Removal of Sulfate Ions From Brine

Various alkali metal halide aqueous solutions or alkaline earth metal halide aqueous solutions, e.g., NaCl brine or MgCl.sub.2 brine, contain sulfate values or sulfate ions which are considered detrimental if the aqueous solution is intended for use in certain applications, such as in electrolytic processes. There are commercial incentives to substantially removing the sulfate ions.

It is known that hydrous zirconium oxide and other zirconium compounds, such as zirconium phosphates, are useful as inorganic ion exchangers.

An amorphous polymeric hydrous zirconium oxide is formed within the resin beads of a macroporous cation exchange resin of the sulfonic acid type and acrylic acid is polymerized within the bead matrix, thereby forming novel ion exchange composite structures which are useful in removing sulfate, borate, bicarbonate, and/or phosphate ions from alkali metal halide brines.

It is known that zirconium hydroxide is prepared by alkali precipitation of an aqueous solution of a zirconyl salt. For the present invention it is preferred that the alkali be ammonia since it is more efficiently washed out than the alkali metal hydroxides or alkaline earth metal hydroxides. Precipitation at cold (ambient) temperature gives a gelatinous product which is substantially Zr(OH).sub.4 containing about 26.5% water or more. A partially dehydrated zirconyl hydroxide, ZrO(OH).sub.2, results from drying it at elevated temperature (e.g., 100.degree. C.), or from hot-precipitation (e.g., 85.degree. C.) followed by hot drying.

Thus, in the present invention the expression "hydrous zirconium oxide" has within the purview of its meaning any of the various amorphous polymeric hydrated forms of zirconium oxide which are substantially or largely insoluble in plain water.

The macroporous cation exchange resin is one which contains sulfonic acid (SO.sub.3.sup.-) functional groups. Such resins are available commercially, such as sulfonated polymers of styrene crosslinked with divinylbenzene. For instance a macroporous cation exchange resin with SO.sub.3.sup.- Na.sup.+ groups affixed to a styrene-divinylbenzene resin structure is sold by The Dow Chemical Company under the tradename DOWEX MSC-1-NA.sup.+. It is within the purview of the present invention to use any porous resin containing sulfonic acid functional groups, so long as the resin is substantially inert or non-reactive, when used in the present invention, except for the reactivity of the sulfonic acid functionality groups.

In general, the polymeric hydrous zirconium oxide is formed within the resin beads by wetting the resin with an aqueous solution of a soluble zirconyl compound, such as ZrOCl.sub.2.8H.sub.2 O. If excessive zirconyl solution is present, it should be drained off and the resin substantially dried, such as by air-drying. The dried resin, containing the zirconyl compound is neutralized, preferably by use of NH.sub.4 OH, thereby forming polymeric ZrO(OH).sub.2 and/or Zr(OH).sub.4. Excess NH.sub.4 OH and NH.sub.4 Cl (which forms) is washed out, such as by repeated contact with water or NaCl brine. The composite is impregnated with acrylic acid which is polymerized in-situ with a free-radical or redox polymerization initiator or catalyst. The composite is then preferably treated with an acid, e.g., with HCl.

In the general process outlined above, the beginning zirconyl compound may be ZrOCl.sub.2.xH.sub.2 O or the like, such as Zr(NO.sub.3).sub.4.5H.sub.2 O, ZrOBr.sub.2.xH.sub.2 O, ZrOI.sub.2.8H.sub.2 O, or Zr(SO.sub.4).sub.2.4H.sub.2 O, or any such zirconium compound which will precipitate to form Zr(OH).sub.4 and/or ZrO(OH).sub.2 when contacted with a base, especially NH.sub.4 OH. The so-formed Zr(OH).sub.4 and/or ZrO(OH).sub.2, also called "zirconium hydrous oxide" or "zirconyl hydrate", is an amorphous, polymeric structure. Following the above alkalizing step, acrylic acid monomer is polymerized in-situ in the composite; the acid used for lowering the pH is preferably HCl, but may also be HBr, HI, HNO.sub.3, and the like.

Polymerization of acrylic acid in aqueous solution is readily and conveniently carried out by application of heat, especially in the presence of a free-radical generator, e.g., a small amount of K.sub.2 S.sub.2 O.sub.8. Redox initiators may also be used. The addition of and in-situ polymerization of acrylic acid may, alternatively, be performed prior to addition of and precipitation of the zirconium compound. Other polymerizable carboxylic acid monomers may be used, e.g., methacrylic acid, but acrylic acid is preferred because it is the smallest in molecular size of the unsaturated carboxylic acids.

As stated above, once the resin has been acidized, the composite is ready to take on SO.sub.4.sup.= values from alkali metal salt brine. This is done, for example, by placing the composite in a vessel, preferably a column, and passing sulfate-containing brine through the composite until the composite is substantially "loaded" with SO.sub.4.sup.= values and is ready for another water-washing.

