Publication: Investigation of reactive crystallization of gypsum in the presence of additives
Abstract
KATKI MADDELERİ VARLIĞINDA REAKSİYON KRİSTALİZASYONU İLE JİPS ELDESİNİN İNCELENMESİ Birçok endüstriyel uygulamada karşılaşılan kalsiyum sülfat dihidratın (Jips, CaSO4.2H2O) kristalizasyonu büyük önem taşımaktadır. Bu çalışmada, saf Ca(OH)2 çözeltisi ve H2SO4 çözeltisinden reaksiyon ile elde edilen kalsiyum sülfat dihitratın; farklı pH, sıcaklık ve sitrik asit konsantrasyonlarında kristalizasyonu incelenmiştir. Ayrıca farklı konsantrasyonlarda Fe2+, Fe3+, Cr6+ ve Cr3+ iyonları gibi safsızlıkların jips kristalizasyonuna etkisi çalışılmıştır. Bunun yanısıra, 2500 ppm sitrik asit varlığında Fe3+ ve Cr3+ iyonlarının jips kristalizasyonuna etkisi ayrıca araştırılmış ve tartışılmıştır. Jips kristallerinin kristal tane dağılımı (CSD), filtrasyon karakteristiği ve şekli açısından bütün parametrelerinin jips kristalizasyonuna etkisi dikkate alınmıştır. Sonuçlar; 20ºC’de pH’ın artması ile jips kristallerinin ortalama tane dağılımının hafifçe arttığını ve pH’a bağlı olarak jips kristallerinin şeklinin iğne yapıdan daha kalın prizma şekline dönüştüğünü göstermiştir. Sıcaklığın etkisinin incelendiği deneylerde sonuçlar, jips kristalizasyonunda sıcaklığın da önemli derecede etkili olduğunu göstermiş ve optimum çalışma sıcaklığı 65ºC seçilmiştir. pH 3.5(±0.1) ve 65ºC’de sitrik asidin jips morfolojisi üzerinde önemli rol oynadığı gözlenmiştir. Özellikle 2500 ppm sitrik asit konsantrasyonu; ortalama tane dağılımı, kristal şekli ve filtrasyon karakteristiği açısından kritik konsantrasyon olarak tanımlanmıştır. Sitrik asit konsantrasyonuna bağlı olarak prizma, plaka, yaprak ve çiçek benzeri çeşitli kristal morfolojileri elde edilmiştir. Zeta potansiyel analizleri jipsin farklı kristal yüzlerinde adsorplandığını göstermiştir. Kristalizasyon ortamında Fe2+ ve Fe3+ iyonlarının varlığı jipsin ortalama tane dağılımını ve CSD’nı etkilememiştir. Morfolojideki değişim ve Fe3+ iyonları varlığında oluşmuş jips kristalleri üzerinde ince ve plaka yapıda kristallerin meydana gelmesi yüksek nükleasyon hızı ile açıklanmıştır. 2500 ppm sitrik asit varlığında, CSD analizleri ve SEM gözlemleri sitrik asidin jips morfolojisi, ortalama tane dağılımı ve filtrasyon karakteristiği üzerindeki etkisinin Fe3+ iyon konsatrasyonunun artması ile azaldığını ve Fe3+ iyonlarının gerçek etkisinin özellikle 500 ppm iyon konsantrasyonunda ortaya çıktığını göstermiştir. Fe3+ iyonları varlığında sitrik asit etkisindeki bu azalma, sitrik asit ve Fe3+ iyonları arasında kompleks oluşumu ile yorumlanabilmektedir. Cr6+ iyonlarının jips kristallerinin büyüyen yüzlerinde adsorplanması ve bloke olması kristal morfolojisini etkilemiş ve yapraksı plaka yapıların oluşmasına neden olmuştur. CSD’daki düzensizliğin ortalama tane dağılımı ve filtrasyon hızının azalmasında önemli etkisi olduğu görülmektedir. Sonuçlar ayrıca ortalama tane dağılımı ve filtrasyon karakteristiğinin Cr3+ iyonları varlığına bağlı olmadığını, fakat jips kristallerinin mekanik dayanımında azalma meydana geldiğini göstermiştir. Jips kristallerinin büyüme özelliği, yüzey morfolojisi ve filtrasyon karakteristiği; kristalizasyon ortamında Cr3+ iyonlarının yanı sıra 2500 ppm sitrik asit bulunmasına bağlıdır. CSD analizleri Cr3+ iyon konsantrasyonuna bağlı olarak jips kristallerinin ikili dağılıma sahip olduğunu ortaya çıkarmıştır. SEM ve EDS analizleri yüksek Cr3+ iyon konsantrasyonunda meydana gelen farklı kristal oluşumların sitrik asit ve adsorplanan Cr3+ iyonlarının kompleks oluşturmasından meydana gelebileceğini kanıtlamıştır. Ayrıca bu yapılar FT/ IR analizleri ile de desteklenmiştir.
