The chemical manipulation of meta-stable brine super-saturated with gypsum: forcing precipitation by overriding the inhibitory effect of antiscalants on crystal formation.

[摘要] ENGLISH ABSTRACT: Desalination, by means of reverse osmosis (RO), in combination with other processes, can producepotable water at high recoveries. Antiscalants are generally used to reduce scaling on equipmentsurfaces and to improve water recovery during RO by slowing down the precipitation kinetics ofsparingly soluble salts in the RO feed, thereby allowing concentration levels in the RO brine atseveral times the solubility limit of these salts. In addition, a fraction of the concentrate may berecycled back to the feed of the RO-membrane to improve the overall recovery, but only after thesuper saturated salts in the concentrate have been precipitated. The inhibitory character of theantiscalants (which are rejected into the concentrate stream) complicates the removal of salt fromthe concentrate and therefore prohibits such recycling.The focus of this study is aimed at properly understanding some of the parameters that influencethe functionality or effectiveness of antiscalants used in high sulphate waters, with the purpose tooverride the effect of the antiscalant in the concentrate stream and force precipitation of the supersaturated salts in solution.A batch crystallization technique, which considers the precipitation of calcium sulphate dehydrate(gypsum) from a solution of changing super saturation, was used to perform precipitation tests 1) onsynthetically prepared solutions, super saturated with gypsum and 2) industrial concentrate, rich insulphate (produced by concentrating acid mine drainage (AMD) by means of a lab scale RO unit).During batch crystallization, the precipitation process was observed by means of monitoring thedepletion of calcium, using a calcium selective electrode (ISE). Deductions concerning the kinetics ofprecipitation were made from observing two kinetic variables (response variables) e.g. the inductiontime and the growth rate (tC80 – inferential variable).Two antiscalants have been evaluated in this study: a phosphonate based antiscalant (HYDREX) anda polyacrylate antiscalant (BULAB), at concentrations of 4 mg/l and 12 mg/l. The objective was tochemically and physically manipulate the antiscalant effectiveness, override its effect and forceprecipitation of gypsum by means of changing parameters in the system, such as the temperature(15°C- 25°C), pH (4-10), ferric chloride concentration (2-10 mg/l) or seeding the solution withgypsum seed at a concentration of 0-2000 mg/l. In addition, lime and a combination of gypsum andlime were also used for seeding at concentrations of 2000 mg/l.The induction time, prior to precipitation, was found to be most strongly affected by the change inseed concentration and pH at a given antiscalant concentration. Seed at a concentration of2000 mg/l was sufficient in most cases to immediately override the effect of HYDREX and BULAB (at4-12 mg/l) and produce ~ 0 minutes induction time. A pH of 10 increased the adsorption capacity ofHYDREX and BULAB, leading to longer induction times (exceeding 24 hours in some cases). At a pH of4 the adsorption capacity was very low for both HYDREX and BULAB (lower) leading to shorterinduction times (zero to 100 minutes). It was especially in the 'no-seed' cases that the effect of pHon the induction time was prominent.The rate of precipitation (crystal growth rate) was increased at a temperature of 25°C, compared to15°C (the rate increased two fold for an increase in 10°C). The addition of lime-seed, instead ofgypsum, (at 2000 mg/l) produced growth rates, two times higher compared to when gypsum wasused at the same conditions. In Addition, seeding with lime produced induction times (150 minutesfor HYDREX and 50 minutes for BULAB) prior to precipitation, compared to zero induction time whengypsum was used at the same conditions. It was proven that an induction time could be eliminatedby adding a combination of gypsum and lime both at a concentration of 2000 mg/l. with the addedbenefit of the higher growth rate.An increase in the calcium concentration increased the crystal growth rate in the presence ofHYDREX. The presence of a high pH, however caused the effect of calcium on the growth (in thepresence of BULAB) to be overshadowed. At a higher pH the growth rate of gypsum slowed down asa result of the increase in adsorption capacity of the polymer onto the crystal surface.The interaction of the antiscalant with FeCl3 seemed to be important with regard to crystal growth.Higher ferric concentrations (10 mg/l) were sufficient to limit the inhibitory effect of 12 mg/lantiscalant (HYDREX and BULAB) on the crystal growth rate. Conversely, low ferric concentrationresulted in slower growth rates in the presence of an antiscalant.The best conditions (within the scope of the current study), sufficient 1) to override the inhibitoryeffect of antiscalants (HYDREX and BULAB) and 2) to produce rapid precipitation of gypsum, lie in theuse of seeding with gypsum and lime (2000 mg/l), adding ferric chloride (10 mg/l), lowering the pHto 4 or lower (which can only be obtained when lime is not added) and setting the solutiontemperature to a moderate value of 25°C or higher.These 'best' conditions were subsequently applied to a concentrate, produced from concentratingAMD in a RO unit, and proved to be even more successful in overriding the effect of HYDREX andBULAB than in synthetic aqueous solutions. The induction times of precipitation of AMD in all cases were ~ 0 minutes, whereas the growth rate increased threefold compared to the synthetic tests.The presence of additional foreign precipitates of aluminum, calcium and magnesium as well as anincreased [SO4ª-] x [Caª+] product of 3.73 (AMD concentrate) vs. 3.46 (synthetic solutions) is thoughtto be responsible for the increase in precipitation kinetics when only gypsum seed was used.The addition of lime caused an increase in the precipitation potential of the brine by increasing thecalcium concentration. Although the addition of lime caused an increase in the pH to 12.3 (at whichpoint the antiscalant was most effective), the increase in pH is likely to cause an increase in thenatural carbonate in the water, which would stimulate CaCO3 precipitation. The CaCO3 precipitatewould be responsible for the adsorption of antiscalants, reducing their efficiency.

[发布机构] Stellenbosch University