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

Gerber, Daniel Hendrik (Stellenbosch : Stellenbosch University, 2011-12)


ENGLISH ABSTRACT: Desalination, by means of reverse osmosis (RO), in combination with other processes, can produce potable water at high recoveries. Antiscalants are generally used to reduce scaling on equipment surfaces and to improve water recovery during RO by slowing down the precipitation kinetics of sparingly soluble salts in the RO feed, thereby allowing concentration levels in the RO brine at several times the solubility limit of these salts. In addition, a fraction of the concentrate may be recycled back to the feed of the RO-membrane to improve the overall recovery, but only after the super saturated salts in the concentrate have been precipitated. The inhibitory character of the antiscalants (which are rejected into the concentrate stream) complicates the removal of salt from the concentrate and therefore prohibits such recycling. The focus of this study is aimed at properly understanding some of the parameters that influence the functionality or effectiveness of antiscalants used in high sulphate waters, with the purpose to override the effect of the antiscalant in the concentrate stream and force precipitation of the super saturated 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) on synthetically prepared solutions, super saturated with gypsum and 2) industrial concentrate, rich in sulphate (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 the depletion of calcium, using a calcium selective electrode (ISE). Deductions concerning the kinetics of precipitation were made from observing two kinetic variables (response variables) e.g. the induction time and the growth rate (tC80 – inferential variable). Two antiscalants have been evaluated in this study: a phosphonate based antiscalant (HYDREX) and a polyacrylate antiscalant (BULAB), at concentrations of 4 mg/l and 12 mg/l. The objective was to chemically and physically manipulate the antiscalant effectiveness, override its effect and force precipitation 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 with gypsum seed at a concentration of 0-2000 mg/l. In addition, lime and a combination of gypsum and lime 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 in seed concentration and pH at a given antiscalant concentration. Seed at a concentration of 2000 mg/l was sufficient in most cases to immediately override the effect of HYDREX and BULAB (at 4-12 mg/l) and produce ~ 0 minutes induction time. A pH of 10 increased the adsorption capacity of HYDREX and BULAB, leading to longer induction times (exceeding 24 hours in some cases). At a pH of 4 the adsorption capacity was very low for both HYDREX and BULAB (lower) leading to shorter induction times (zero to 100 minutes). It was especially in the ‘no-seed’ cases that the effect of pH on the induction time was prominent. The rate of precipitation (crystal growth rate) was increased at a temperature of 25°C, compared to 15°C (the rate increased two fold for an increase in 10°C). The addition of lime-seed, instead of gypsum, (at 2000 mg/l) produced growth rates, two times higher compared to when gypsum was used at the same conditions. In Addition, seeding with lime produced induction times (150 minutes for HYDREX and 50 minutes for BULAB) prior to precipitation, compared to zero induction time when gypsum was used at the same conditions. It was proven that an induction time could be eliminated by adding a combination of gypsum and lime both at a concentration of 2000 mg/l. with the added benefit of the higher growth rate. An increase in the calcium concentration increased the crystal growth rate in the presence of HYDREX. The presence of a high pH, however caused the effect of calcium on the growth (in the presence of BULAB) to be overshadowed. At a higher pH the growth rate of gypsum slowed down as a 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/l antiscalant (HYDREX and BULAB) on the crystal growth rate. Conversely, low ferric concentration resulted 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 inhibitory effect of antiscalants (HYDREX and BULAB) and 2) to produce rapid precipitation of gypsum, lie in the use of seeding with gypsum and lime (2000 mg/l), adding ferric chloride (10 mg/l), lowering the pH to 4 or lower (which can only be obtained when lime is not added) and setting the solution temperature to a moderate value of 25°C or higher. These ‘best’ conditions were subsequently applied to a concentrate, produced from concentrating AMD in a RO unit, and proved to be even more successful in overriding the effect of HYDREX and BULAB 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 an increased [SO4ª-] x [Caª+] product of 3.73 (AMD concentrate) vs. 3.46 (synthetic solutions) is thought to 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 the calcium concentration. Although the addition of lime caused an increase in the pH to 12.3 (at which point the antiscalant was most effective), the increase in pH is likely to cause an increase in the natural carbonate in the water, which would stimulate CaCO3 precipitation. The CaCO3 precipitate would be responsible for the adsorption of antiscalants, reducing their efficiency.

