In this work, an in-depth analysis of crystalline characteristics has been performed for a unique set of strictly monodisperse poly-ε-caprolactone (PCL) oligomers. The molecules have different sets of end groups with various degrees of bulkiness and hydrogen bonding potential, affecting their aptitude to pack in ordered crystal structures. The oligomers also have different numbers of repeating units (n = 2-64), affecting the degree to which end groups influence overall molecular characteristics. The presence of bulky end groups leads to an innate confinement effect on crystallisation which in turn makes it possible to utilize the set of PCL oligomers to study confined space crystallisation. Confined space crystallisation is explored as a route to gain further understanding about the early metastable phases in crystal formation.   The monodisperse nature of the samples made it possible to collect very precise small-angle and wide-angle X-ray scattering data (SAXS and WAXS) as well as calorimetric data. Computer modeling studies were performed to support experimental findings. It was shown that end groups strongly affected crystallisation features for the shorter oligomers (n ≤ 8) but to a lesser extend for the longer oligomers (n ≥ 16). The presence of a bulky end group at one end of an oligomer could inhibit the formation of hydrogen bonds on the other end. Short oligomers (n = 8) with OH-end groups exhibited novel packing characteristics. At one isothermal crystallisation temperature the molecules exhibited not only lamellar ordering but also an additional, likely rectangular or slanted, ordering. The sample was packed in a unique structure with molecular chains lying parallel but not aligned head to head with each other. At a higher crystallisation temperature the molecules packed in a double layered structure and at an even higher temperature in a typical non-folded but tilted single-molecular layer pattern.   Unit cell determination was performed for a short oligomer with two bulky end groups, showing the existence of a tetragonal unit cell with different dimensions than the orthorhombic unit cells previously reported for linear PCL without end groups. To gain greater insight into the earliest stages of molecular packing, in situ WAXS measurements were performed using a synchrotron radiation beam and measuring data each 12 s whilst very slowly going from melt to isothermal crystallisation. It was shown that the crystal unit cell was distorted during the first minutes of slow crystallisation, which might either represent a metastable phase or else a highly distorted orthorhombic phase. / QC 20101105

The research described in this dissertation mainly covers the development of novel technologies for co-crystallisation along with the discovering of plumbagin co-crystal and thermodynamic interrelationship between the co-crystal polymorphs. Co-crystallisation is a fast growing field in the area of crystal design and has shown potential advantages in the field of pharmaceutical. Currently, many research groups are working on the development of new technologies for the synthesis of pure and stoichiometrically controlled co-crystals. In present study, three novel technologies have been developed for co-crystallisation, which include microwave assisted co-crystallisation, spherical crystallisation and microwave assisted sub-critical water processing. The microwave assisted co-crystallisation is a slurry based technology where, effects of drug solubility and dielectric properties of the solvent were investigated using caffeine / maleic acid as a model co-crystal pair. The mechanism of co-crystallisation under microwave irradiation has been proposed. The co-crystals of plumbagin with improved solubility were obtained with the coformers such as hydroquinone, resorcinol and urea using microwave assisted co-crystallisation technique. The spherical crystallisation technology was developed for co-crystallisation of carbamazepine / saccharin co-crystal pair and demonstrated its application for polymorphic control and as a potential technique for the purification of desired crystal form through surface energetic based separation. The thermodynamic interrelationship between Form I and Form II of carbamazepine / saccharin co-crystal was studied using different thermodynamic tests. The results obtained suggest that the carbamazepine / saccharin co-crystal polymorphs are monotropic. Microwave assisted sub-critical water processing has been explored as a green technology for the synthesis of co-crystals. Carbamazepine / saccharin co-crystal pair has been used as a model pair and effects of processing variables on the resulting crystal form and degradation of an API have been studied.

