molecules Article Differential Precipitation of Mg(OH)2 from CaSO4·2H2O Using Citrate as Inhibitor—A Promising Concept for Reagent Recovery from MgSO4 Waste Streams Szilveszter Ziegenheim 1,2,Márton Szabados 2,3, Zoltán Kónya 4,5 , Ákos Kukovecz 4, István Pálinkó 2,3 and Pál Sipos 1,2,* 1 Department of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, H-6720 Szeged, Hungary; [email protected] 2 Material and Solution Structure Research Group, Institute of Chemistry, University of Szeged, Aradi Vértanúk tere 1, H-6720 Szeged, Hungary; [email protected] (M.S.); [email protected] (I.P.) 3 Department of Organic Chemistry, University of Szeged, Dóm tér 8, H-6720 Szeged, Hungary 4 Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Béla tér 1, H-6720 Szeged, Hungary; [email protected] (Z.K.); [email protected] (Á.K.) 5 MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich B. tér 1, H-6720 Szeged, Hungary * Correspondence: [email protected]; Tel.: +36-62-54-4045 Academic Editor: Nikolay Gerasimchuk Received: 8 September 2020; Accepted: 27 October 2020; Published: 29 October 2020 Abstract: In hydrometallurgical processing and acidic wastewater treatment, one of the neutralizing agents employed is MgO or Mg(OH)2. At the end of this process, the resulting solution, which is rich 2 2+ in SO4 − and Mg is treated with lime to remove (or minimize the amount) of these ions via the precipitation of Mg(OH) and CaSO 2H O (gypsum). In our work, an attempt was made to separate 2 4· 2 the two solids by increasing the induction time of the gypsum precipitation, thus regenerating relatively pure Mg(OH)2 which could be reused in wastewater treatments or hydrometallurgical processing circuits, and in this way, significantly enhancing the economic viability of the process. During our experiments, the reaction of an MgSO4 solution with milk of lime prepared from quicklime was studied. The effects of a range of organic additives, which can slow down the precipitation of gypsum have been assessed. The process was optimized for the most promising inhibiting agent—that is, the citrate ion. The reactions were continuously monitored in situ by conductometric measurements with parallel monitoring of solution pH and temperature. ICP-OES measurements were also carried out on samples taken from the reaction slurry. The composition of the precipitating solids at different reaction times was established by powder XRD and their morphology by SEM. Finally, experiments were carried out to locate the additive after the completion of the precipitation reaction to get information about its potential reuse. Keywords: gypsum; magnesium hydroxide; inhibition; citric acid; precipitation separation 1. Introduction Solid precipitation from supersaturated solutions is a common phenomenon; however, in industrial processes it can cause several nuisances—e.g., via scaling or as a co-precipitating side product. Scaling can reduce the efficiency of heat-exchanging surfaces (water cooling systems), clog membranes and tubes (reverse osmosis units, water injection systems in oil and gas production), while co-precipitating Molecules 2020, 25, 5012; doi:10.3390/molecules25215012 www.mdpi.com/journal/molecules Molecules 2019, 24, x FOR PEER REVIEW 2 of 16 Molecules 2020, 25, 5012 2 of 16 membranes and tubes (reverse osmosis units, water injection systems in oil and gas production), while co-precipitating solids are present as impurities in the final products. One of the most solidscommonly are present precipitating as impurities solids inis theCaSO final4·2H products.2O, gypsum; One oftherefore, the most the commonly detailed precipitatingunderstanding solids of its is CaSO 2H O, gypsum; therefore, the detailed understanding of its crystallization is necessary in a crystallization4· 2 is necessary in a variety of processes, which are adversely affected by its precipitation. varietyAn of important processes, field which where are adverselygypsum is a ffpresentected by in itshigh precipitation. quantities is acidic wastewater treatment. Normally,An important these acidic field waters where contain gypsum heavy is present metal in and high sulfate quantities ions, which is acidic are wastewater precipitated treatment. as metal Normally,hydroxides these and calcium acidic waters sulfate contain using lime heavy for metal neutralization and sulfate [1]. ions, This which way, arethe precipitatedremaining water as metal can hydroxidesbe released to and natural calcium waters. sulfate However, using lime the for sludge neutralization, which forms, [1]. This requires way, further the remaining treatment water and can must be releasedbe disposed. to natural Using waters.magnesium However, oxide/hydroxide the sludge, whichas the forms,first