It is within the purview of this invention that the alkali metal salt brine may be a natural brine, such as seawater or mineral brine, a LiCl brine, a KCl brine, or an alkali metal salt brine which comes from an ore dressing, ore leaching, mineral dressing, and the like. The brine may contain only a small amount, say less than about 1% of hardness values, preferably less than about 0.1% hardness values. The present novel composite exhibits a high affinity for, and a strong preference for, SO.sub.4.sup.= ions, but is also effective for borate, bicarbonate, and/or phosphate ions.

In certain embodiments the product may be described as a macroporous ion exchange resin of the sulfonated styrene-divinyl benzene type (e.g., DOWEX.RTM. MSC-1) containing throughout its bead structure gelatinous zirconium hydroxide or zirconyl hydroxide, and further modified by polyacrylic acid formed by absorbing monomeric acrylic acid into the structure and polymerizing in situ. The resin is initially activated for SO.sub.4.sup.= pick-up by acidizing to 2-3 pH with HCl, thus converting the polymeric Zr(OH).sub.4 to a polymer of the composition Zr(OH).sub.3 Cl. The SO.sub.4.sup.= pick-up is an exchange of 2Cl.sup.- for 1SO.sub.4.sup.=. Water regeneration removes Na.sub.2 SO.sub.4 ; the SO.sub.4.sup.= comes from the (Zr(OH).sub.3.sup.+).sub.2 SO.sub.4.sup.= and the Na.sup.+ from the --SO.sub.3.sup.- Na.sup.+ and the --COO.sup.- Na.sup.+.

The initial resin generally contains on the order of at least about 1 meq. --SO.sub.3.sup.- H.sup.+ groups per ml, Zr(OH).sub.4 is added to about 0.8 atom Zr per atom S, and acrylic acid is added to approximately 1 molecule per atom Zr.

Preferred operation is counterflow at rates of 0.01-0.1 V.sub.B /min. at 40.degree.-60.degree. C. with about 2 to about 3 pH. The flow rate may be increased to 0.2-0.25 V.sub.B /min. at 100.degree.-120.degree. C. where hot salt brine and water are available. At pH values of much less than 2 the COONa will be present as COOH and thus will reduce the efficiency of H.sub.2 O regeneration; at pH values much above about 3 some of the Zr will exist as Zr(OH).sub.4 and pick up much less SO.sub.4.sup.=.

The following is intended to illustrate the present invention, but the invention is not limited to the particular embodiment examples shown.

Performance in the removal of SO.sub.4.sup.= from 26% NaCl with water regeneration is compared between resins with and without --COOH groups. Detailed preparation and operating data are given below.

About 150 ml of DOWEX.RTM. MSC-1(8% DVB, 50-100 mesh ground) in H.sup.+ form was washed and put in a glass column. 32% aqueous ZrOCl.sub.2 was run through the column downflow until the effluent density was the same as the influent. N.sub.2 gas was run through the resin until it was dry and free flowing. Dry weight was 119.7 gms. This resin was poured into 120 ml of 30% aqueous NH.sub.3 and allowed to react for 15 minutes. The resin was washed well with water and then with 26% NaCl. Immersed in excess 26% NaCl it was titrated with N/l HCl to 2.4 pH requiring 85 meg. HCl. The resin volume was now 130 ml. 116 ml of resin was put in a column and operated upflow with 25% NaCl containing 1800 mg/l SO.sub.4.sup.= at 2.4 pH at 10 ml/min and 72.degree. C. 715 ml of brine effluent was obtained with the last 100 ml analyzing 1230 mg/l SO.sub.4. The resin was regenerated with water downflow at 3.3 ml/min and 72.degree. C., taking cuts of the effluent for analysis.

Brine was then run upflow again at 10 ml/min and 72.degree. C., with effluent cuts.

The resin (130 ml) was then well washed with water and sucked almost dry. It was added to a stirred pot with 75 ml H.sub.2 O and a solution containing 20 ml H.sub.2 O, 5 gm. acrylic acid, and 0.2 gm. K.sub.2 S.sub.2 O.sub.8. The mix was stirred for 60 minutes at 25.degree. C. to initiate polymerization. The mix was then put in a beaker, covered with aluminum foil and put in a 95.degree. C. oven overnight to complete polymerization. It boiled to dryness with a small amount of overflow and loss of resin. It was washed with water to give a resin volume of 124 ml (hence a loss of 6 ml). Immersed in 26% NaCl and titrated up to 2.4 pH with 3 ml N/l NaOH. 116 ml of resin was put in a column and saturated upflow with 900 ml of 2.4 pH 26% NaCl containing 1800 mg/l SO.sub.4.sup.= , at a flow rate of 10 ml/min and a temperature of 72.degree. C. The resin was then regenerated downflow with water at 3.2 ml/min and 72.degree. C., taking cuts of the effluent for SO.sub.4 analysis:

Other embodiments will become apparent to persons skilled in the art without departing from the scope of the presently disclosed invention.