This thesis has made two significant contributions to the field of reactive crystallisation. First, new data from batch cooling crystallisation and semi-batch reactive crystallisation experiments of mono-ammonium phosphate (MAP) were obtained to describe the key factors that influence crystal nucleation and growth rates, crystal size distribution (CSD), and crystal shape. The second contribution is the development of a numerical scheme for solving the population balance equations, which can be used to describe the evolution of CSD during the crystallisation process. This scheme combines the finite difference method with a wavelet method, and is the first reported application of this approach for crystallisation modelling and simulation. / Experiments into the batch cooling crystallisation of MAP were conducted both with and without seed crystals. The effects of key factors such as cooling rate, initial level of supersaturation and seeding technique, including seed concentration and seed size, on the real time supersaturation, final CSD, crystal yield and crystal shape were investigated. It was found that a seed concentration of 20-30% effectively suppressed nucleation. The growth and nucleation rate were estimated by using an isothermal seeded batch approach and their parameters were calculated by non-linear optimisation techniques. / The second series of experiments involved the semi-batch reactive crystallisation of MAP. Both single-feed and dual-feed systems were investigated. In the single-feed arrangement, an ammonia solution was fed into a charge of phosphoric acid. In the dual-feed system, phosphoric acid and ammonia solution were fed into a charge of saturated MAP solution. The molar ratio of the reactants, initial supersaturation, presence or absence of seed crystals, initial MAP concentration, reactants’ flow rate, feeding time and stirring speed were varied, and the effects upon the real time supersaturation, final CSD, crystal yield, crystal shape and solution temperature were measured. X-ray diffraction analysis showed that MAP can be produced in both the single-feed and dual-feed arrangements. For the single feed system, the N/P mole ratio controlled the degree of reaction and the CSD of the product. Di-ammonium phosphate (DAP) was not be observed in the single-feed system due to its high solubility. In the dual-feed system, a seeded solution with slow feed addition, moderate stirring speed and a low initial supersaturation provided the most favourable conditions for generating a desirable supersaturation profile, and thus obtaining a product with good CSD and crystal shape. / A comparative numerical study was undertaken in order to evaluate the existing numerical schemes for solving the population balance equations (PBE) that describe crystallisation. Several analytical solutions to the PBE were used to benchmark the following numerical schemes: Upwind Finite Difference, Biased Upwind Finite Difference, Orthogonal Collocation with Finite Elements, and Wavelet Orthogonal Collocation. The Wavelet Finite Difference (WFD) method has been applied here for the first time for solving PBE problems. The WFD scheme was adapted to solve the batch cooling and the semi-batch reactive crystallisation models, and the solutions were validated against experimental data that we obtained. / In summary, the experimental data provide an improved understanding of MAPreaction and crystallisation mechanisms. The adaptability of the WFD method has beendemonstrated validating the two crystallisation systems, and this should help extendthe application of wavelet-based solutions beyond crystallisation processes and intomore diverse areas of chemical engineering.

Occasionally, crystallisation of proteins works in mysterious ways! One might obtain crystals of a protein of unknown identity in place of the protein for which crystallisation experiments were performed. If the investigator is not aware of such possibilities, valuable time and resources might be lost in attempting to determine the structure of such proteins. Instances of non-target protein getting crystallised may not come to light at all or may be realised only when attempts to determine the structure completely fail by conventional procedures after collecting and processing the diffraction data. Usually, it is not possible to reproduce the crystals of the same protein as their occurrence is serendipitous. Such rare instances of crystallisation are probably caused by fluctuating environmental or crystallisation conditions and are not reproducible. It could also be due to contaminating microbes, which is more likely when the experimentalist is not well experienced. Therefore, experimental phasing of the data collected on serendipitously obtained crystals could be a challenging task. With the rapid increase in the number of structures deposited in the protein data bank (PDB), molecular replacement has become the method of choice for structure determination in macromolecular X-ray crystallography. This is due to the fact that it is possible to select a suitable phasing model for most target proteins based on their sequence information. However, if the identity of the target protein itself is uncertain, all attempts of structure determination using phasing models selected on the basis of target protein sequence-dependent search would fail. Sequence-independent ab initio phasing techniques such as ARCIMBOLDO (Meindl et al., 2012), which has recently become available, could provide leads only if the non-target protein is an all-α-protein and the associated diffraction data extends to a resolution better than 