An investigation into treatment options to improve the performance of re-used seed crystals for gypsum precipitation in the presence of an anti-scalant

Cronje, Izak Adolf (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Many mines in South Africa generate large volumes of acid mine drainage (AMD), containing relatively high concentrations of sulphates. Several mine water treatment plants utilise intermediate seeded slurry crystallisation between stages of reverse osmosis (RO), where gypsum (CaSO42H2O) is precipitated and removed to enable further desalination by RO in the presence of antiscalants. This study aimed to determine the effect of various methods of treatment on re-used seed crystals, considering the efficiency and rate of gypsum precipitation in the presence of antiscalants. Four different treatment methods were evaluated. These were two physical and two chemical treatments: 1.vigorous mixing; 2. air scouring; 3. addition of hydrogen peroxide; 4. addition of aluminum. The experiments were performed in sets of three 2.5 hour experiments, while 2000 ppm seed crystals were re-used and introduced into a three-times supersaturated gypsum solution in the presence of 9 ppm polycarboxylic acid antiscalant. The effectiveness of gypsum precipitation was characterised in terms of calcium removal by means of precipitation, while assuming an equivalent sulphate removal. In the presence of antiscalant, the calcium removal decreased with the re-use of seed crystals, indicating that the seeding became less effective with every re-use cycle. The average calcium removal over three runs with re-used seed crystals decreased from 42.8% in the absence of antiscalant to 15.3% in the presence of antiscalant. Mixing of the seed crystal slurry for 10 minutes at a G-factor of 188 s-1 was the most effective physical treatment method, removing an average of 31.4% calcium, compared to the control in the presence of antiscalant where only 15.3% calcium was removed without treatment. Air scouring as a treatment method was found to be less effective, with a maximum calcium removal of 18.6%. The dosing of aluminum proved to be very effective. In the presence of 4.5 ppm aluminum in the form of AlCl3, 41.2% of the calcium was removed, compared to maximum removal of only 25.3% in the presence of 90 ppm hydrogen peroxide. It was found that the presence of hydrogen peroxide, the presence of aluminum and mixing as seed-crystal treatment method increased the average calcium removal significantly when seed crystals were re-used, and are viable options to improve the crystallisation process in the presence of antiscalant when seed crystals are re-used.

AFRIKAANSE OPSOMMING: Baie myne in Suid-Afrika genereer groot volumes suurmynafloop wat hoë konsentrasies sulfate bevat. Verskeie mynwater behandelingsaanlegte gebruik intermediëre saadkristalslik kristallisasie tussen omgekeerde osmose (RO) fases, waar gips (CaSO42H2O) gepresipiteer en verwyder word om verdere ontsouting deur RO in die teenwoordigheid van teenskaalmiddels moontlik te maak. Die studie het gepoog om die effek van verskillende behandelingsmetodes op hergebruikte saadkristalle vas te stel terwyl die effektiwiteit en tempo van gips presipitasie in die teenwoordigheid van teenskaalmiddels in ag geneem word. Vier verskillende behandelingsmetodes is geëvalueer. Dit het twee fisiese en twee chemiese metodes ingesluit: 1. Kragtige vermenging; 2. Lugskuring; 3. Byvoeging van waterstof peroksied; 4. Byvoeging van aluminium. Die eksperimente is uitgevoer in stelle van drie 2.5 uur eksperimente, terwyl 2000 ppm saadkristalle hergebruik is en bygevoeg is in ‘n drie keer superversadigde gips oplossing in die teenwoordigheid van 9 ppm polikarboksielsuur teenskaalmiddel. Die effektiwiteit van gips presipitasie is gekarakteriseer in terme van kalsium verwydering deur presipitasie, terwyl die aanname gemaak is van ekwivalente sulfaatverwydering. In die teenwoordigheid van teenskaalmiddel het die kalsiumverwydering afgeneem met hergebruik van die saadkristalle, wat aandui dat die saadkristalle minder effektief geword het met elke hergebruik siklus. Die gemiddelde kalsiumverwydering oor drie eksperimentele lopies met hergebruikte saadkristalle het afgeneem van 42.8% in die afwesigheid van teenskaalmiddel tot 15.3% in die teenwoordigheid van teenskaalmiddel. Vermenging van die saadkristal slik vir 10 minute teen ‘n G-faktor van 188 s-1 was die mees effektiewe fisiese behandelingsmetode, wat ‘n gemiddeld van 31.4% kalsium verwyder het, in vergelyking met die kontrole in die teenwoordigheid van teenskaalmiddel waar slegs 15.3% van die kalsium verwyder is sonder behandeling. Lugskuring as ‘n behandelingsmetode was minder effektief, met ‘n maksimum kalsiumverwydering van 18.6%. Dosering met aluminium was baie effektief. In die teenwoordigheid van 4.5 ppm aluminium in die vorm van AlCl3, is 41.2% van die kalsium verwyder in vergelyking met die maksimum kalsiumverwydering van 25.3% in die teenwoordigheid van 90 ppm waterstof peroksied. Daar is gevind dat vermenging as die saadkristal behandelingsmetode, in die teenwoordigheid van waterstof peroksied en aluminium, die gemiddelde kalsiumverwydering noemenswaardig verhoog het wanneer saadkristalle hergebruik is. Hierdie benadering is geïdentifiseer as ‘n lewensvatbare opsie om die kristallisasieproses te verbeter in die teenwoordigheid van teenskaalmiddel wanneer saadkristalle hergebruik word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108231
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