Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment
Reexamination Certificate
2001-05-21
2002-10-08
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
C429S010000, C429S010000, C429S010000, C429S006000
Reexamination Certificate
active
06461495
ABSTRACT:
The present invention relates to a process for the removal of sulfate ions and, in particular, to a process for the removal of sulfate ions from an electrolyte stream containing halide ions in which they are an interferant or unwanted species.
U.S. Pat. No. 4,485,154 discloses an electrically chargeable anionically active reduction-oxidation system using a sulfide/polysulfide reaction in one half of the cell and an iodine/polyiodide, chlorine/chloride or bromine/bromide reaction in the other half of the cell. The two halves of the cell are separated by a cation exchange membrane.
The overall chemical reaction involved, for example for the bromine/bromide-sulfide/polysulfide system is
Br
7
+S
2−
=2Br+S Equation 1
The electrochemical reaction takes place in separate but dependent bromine and sulfur reactions. When discharging, the bromine reaction takes place on the +ve side of the membrane and the sulfur reaction takes place on the −ve side of the membrane. The reaction goes from left to right and metal ions flow from the −ve side of the membrane to the +ve side of the membrane to complete the circuit. When charging occurs the reaction goes from right to left and metal ions flow from the +ve side of the membrane to the −ve side of the membrane to complete the circuit. During extended cycling of the cell anionic species diffuse through the membrane. Thus, although a cation. selective exchange membrane is used, sulfide ions diffuse from the sulphide/polysulfide electrolyte into the bromine/bromide electrolyte where they will be oxidised by the bromine to form sulfate ions according to the reaction scheme below.
HS
−
+4Br
2
+4H
2
O=8Br
−
+SO
4
2−
+9H
+
Equation 2
The occurrence of this reaction is disadvantageous because the presence of sulfate ions in the bromine/bromide electrolyte can result in the precipitation of a sulfate salt. This precipitate can cause scaling within the apparatus, blockage of electrolyte ducts and contamination of the electrodes. In the system described above sodium ions are commonly used as counter-ions and so in this case sodium sulphate is precipitated.
A number of methods are known in the art for the removal of sulfate ions from solution or for the prevention of their precipitation as a salt.
The chlor-alkali industry uses a method known as “purging” wherein the solution is simply diluted to prevent the sulfate salt from reaching its solubility limit and precipitating out of solution. This method is not suitable in the present process however since the electrolyte is used repeatedly over a large number of cycles whereas in the chlor-alkali processes it may only be recirculated for a few cycles.
Another option is the addition of soluble barium hydroxide and filtration of the resulting barium sulfate precipitate which is highly insoluble in aqueous solutions. This method suffers from the disadvantages that barium salts are very expensive and their toxicity presents environmental problems on disposal. Furthermore, if too much soluble barium is added to the electrolyte it can adversely affect the cation exchange membrane by substituting onto the cation-binding groups (commonly sulfonic acid groups) causing a consequential increase in resistivity, of the membrane. Addition of a soluble calcium salt such as calcium chloride and filtration of the resultant calcium sulfate precipitate is also possible. Calcium salts are less expensive than barium salts and do not cause the environmental hazards associated with the use of barium. However, calcium sulfate is some 1200 times more soluble than barium sulfate, increasing the risk of damage to the membrane and making the process less efficient for reducing the concentration of sulphate. U.S. Pat. No. 4,747,917 discloses a process which includes adding a brine-soluble calcium salt to a brine solution in order to reduce the sulfate ion concentration.
Another possibility for the removal of sulfate is the use of anion exchange resins which selectively remove the sulfate ions. These exchange sulfate ions with, for example, bromide or chloride ions. However, these resins are costly and at present they are not very selective and may remove bromide ions in addition to sulfate ions. They also require periodic regeneration which is a costly process. U.S. Pat. No. 4,556,463 shows the use of an anion exchange medium to remove the sulfate ion prior to returning the solution to the cell.
U.S. Pat. No. 4586993 shows the use of calcium salt addition to form a precipitate of calcium sulfate followed by the use of an ion exchange column as described above.
U.S. Pat. No. 5,587,033 discloses a nanofiltration process for selectively removing multivalent ions from aqueous solution. However, as with ion exchange resins the selectivity is not perfect and considerable amounts of bromide are still lost in the process.
Another possibility for removal of sulfate ions is the crystallisation and separation of a sulfate salt such as sodium sulfate. EP 498919 discloses the use of combinations of refrigeration and crystallisation, and brine recirculation techniques in systems to make brine for electrolysis, which techniques reduce the sulfate ion content of depleted brine. DE 3216418 discloses a process for the cooling and refrigeration of a side-stream of depleted brine so as to crystallise sodium sulfate from the solution. SU 1520012 discloses a process for removing sodium sulfate from brine wherein the brine is subjected to alkali treatment to pH 7.5-9.0 and saturation with NaCl, to bring the ratio of sulfate to chloride to 1:(3-6). Then it is cooled to −20° C. and the crystals of sodium sulfate obtained are separated. However, the crystallisation techniques described in the prior art all involve cooling the sulfate contaminated solution which represents a considerable expense.
It is an object of the present invention to provide a method for the removal of sulfate ions from a halogen/halide electrolyte which addresses the problems associated with the prior art.
It is a further object of the present invention to provide a method for the removal of sulfate ions from the halogen/halide electrolyte of a halogen/halide sulfide/polysulfide electrochemical reduction-oxidation system in which the sulfate ions are a contaminant or interferant wit in the halogen/halide electrolyte without interrupting the normal operation of sale system.
Accordingly, the present invention provides a method for the removal of sulfate ions from an electrolyte of an electrochemical reduction-oxidation system wherein said electrolyte comprises a halogen and in which the sulfate ions are a contaminant or interferant, which method comprises the steps of:
(i) increasing the halide concentration in the electrolyte by electrochemical reduction of the halogen,
(ii) crystallising a sulfate salt out of the electrolyte, and
(iii) separating the electrolyte from the crystallised sulfate salt.
Increasing the halide concentration in the electrolyte in step (i) decreases the solubility of the sulfate salt in the electrolyte and therefore promotes the crystallisation of the sulfate salt in step (ii).
The maximum halide concentration attainable will depend on the halogen content of the electrolyte, however, it is preferable that the halide concentration be increased to at least approximately 4M, preferably approximately 5M, and most preferably approximately 6M.
Preferably, before or during step (ii), a seed crystal is added to the electrolyte. In the presence of a seed crystal the sulfate crystallises into relatively large crystals of the sulfate salt which are easier to separate and can be washed free of any bromide contained in the mother liquors. In the absence of a seed crystal, crystallisation does occur but it takes place by spontaneous nucleation which is less desirable and produces smaller crystals which are more difficult to separate. Also, in the absence of a seed crystal, crystallisation tends to occur on the surfaces of the apparatus causing sc
Male Stewart Ernest
Mitchell Philip John
Morrissey Patrick John
Antonelli Terry Stout & Kraus LLP
Phasge Arun S,.
Regenesys Technologies Limited
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