2 Å. Even then, the success rate with this technique is low. Hence, it becomes important to employ a sequence-independent method of structure determination for such mysteriously obtained crystals. This thesis reports crystal structures of proteins which are serendipitously crystallised using a large-scale application of Molecular Replacement (MR) technique (referred in this thesis as MarathonMR). This thesis also presents an evaluation of molecular replacement strategies for structure determination. The thesis begins with an overview of crystallographic methods of structure determination with an emphasis on the method of molecular replacement (Chapter 1). The most prominent of the results obtained in the course of these investigations pertains to a crystal obtained during routine crystallisation of a viral protein mutant in the year 2011. The cell parameters were different from cell constants of crystals obtained with other known viral protein mutants crystallised earlier in the same laboratory. Unfortunately, this crystal could not be reproduced in the same form in subsequent crystallisation trials. All attempts to determine the structure through conventional molecular replacement techniques using a combination of domains from a nearly identical virus coat protein protomer as the phasing model had failed. The data was shelved as “not-solvable” in late 2011. However, the crystal had diffracted to 1.9 Å and had excellent merging statistics. Therefore, the data was retrieved recently and additional attempts were made to determine the structure through phasing techniques that have become available recently. Techniques such as AMPLE (Bibby et al., 2013) and Rosetta (DiMaio, 2013), which use large-scale homology models coupled with molecular replacement, did not lead to meaningful solutions. A couple of helices identified by ARCIMBOLDO (Meindl et al., 2012) were neither correct (retrospectively) nor sufficient to determine the entire structure. Given the excellent merging statistics of the crystal data, there was significant motivation to determine the structure, though it meant developing a fresh protocol. It was at this time that we came across the work of Stokes-Rees and Sliz (2010) in which they had demonstrated that it is possible to determine structure of proteins of unknown identity by employing almost every known protein structure as a potential phasing model. The work reported in the thesis is a result of an earlier project to examine the relationship between properties of phasing models and the quality of target protein model generated through MR by employing large scale molecular replacement runs. This project was initiated because of the realisation that the recent explosion in crystallographic structural studies has resulted in near complete exploration of the “fold-space” of proteins and PDB now has a representative structure for most plausible folds of proteins. Some folds are highly represented in the PDB. Hence, it is likely that there would be at least one homologue in the PDB which could be used as a phasing model to successfully determine the structure of a protein of unknown identity if the diffraction dataset is of excellent quality. Hence, the single dataset which had diffracted to 1.9 Å resolution was used to develop a MarathonMR procedure for structure determination. MarathonMR procedure takes sequence-independent approach to structure determination and employs large-scale molecular replacement calculations to identify the closest homologue (in structural terms initially). This protocol is described in Chapter 2 (Materials and methods) of the thesis. Through MarathonMR, structure of the dataset which had remained unsolved for 5 years was finally determined. Nearly complete sequence of the polypeptide could be deduced by inspecting the electron density map due to the high resolution and quality of the map. The protein was found to be a phosphate binding protein from a soil bacterium Stenotrophomonas maltophilia (SmPBP). The way in which the structure was determined and possible explanations for the mysterious source of this protein which had crystallised instead of the target protein is discussed in Chapter 3. Though MarathonMR procedure was developed to solve a single dataset, it was soon realised that the same procedure could be applied to other similar datasets, all of which had diffracted to reasonable resolutions with good merging statistics but had remained unsolved for unknown reasons. Among such datasets, one of the datasets which was collected in 2007 and had diffracted to 2.3 Å resolution had cell parameters very close to that of SmPBP. Hence, a poly-alanine model of the structure of SmPBP, which was determined by then, was used as the phasing model to run molecular replacement and the structure was readily solved. It was surprising to note that SmPBP had crystallised serendipitously not once but twice, once in 2011 resulting in crystals that diffracted to 1.9 Å resolution and earlier in 2007 in crystals that diffracted to 2.3 Å resolution independently by two different investigators in the same laboratory. Both the structures are nearly identical and a comparison of these structures is presented in Chapter 4. Structure of SmPBP determined at 2.3 Å resolution by MarathonMR also corresponds to the dataset that had remained unsolved for the longest period of time (9 years). This success of structure determination after the lapse of such a long period emphasises the importance of carefully preserving X-ray diffraction data irrespective of its immediate outcome. In Chapter 5 of the thesis, another instance of non-target protein