The effect of humic substances on the crystallisation of gypsum

Bock, Heinrich (2017-03)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH SUMMARY: High recovery mine water treatment plants generate brine streams that are highly supersaturated with inorganic salts. Intermediate crystallisation of gypsum is required for further treatment of these brine streams. The crystallisation of gypsum is influenced by various factors such astemperature, supersaturation, additives orimpurities, pH and seeding. The presence of natural organic matter (NOM), consisting of humic substances (HS), can prevent the onset of crystallisation. These substances, mainly composed of humic (HA) and fulvic acids (FA), are considered to be weak polyelectrolytes due to their carboxylic and phenolic functional groups. The aim of this study is to investigate the effect of HS on the crystallisation of gypsum to understand the mechanisms (nucleation and crystal growth) of crystallisation better. This knowledge can be used to improve the sizing and operation of crystallisers. The effect of HS was investigated at supersaturation (SS) of 2-4, pH of 4.5–9.5 and seed loading of 200, 1000 and 2000 mg/l through a batch crystallisation process. An increase in HS concentration resulted in an increase in induction times due to the increased inhibitory effects of HA and FA through their functional groups. Induction times increased from 25 to 295 minutes with an increase in HA concentration from 0 to 15 mg/l at SS3 (0.0419 mol/l). At a HA concentration of 15 mg/l, an increase to SS4 (0.0566 mol/l) resulted ina decrease of induction times from 295 to 15 minutes, indicating the driving force of supersaturation. Increase in initial pH enhanced the inhibitory abilities of both HA and FA. Induction time increased from 115 to 415 min with an initial pH increase from4.5to9.5 in the presence of 15 mg/l HA at SS3 (0.0419mol/l). The effect of FA was far greater than HA, with crystallisation completely inhibited for a period of 2 days at a FA concentration of 5 and 15 mg/l in the absence of any seed crystals. At FA concentrations of 1.0 and 2.5 mg/l, induction times were 185 and 480 minutes, respectively. The greater effect of FA is attributed to an increase inthe number of functional groupswith a decrease in molecular weight. Seeding the crystallisation process successfully overcame the inhibitory effects of HS (both HA and FA) at concentrations of 1000 and 2000 mg/l gypsum seed crystals. With a seed concentration of 200 mg/l, an induction period of 50 min was observed in the presence of HA at 15 mg/l. With FA at 10mg/l and a gypsum seeding of 200 mg/l, no crystallisation was induced. This again illustrated the enhanced effect of FA to block active growth sites successfully. In the presence of seed crystals pH has no effect, suggesting that only surface interaction is taking place. With HA, an increase in seed crystals resulted in an increased growth rate (from 0.50 to 4.91 litre.mol-1.min-1) due to an increase in available growth sites. The inhibiting and retarding effect of HS on crystallisation is significant. Increasing supersaturation can override the inhibitory abilities of HS, while the presence of sufficient seed material will completely override the inhibitory abilities of HS and minimise the effects of these substances on crystallisation.

AFRIKAANS OPSOMMING: Hoë herwinning mynwater behandelingsaanlegte genereer pekelstrome wat hoogs oorversadig is met anorganiese soute. Intermediêre kristallisasie van gips word vereis vir verdere behandeling vanhierdiepekelstrome. Die kristallisasie van gips word beïnvloed deur verskeie faktore soos temperatuur, oorversadigdheid, bymiddels of onsuiwerhede, pH en saad kristalle. Die teenwoordigheid van natuurlike organiese materiaal (NOM), wat bestaan uit humus stowwe (HS), kan die aanvang van kristallisasie voorkom. Humiese stowwe, hoofsaaklik humussuur (HA) en fulviensuur (FA), word beskou as swak poli-elektroliete as gevolg vanhulle karboksiel en fenoliese funksionele groepe. Hierdie studie se doel is om die effek van HSop die kristallisasie van gips te ondersoek en die meganismes(kernvormingen kristal groeitempo)van kristallisasie beter te verstaan. Die kenniswat hiervoorgelêwordkan gebruik word om die ontwerp en werking van kristalliseerderste verbeter. Die effek van HS was ondersoek by ‘n oorversadigingsvlak (SS) van 2-4, pH van 4.5–9.5 en saad konsentrasie van 200,1000 en 2000 mg/l deur middel van ‘n lot (“batch”) kristallisasie proses. ‘n Toename in HS konsentrasie het tot ‘n toename in induksie tyd gelei as gevolg van ‘n toename in die vermoë van HA en FAom kristallisasie te onderdruk deur middel van hul funksionele groepe. Induksie tye het toegeneem vanaf 25 tot 295 minute met ‘n toename in HS konsentrasie vanaf 0 tot 15 mg/l by SS3(0.0419mol/l). ‘n Verhoging van die oorversadigingskonsentrasie tot SS4 (0.0566mol/l), in die teenwoordigheid van 15mg/lHA, het gelei tot n afname in induksie tye, vanaf 295 tot15minutes. Dit dui dat oorversadiging ‘n groot dryfkrag vir kristallisasie is. Verhoodge aanvanklike pH, van 4.5tot9.5, het die kristallisasieonderdrukkingsvermoë van beide HA en FA verbeter, met induksie tye wat toegeneem het vanaf 115 tot 415 minute, in die teenwoordigheid van 15mg/l HAby SS3(0.0419mol/l). Dieeffek van FA was veel groter as HA, metkristallisasie wat vir ‘n tydperk van 2 dae ten volle onderdruk is by ʼn FA konsentrasie van 5 en 15 mg/l, in dieafwesigheidvan enige saad kristalle. Die induksie tye by ‘n FAkonsentrasie van 1.0 en 2.5mg/l was onderskeidelik, 185 en 480 minute. Die groter effek van FA is toegeskryf aan die toename in funksionele groepe met ‘n afname in molekulêre massa. Die vermoë van HS (beide HA en FA) om kristallisasie te onderdruk is oorkom deur die kristallisasie proses met gips saad kristalle te voed by konsentrasies van 1000 en 2000mg/l. ‘n Induksie periode van 50 min was waargeneem in die teenwoordigheid van 15mg/l HS en by ‘n saad konsentrasie van 200 mg/l. Met ‘n FA konsentrasie van 10 mg/l en gips kristalle van 200 mg/l, het geen kristallisasie plaasgevind nie. Dit het weereens die hoër vermoë van FA om aktiewe groeipunte suksesvol te blok, beklemtoon. pH het geen effek in die teenwoordigheid van saad kristalle nie, wat daaorp dui dat slegs oppervlak interaksie plaasvind. In die teenwoordigheid van HAhet ‘n toename in saad kristalle gelei tot ‘n toename in groeitempo (van 0.50tot4.91liter.mol-1.min-1), as gevolg van ‘n toename in beskikbare groeipunte. Die vermoë van HS om kristallisasie te onderdruk en vertraag was beduidend gevind. Deur die vlak van oorversadiging te verhoog, kan die vermoë van HS om kristallisasie te onderdruk oorkom word, terwyl die teenwoordigheid van genoeg saad kristalle die onderdrukkende vermoë van HS heeltemal sal oorkom en sodoende die effekte van hierdie stowwe op kristallisasie te minimiseer.

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