Browsing by Author "Cronje, Izak Adolf"

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    An investigation into treatment options to improve the performance of re-used seed crystals for gypsum precipitation in the presence of an anti-scalant
    (Stellenbosch : Stellenbosch University., 2020-03) Cronje, Izak Adolf; Burger, A. J.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    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.