crystallisation, the structure of which was determined using the MarathonMR procedure is described. The crystal was obtained while carrying out crystallisation of mutants of a survival protein (SurE) expressed in Salmonella typhimurium when the bacterium is subjected to environmental or internal stresses. The original investigator had used the structure of SurE as the phasing model to determine structure of the mutant crystals and obtained a model with R and Rfree of 35% and 40%, respectively. However, the model did not refine further to lower R-factors suggesting that the solution obtained may not be correct. MarathonMR indicated that the fold of the crystallised protein could be similar to that of glycerol dehydrogenase. As SurE shares some fold similarity with one of the domains of GlyDH, the original investigator might have been able to achieve a limited success with R/Rfree factors of 35% and 40%, respectively. As the merging statistics for this diffraction data set was poor, the diffraction images were reprocessed in XDS program on Xia2 automated spot processing pipeline. The data statistics indicated merohedral twinning (14%). However, using appropriate parameters, it was possible to refine the structure obtained by MarathonMR to acceptable R/Rfree using the Refmac program. Four protomers were present in the crystal asymmetric unit (ASU). Non-crytsallographic symmetry averaging of electron density over these four molecules further improved the electron density. As the data was limited to 2.7 Å resolution, it was not possible to deduce the identity of every residue of the protein unambiguously based solely on the resulting electron density map. With the identity of the amino acids that could be deduced with certainty, it was clear that the protein belongs to glycerol dehydrogenase from a species of Enterobacteriacea family. Though a similar structure of glycerol dehydrogenase has been reported from Serratia, there are clear differences in many unambiguously determined residues which suggest that the protein is not from Serriatia. The protein has been named EnteroGlyDH as the source of the protein is likely to be from a species of Enterobacteriacea family. The structure of the protein, its biochemical implications and possible reasons for the serendipitous crystallisation of a non-target are discussed. Chapter 6 discusses the structure determination of an inorganic pyrophosphatase and catalytic domain of Succinyl transferase, the crystals of which had diffracted to 2.3 Å and 3.1 Å, respectively, but had remained unsolved. Neither of the datasets corresponds to the intended target proteins. The dataset corresponding to the protein whose structure was determined as that of an inorganic pyrophosphatase was provided by a colleague from a different laboratory in the Indian Institute of Science. It is interesting to note that the investigator had carried this dataset to one of the CCP4 workshops and had tried to determine the structure with the help of experts in the workshop. The attempts to determine its structure had however failed for reasons that are obvious now. The original investigator was unfortunately making efforts with an erroneous assumption on the identity of the target protein. As these enzymes are well studied, their structures and functions are briefly discussed. It is already well established that molecular replacement is being used with increasing frequency as the phasing technique when compared to other experimental phasing techniques. With the ever growing number of structures in the PDB, high population of certain folds and a near-plateau attained in the identification and growth of new folds, it is reasonable to expect that molecular replacement will be used even more frequently in the years to come. Therefore, for carrying out molecular replacement for a given diffraction dataset of a target protein, it is very likely that several homologous structures would be available in the PDB that could be used as potential phasing models. Hence, it becomes important to understand the influence of phasing model on the quality and accuracy of model generated through MR to achieve the best structure solution. To understand this relationship between phasing model and model obtained by MR protocol, re-determination of already known structures deposited in the PDB starting with their respective structure factors and various phasing models was initiated. Structures belonging to TIM beta/alpha-barrel (SCOPe ID: c.1) and Lysozyme-like (SCOPe ID: d.2) folds were chosen as targets. The structure of each target was re-determined serially starting with poly-alanine models of all available unique homologues as phasing models. Due to the multi-dimensional nature of this study, the results obtained were represented in a graphical form with nodes and edges. Detailed methodology of the work carried out and the data representation model are discussed in the Chapter 2 (Materials and methods). It was found that after a certain sequence identity cut-off, sequence identity between phasing model and target seems to have little influence on the quality and accuracy of the model generated through MR. Instead, other qualities of the phasing model such as Rfree and RSCC influence the quality of MR models. These results are discussed in Chapter 7. Learning from the work reported in this thesis are discussed in concluding chapter. The possible logical and programmatic upgrades to MarathonMR protocol and future path in which the relationship between phasing models and models generated through MR can be studied are discussed in Chapter 8 (Conclusion and future prospects).

