated fraction. 
Still other objects and advantages of the present invention will become 
readily apparent by those skilled in the art from the following detailed 
description, wherein it is shown and described only the preferred 
embodiments of the invention, simply by way of illustration of the best 
mode contemplated of carrying out the invention. As will be realized, the 
invention is capable of other and different embodiments, and its several 
details are capable of modifications in various obvious respects, without 
departing from the invention. Accordingly, the drawings and description 
are to be regarded as illustrative in nature and not as restrictive.

DETAILED DESCRIPTION OF THE INVENTION 
According to an embodiment of the process according to the invention as 
defined hereinabove, an additional preliminary stage of neutralization can 
be provided consisting of a chemical modification of the solution for the 
purpose of reaching the solubility product of the ion or ions to be 
removed. According to the invention, this additional stage is carried out 
by introducing reagents, for example addition of calcium in the form of 
calcium chloride or calcium hydroxide, in order to precipitate the 
sulphates or fluorides. According to the invention, this optional 
additional preliminary stage of neutralization may or may not be followed 
by a clarification stage. 
According to the invention, a final additional stage can be provided 
consisting of recycling the concentrated fraction at the head of the 
clarification stage, before or after the stage of 
precipitation/crystallization of the supersaturated salts. 
According to the present invention, the inhibitors of the precipitation of 
the salts removed are chosen from the group comprising phosphonates, 
polyacrylates and polycarboxylates. 
According to an embodiment of the process according to the invention, the 
stage of demineralization of the solution and of concentration of the 
salts is preferably carried out by filtration through reverse osmosis or 
nanofiltration membranes, which are selective or otherwise with respect to 
the ions to be removed, or by evaporation and/or crystallization and/or 
dialysis and/or electrodialysis and/or electro-electrodialysis. 
According to the invention, the reduction, indeed removal, of the effect of 
the precipitation inhibitor can be carried out using a metal with a 
valency which may or may not depend on the quality of the precipitation 
inhibitor, in particular a trivalent metal, and/or by increasing the pH 
and/or by increasing the temperature and/or by the action of a chemical 
oxidizing agent and/or by the action of any other chemical additive, 
inorganic or organic, which makes it possible to neutralize, in all or in 
part, the activity of the precipitation inhibitor. 
As a non-limiting example, results obtained experimentally during the 
implementation of the process according to the invention on a 
semi-industrial scale have been given hereinbelow. The effluents treated 
during this operation were waters originating from the mining industry. In 
this implementational example, which is without any limiting nature, the 
following steps were combined: conditioning the waters, saturated with 
calcium sulphate, with a polyacrylate, filtration through a nanofiltration 
membrane and destruction of the polyacrylate with a ferric iron salt. 
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The starting raw water exhibited the following characteristics: 
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conductivity: 7 .mu.S/cm 
calcium: 725 mg/l 
sulphates: 2200 mg/l 
turbidity: 0.25 NTU 
pH: 7.9 
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This water was conditioned using an inhibitor of precipitation of calcium 
sulphate composed of 2.5 mg/l of polyacrylate. 
The water, thus conditioned, was subsequently filtered through a 
nanofiltration membrane (operating pressure: 7 bar; degree of conversion: 
66%). 
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The characteristics of the permeate and of the 
concentrate obtained were respectively as follows: 
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permeate: 
flow: 66% of the feed flow 
conductivity: 5 .mu.S/cm 
sulphates: 100 mg/l 
turbidity: 0.2 NTU 
concentrate: 
flow: 34% of the feed flow 
conductivity: 12.5 .mu.S/cm 
calcium: 2150 mg/l 
sulphates: 6600 mg/l 
turbidity: 0.35 NTU 
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The reduction, indeed elimination, of the inhibiting effect on 
precipitation of the polyacrylate was achieved using 16 mg/l of ferric 
iron in the concentrate. After self-crystallizing for one hour, the 
content of soluble polymer was below the detection threshold of the 
analytical method, i.e. 0.2 mg/l. The content of soluble calcium sulphate 
was in the region, to within about 5%, of the theoretical solubility of 
calcium sulphate. 
Two implementational examples of plants employing the process according to 
the present invention have been represented diagrammatically in the 
appended drawings, as non-limiting illustration. 
FIG. 1 refers to a closed-circuit desulphatation plant with recycling of 
the concentrate from the nanofiltration stage. 
It is seen, in this FIG. 1, that the water to be treated is subjected 
beforehand in 1 to a neutralization-precipitation treatment with 
introduction of calcium compounds (Ca(OH).sub.2, CaCl.sub.2), followed by 
flocculation in 2 and then by settling in 3 (in this implementational 
example, a lamellar settling tank can be used). In this device, a portion 
of the sludges is recycled at the head of the plant, the remainder being 
subjected to a dewatering stage as shown in this FIG. 1. The clarified 
water is conditioned by the inhibitor of precipitation of the calcium 
sulphate, before being filtered, for example, through a sand filter 4. The 
stage of desulphatation of the water and of concentration of the sulphate 
salts is here carried out through a nanofiltration membrane 5. The 
permeate is subsequently discharged and the concentrate is recycled at the 
head of the process, as has been shown in FIG. 1. The crystallization of 
the supersaturated salts in the concentrate is promoted by virtue of 
mixing the crystals from the settling tank with the concentrate, for 
example via a jet pump, thus using the available energy on the 
concentrate. 
FIG. 2 refers to a desulphatation plant operating in an open and/or 
semi-open circuit, with independent treatment and total or partial 
discharge of the concentrate from the nanofiltration. 
The same plant is found in this FIG. 2 as that described above with 
reference to FIG. 1, the difference being that the concentrate from the 
nanofiltration is subjected to crystallization-coagulation treatment in 7, 
then flocculation in 8 and finally settling in 9, the water saturated with 
calcium sulphate subsequently being discharged. 
Mention may be made, among the advantages of the process which is the 
subject-matter of the present invention, of the following in particular: 
low operating costs with respect to reagents (calcium salts, calcium 
chloride or calcium hydroxide, inhibitor of precipitation of salts, metal 
salt), 
removal of anions, such as sulphates, phosphates, carbonates or fluorides, 
and of silica in the form of precipitates of metal salts or of hydroxides 
which can be easily dewatered (this list not being limiting), 
removal of cations, such as calcium, strontium, barium and other heavy 
metals, in the form of simple or complex metal salts or in the form of 
hydroxides, 
possibility of producing a flow of treated water (degree of conversion) in 
the region of 100% of the flow of raw water (the difference arising from 
the water discharged with the metal precipitates). 
It remains, of course, that the present invention is not limited to the 
implementational or production examples described and/or represented here 
but that it encompasses all the alternative forms thereof.