AFRIKAANSE OPSOMMING: Ontsouting by wyse van tru-osmose (TO), in samewerking met ander prosesse, kan help om drinkwater te lewer teen verhoogte herwinning. Tipies word antiskaalmiddels gebruik om bevuiling op die oppervlak van toerusting te verminder en terselfdetyd herwinning te verhoog deurdat dit die presipitasiekinetika van superversadigde soute in die TO voerwater vertraag. Dit lei daartoe dat water (superversadig met soute) deur die membraansisteem kan beweeg, sonder om bevuiling te veroorsaak. ‘n Breukdeel van die konsentraat kan herwin word na die TO voer om sodoende die algehele waterherwinning te verhoog. Dit kan egter eers gebeur nadat die soute in die konsentraat neergeslaan en verwyder is. Die inhirente ‘vertragingskarakter’ van antiskaalmiddels (wat ook in die konsentraat stroom beland) kompliseer die verwydering van sout vanuit die konsentraat en verhoed so herwinning. Die fokus van hierdie studie is daarop gemik om die parameters wat die funksionaliteit of effektiwiteit van antiskaalmiddels (wat in sulfaatryke waters gebruik word), beter te verstaan. Die doel is daarop gemik om die betrokke antiskaalmiddel se effek te kanselleer asook presipitasie van die superversadigde soute in oplossing aan te help. ‘n Lot (‘batch’) kristallisasietegniek wat die presipitasie van kalsiumsulfaatdehidraat (gips) beskou vanuit ‘n oplossing waar die konsentrasie verander soos presipitasie plaasvind, is gebruik om presipitasietoetse uit te voer 1) op oplossings wat sinteties versadig is met gips en 2) op sulfaatryke AMD (gekonsentreer met behulp van ‘n laboratoriumskaal TO eenheid). Die presipitasie proses is in elke geval waargeneem, deur die vermindering van die kalsium konsentrasie in die oplossing dop te hou, met die gebruik van ‘n kalsiumselektiewe elektrode. Afleidings rakende die kinetika van presipitasie is gemaak deur twee responsveranderlikes dop te hou: die induksietyd en die kristal groeitempo (tC80). Twee antiskaalmiddels by ‘n konsentrasies van 4 dpm (deetjies per miljoen) en 12 dpm is evalueer: ‘n fosfonaat (HYDREX) and poliakrilaat (BULAB). Die doel was om die antiskaalmiddel se werking chemies en fisies te manipuleer, hul werking teen te werk en presipitasie van gips te forseer. Die manipulasie het geskied deur die volgende parameters te verander: temperatuur (15°C-25°C), pH (4-10), FeCl3 (2-10 mg/l) of saad byvoeging (gips: 2000 mg/l). Kalsiumhidroksied (gebuste kalk) en ‘n kombinasie van gips en gebluste kalk is ook gebruik by konsentrasies van 2000 mg/l. Die induksietyd (by ‘n spesifieke antiskaalmiddel konsentrasie) is die sterkste beïnvloed deur ‘n verandering in saad konsentrasie en pH verandering. In die meeste gevalle was ‘n saad konsentrasie van 2000 mg/l voldoende om die induksie effek van beide HYDREX en BULAB te vernietig en nulminute induksietyd is verkry. ‘n pH van 10 het gelei tot die verhoging van die adsorpsiekapasiteit van HYDREX en BULAB wat gelei het tot langer induksietye (in sommige gevalle het dit 24 uur oorskry). By ‘n pH van 4 was die adsorpsie kapasiteit van beide antiskaalmiddels baie laag (laer vir BULAB) en induksie-tye is beperk tot 100 minute. Dit is veral wanneer geen saad toegevoeg is nie wat die effek van pH prominent was. Die tempo van presipitasie was verhoog by ‘n temperatuur van 25°C (2 keer hoër as by 15°C). Die byvoeging van gebluste kalk teen 2000 mg/l het ‘n kristal groeitempo, 2 keer hoër as in die teenwoordigheid van gips gelewer. Gebluste kalk saad byvoeging het egter gelei tot ‘n indukisetyd (150 minute vir HYDREX en 50 minute vir BULAB). Hierdie probleem is oorkom deur ‘n kombinasie van gips en gebluste kalk te gebuik teen ‘n konsentrasie van 2000 mg/l. Geen induksie tyd is waargeneem met die voordeel van ‘n hoër presipitasietempo (kristal groei). ‘n Verhoging van kalsium konsentrasie verhoog die kristal groei tempo in die teenwoordigheid van HYDREX. Nietemin, die invloed van pH oorskadu die invloed van kalsium op die groei tempo (in die teenwoordigheid van BULAB). By ‘n hoë pH word die kristal groei tempo vertraag as gevolg van die verhoging van die adsorpsiekapasiteit van die antiskaalmiddel. Die interaksie van FeCl3 met die antiskaalmiddel blyk van belang te wees. By hoë FeCl3 konsentrasies (10 dpm), is die werking van beide HYDREX en BULAB (12 dpm) beperk. Die ‘beste’ kondisies (verkry binne die konteks van hierdie studie), 1) om die vertragingseffek van HYDREX en BULAB teen te werk en 2) spoedige presipitasie van gips te bewerk, lê in die gebruik van saad (gips en gebluste kalk teen 2000 mg/l), die byvoeging van FeCl3 (10 mg/l), ‘n lae pH (4 of laer, wat natuurlik net tersprake is wanneer slegs gips as saad gebruik word aangesien geluste kalk die pH sal lig) asook ‘n relatiewe hoë temperatuur (25°C). Hierdie ‘beste’ kondisies is toegepas in AMD konsentraat om die effek van HYDREX en BULAB te vernietg en gips te presipiteer en die gevolg was dat dit selfs meer suksesvol was as in sintetiese oplossings. In elke geval is die induksietyd na nul minute toe verminder, terwyl die kristal groei tempo 3 maal verhoog het in vergelyking met die sintetiese toetse. Die teenwoordigheid van onsuiwerhede insluitende aluminium, kalsium, magnesium sowel as ‘n verhoging in die [SO4ª-]x[Caª+] produk (3.73 teenoor 3.46 vir sintetiese toetse), blyk verantwoordelik te wees vir die versnelling van die kinetika. Met die byvoeging van gebluste kalk is dit waarskynlik dat die verhoging van die pH (12.3) lei tot die verhoging van natuurlike karbonate in die water wat weer CaCO3 stimlueer. Die teenwoordigheid van CaCO3 kan verantwoordelik gehou word vir bykomende nukleasie en groei, sowel as die deaktivering van antiskaal effektiwiteit.

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