Polymeric additives: Effects on crystallization of hydroxyapatite scales

The effects of some commercial polymeric additives on the nucleation (induction period), crystal growth, and morphology of hydroxyapatite precipitated from pure solutions of the lattice ions have been examined. The inhibitory capacities of these additives for hydroxyapatite scale formed from whole juice samples under conditions similar to those operating in sugar mills have also been determined. The results show that poly (aspartic acid) with Mw of 5700 is the most effective scale inhibitor.</p

Polymeric AdditiveS' Effect on Crystallization of Calcium Oxalate Scales

Laboratory studies concerned with the effects of polymeric additives on the crystallization of calcium oxalate dihydrate and calcium oxalate monohydrate are described and the results obtained are discussed. These compounds are major components of scales formed in some Australian sugar mills. The studies involved the measurement of crystallization induction periods and rates, in the presence and absence of additives. Crystal morphologies were also examined. A range of experimental conditions were employed, from studies in pure solutions of the lattice ions to those more directly comparable with the mill operation.</p

The preparation of calcium oxalate dihydrate crystals

Two simple related methods for exclusively preparing well‐developed tetragonal bipyramidal crystals of calcium oxalate dihydrate at moderate supersaturations and at ambient temperature are reported. These crystals have an average length of 5.7–7.2 μm and are easy to wash and filter. One of the methods is suitable for studies of the kinetics of spontaneous precipitation and crystal morphology of the oxalate in the presence and absence of additives. A tentative mechanism is given to account for the sole formation of this particular hydrate. At 70 °C a mixture of the monohydrate and dihydrate phase were formed with various crystal shapes and sizes. These phases were also identified in the presence of sucrose at this temperature, although some of the crystals contained tetragonal habit as obtained at ambient temperature.</p

The Effects of Polymeric Additives on the Crystallization of Compounds Found in the Evaporator Scales of Australian Sugar Mills (I), Compositions of the Scale Deposits

The composition of scales formed in multiple evaporator units at three Australian sugar mills have been determined using X‐ray fluorescence (XRF), high performance liquid chromatography (HPLC), X‐ray diffraction (XRD), thermogravimetry/differential thermal analysis (TG/DTA) and other analytical methods. The composition of the scales formed in each mill have been found to be significantly different and to depend on whether they originated from early or later stages of the evaporation process. Calcium oxalate (present as the di and monohydrate) was identified as a major component of the scales from one mill but was found to be only a minor component of the scales from the other two. The latter were shown to contain two or more forms of calcium phosphate as dominant phases. Amorphous or crystalline silica was found to be present in most of the scales examined.</p

Studies on the dehydration and decomposition of calcium and dicalcium magnesium aconitate hydrates

The thermal decomposition of calcium and dicalcium magnesium aconitate hydrates were studied by TG/DTG, DTA, EGA, SEM and other physico-chemical techniques. The decomposition proceeds in four stages: dehydration; oxidation of the carboxylic acid portion of the salt; complete fragmentation of the hydrocarbon portion; and finally, decarboxylation of the metal carbonate to the oxide. The crystal morphologies of the hydrate and anhydrous salts of each compound are very similar. Tricalcium aconitate consists of well-developed twinned crystals and stellate clusters intergrown with flat platy crystals. On the other hand, dicalcium magnesium aconitate crystals are monoclinic with well-developed pinacoidal faces. The activation energy, Ed (43±2 kJ mol-1 water), calculated from Borchardt and Daniels' method, for the dehydration process of calcium aconitate trihydrate is of the same order of magnitude as some simple metal salt hydrates. The rate constant, kd increased from 0.04/min at 238°C to greater than 0.86/min at 295°C. It is concluded that the dehydration process is due to cation bound water.</p