step requires of a two-st furtherep treatmentneutralization and mustprocess be disposed.can provide Using an alternative, magnesium and oxide this way,/hydroxide gypsum as can the be first precipitated step of a two-stepseparately neutralization from the heavy process metal canhydroxides provide [2,3], an alternative, as shown andin Scheme this way, 1. gypsumThe proce canss could be precipitated also be useful separately in the fromdesalination the heavy process metal hydroxides[4]. [2,3], as shown in Scheme1. The process could also be useful in the desalination process [ 4]. Scheme 1. Two-step neutralization of acidic mine drainagedrainage using magnesium oxideoxide/hydroxide/hydroxide for separate precipitation of metal hydroxides. In thisthis work, work, an an attempt attempt is made is made to improve to improv the lattere the method latter furthermethod by separatingfurther by theseparating precipitation the ofprecipitation magnesium of hydroxide magnesium and gypsumhydroxide in solutionand gypsumby using in additives.solution by This using has the additives. potential This of enhancing has the thepotential economic of enhancing and environmental the economic viability and environmental of the process, viability and the resultsof the process, can be extended and the toresults separate can gypsumbe extended precipitation to separate in timegypsum from precipitation rapidly precipitating in time from solids rapidly other than precipitating Mg(OH)2. solids other than 11 Mg(OH)The2 solubility. of Mg(OH)2 (Ksp is around 1.38 10− ) is significantly lower than that of CaSO4 5 × (Ksp Theis around solubility 2.5 of 10Mg(OH))[5,62]. (K Thesp is nucleationaround 1.38 of ×Mg(OH) 10−11) is significantlyis also highly lower dependent than that on of theCaSO ion4 × − 2 concentrations,(Ksp is around and2.5 is× very10−5) fast[5,6]. (practically The nucleation instantaneous) of Mg(OH) at high2 is supersaturation also highly dependent [7–10]. The on inhibition the ion ofconcentrations, the crystallization and ofis Mg(OH)very fast2 was(practically found to in bestantaneous) rather difficult at whenhigh supersaturation conventional additives [7–10]. wereThe usedinhibition [10–13 of]. the crystallization of Mg(OH)2 was found to be rather difficult when conventional additivesThe kineticswere used of [10–13]. gypsum precipitation, which is significantly slower than that of Mg(OH)2, was investigatedThe kinetics of in gypsum several studiesprecipitation, [6,14– 18which]. As is itsignificantly is a common slower scaling than and that precipitating of Mg(OH)2 solid,, was theinvestigated inhibition in of several its crystallization studies [6,14–18]. has been theAs focusit is a of common various studiesscaling since and theprecipitating 1950s [19]. solid, The eff theect ofinhibition various of additives its crystallization has been has scrutinized been the overfocus recent of various decades. studies Various since the metal 1950s ions [19]. were The found effect toof influencevarious additives the precipitation has been of gypsum,scrutinized aff ectingover rece bothnt crystal decades. growth Various and morphologymetal ions [were20–23 found], even to in theinfluence presence the of precipitation certain amino of acidsgypsum, [24]. affecting Organic phosphonatesboth crystal growth were also and demonstrated morphology to[20–23], be effi evencient inhibitorsin the presence in gypsum of certain precipitation amino [acids25–28 [24].]. Polymeric Organic compounds phosphonates with were carboxylic also demonstrated groups also proved to be toefficient be effective inhibitors in the in inhibition gypsum ofprecipitation gypsum precipitation—the [25–28]. Polymeric most compounds popular is perhapswith carboxylic polyacrylic groups acid; however,also proved some to be other effective “green” in the alternatives inhibition were of gypsum also effi precipitation—thecient [29–32]. Looking most into popular small-molecule is perhaps simplepolyacrylic carboxylic acid; however, acids, the some performance other “green” of citric altern acidatives seemed were to bealso remarkable efficient [29–32]. [33] and Looking accordingly into itssmall-molecule inhibiting eff ectssimple were carboxylic investigated acids, in the more performance detail [34–36 of]. citric acid seemed to be remarkable [33] and accordinglyIt seems plausible its inhibiting that the effects inhibition were of investigated the precipitation in more of gypsum detail [34–36]. is much more promising than that ofIt magnesiumseems plausible hydroxide. that the Accordingly, inhibition of scouting the precipitation experiments of weregypsum performed is much in more a stoichiometric promising Cathan2+ +thatSO of2 reactionmagnesium at pH hydroxide.7