INVESTIGATION OF REACTIVE CRYSTALLIZATION OF GYPSUM IN THE PRESENCE OF ADDITIVES Crystallization of calcium sulfate dihydrate (Gypsum, CaSO4.2H2O) is considerably important since it is encountered in a variety of industrial applications. The crystallization of calcium sulfate dihydrate produced by the reaction between pure Ca(OH)2 suspension and H2SO4 solution was investigated at different pH values, temperatures and citric acid concentrations. Also the effect of additives such as ; Fe2+, Fe3+, Cr6+ and Cr3+ ions at different concentrations was studied on gypsum crystallization. Besides, the influence of Fe3+ and Cr3+ ions in the presence of 2500 ppm citric acid on the crystallization of gypsum were also investigated and discussed. The influence of all parameters on gypsum crystallization was considered in terms of crystal size distribution (CSD), filtration characteristics and shape of gypsum crystals. The results indicate that the average particle size of gypsum crystals slightly increases with increasing pH, and the shape of gypsum changed from needle-like to thicker tubular form depending on pH at 20ºC. The results obtained from the experiments which, the effect of temperature was investigated, showed that the temperature had also an important effect and 65ºC was selected as an optimum working temperature for this study. Citric acid played an important role for gypsum morphology (size and shape) at pH 3.5 and 65ºC. Especially, 2500 ppm citric acid concentration was determined as a critical concentration in terms of average crystal size, crystal shape and filtration characteristics. Various crystal morphologies such as tubular, plate-like, double-taper leaf-like and flower-like were obtained depending on the citric acid concentration. Zeta potential values showed that citric acid adsorbed preferentially on the different crystal faces of gypsum. Presence of Fe2+and Fe3+ ions in crystallization media did not affect the average particle size and CSD of gypsum. The change of morphology and appearance of fine and tubular crystals on the surface of already formed crystals in the presence of Fe3+ ions were explained by high nucleation rate. In the presence of 2500 ppm citric acid, CSD analysis and SEM observations show that the effect of citric acid on morphology, average particle size and filtration characteristics of gypsum is lessened with increasing Fe3+ ion concentration and real effect of Fe3+ ion is appeared especially at 500 ppm ion concentration. Diminishing of the effect of citric acid due to the presence of Fe3+ ion could be attributed to the complex formation between citric acid and Fe3+ ion. Adsorption and blocking of Cr6+ ions on growth units of gypsum crystals impact the crystal morphology of gypsum resulting in defective leaf-like plate form. Irregularity in CSD seems to play a greater role in decreasing average particle size and filtration rate. Results also indicate that the average particle size and filtration characteristics was not very strongly dependent on the presence of Cr3+ ion, but weakness in mechanical strength of gypsum crystals was observed. The growth habit, surface morphology and filtration characteristics of the gypsum crystals depend on the Cr3+ ion besides 2500 ppm citric acid in crystallization media. Analyses of the CSD reveal that gypsum crystals have bimodal CSD distribution depending on Cr3+ ion concentration. SEM and EDS analyses further confirmed that uncommon crystal formations, which might come from the formation of complex between citric acid and adsorbed Cr3+ ion, as evidenced by FT/ IR, occurred at high Cr3+ ion concentrations.
INVESTIGATION OF REACTIVE CRYSTALLIZATION OF GYPSUM IN THE PRESENCE OF ADDITIVES Crystallization of calcium sulfate dihydrate (Gypsum, CaSO4.2H2O) is considerably important since it is encountered in a variety of industrial applications. The crystallization of calcium sulfate dihydrate produced by the reaction between pure Ca(OH)2 suspension and H2SO4 solution was investigated at different pH values, temperatures and citric acid concentrations. Also the effect of additives such as ; Fe2+, Fe3+, Cr6+ and Cr3+ ions at different concentrations was studied on gypsum crystallization. Besides, the influence of Fe3+ and Cr3+ ions in the presence of 2500 ppm citric acid on the crystallization of gypsum were also investigated and discussed. The influence of all parameters on gypsum crystallization was considered in terms of crystal size distribution (CSD), filtration characteristics and shape of gypsum crystals. The results indicate that the average particle size of gypsum crystals slightly increases with increasing pH, and the shape of gypsum changed from needle-like to thicker tubular form depending on pH at 20ºC. The results obtained from the experiments which, the effect of temperature was investigated, showed that the temperature had also an important effect and 65ºC was selected as an optimum working temperature for this study. Citric acid played an important role for gypsum morphology (size and shape) at pH 3.5 and 65ºC. Especially, 2500 ppm citric acid concentration was determined as a critical concentration in terms of average crystal size, crystal shape and filtration characteristics. Various crystal morphologies such as tubular, plate-like, double-taper leaf-like and flower-like were obtained depending on the citric acid concentration. Zeta potential values showed that citric acid adsorbed preferentially on the different crystal faces of gypsum. Presence of Fe2+and Fe3+ ions in crystallization media did not affect the average particle size and CSD of gypsum. The change of morphology and appearance of fine and tubular crystals on the surface of already formed crystals in the presence of Fe3+ ions were explained by high nucleation rate. In the presence of 2500 ppm citric acid, CSD analysis and SEM observations show that the effect of citric acid on morphology, average particle size and filtration characteristics of gypsum is lessened with increasing Fe3+ ion concentration and real effect of Fe3+ ion is appeared especially at 500 ppm ion concentration. Diminishing of the effect of citric acid due to the presence of Fe3+ ion could be attributed to the complex formation between citric acid and Fe3+ ion. Adsorption and blocking of Cr6+ ions on growth units of gypsum crystals impact the crystal morphology of gypsum resulting in defective leaf-like plate form. Irregularity in CSD seems to play a greater role in decreasing average particle size and filtration rate. Results also indicate that the average particle size and filtration characteristics was not very strongly dependent on the presence of Cr3+ ion, but weakness in mechanical strength of gypsum crystals was observed. The growth habit, surface morphology and filtration characteristics of the gypsum crystals depend on the Cr3+ ion besides 2500 ppm citric acid in crystallization media. Analyses of the CSD reveal that gypsum crystals have bimodal CSD distribution depending on Cr3+ ion concentration. SEM and EDS analyses further confirmed that uncommon crystal formations, which might come from the formation of complex between citric acid and adsorbed Cr3+ ion, as evidenced by FT/ IR, occurred at high Cr3+ ion concentrations.