Active pharmaceutical ingredients (APIs) are frequently delivered to the patient in the solid-state as part of an approved dosage form (tablets, capsules, etc.). Understanding and controlling the solid-state chemistry of APIs is therefore an important aspect of the drug development process. APIs can exist in a variety of distinct solid forms, including polymorphs, solvates, hydrates, co-crystals and amorphous solids. Each form displays unique physicochemical properties that can profoundly influence the bioavailability, manufacturability, stability and other performance characteristics of the drug. Most APIs are purified and isolated by crystallisation from an appropriate solvent during the final step in synthetic process. The main objective of a crystallisation process is to produce crystals with desired properties such as particle size distribution (PSD), shape and purity. All pharmaceutical dosage forms must be produced in uniform units, and good content of uniformity is only possible when the size of the active component is carefully controlled. For on-line control of crystallisations of Quetiapine Fumarate to achieve desired PSD and no changed physicochemical purity was used the Lasentec Focus Beam Reflectance Measurement (FBRM) system.

Metallic glass is a class of materials which have a disordered structure of atoms, due to this, glasses lack grains and grain boundaries, which are present in their crystalline counterparts. Metallic glasses have many interesting properties worth investigating, such as high corrosion resistance or high mechanical strength. However, metallic glasses are metastable and will therefore crystallise if heated above the crystallisation temperature. MoSiZr alloys have been studied and to gain knowledge of how the composition affects the crystallisation temperature, which enables further improvement of thermal stability. Crystallisation temperatures of the MoSiZr alloys were investigated by heat treatments in vacuum and ex-situ X-ray diffraction and X-ray reflectivity analysis. The highest thermal stability of the alloys was exhibited by M48Si48Zr4, Mo43Si50Zr7, Mo50Si40Zr10 and Mo45Si43Zr12, they remained amorphous after heat treatment at 1073 K. The resulting crystalline phases are Mo3Si, Mo5Si3 and ZrO2. Oxidation of Zr in the alloys is present only when the Zr content is at least 10 at%, crystallisation is otherwise mainly driven by formation of Mo3Si. Further improvement of the thermal stability is possible by introducing new alloying elements at the cost of those that promote crystallisation. Keeping the content of Zr below 10 at% is of great importance to prevent oxidation.

The thesis provides a background on crystallisation, the effects of applied fields and summarises the techniques used for characterisation and analysis. The study of applied magnetic fields was carried out on three crystallising systems (a) sucrose, (b) lactose and (c) cocoa butter. Both sucrose and lactose were crystallised from aqueous solutions in incubators at 50°C in applied magnetic fields and the resulting crystals compared to the those obtained under zero field conditions. The results for the sucrose study where the magnetic treatment was carried out under static, dynamic pumped and dynamic syphoned conditions domonstrated that changes in phase, crystallinity, morphology and microcrystallinity were a result of the applied magnetic fields and additional strongly bound water was found to be present within the sucrose crystals most likely to be sucrose hydrates. The resulting sucrose crystals were dependant on the type of field applied, the purity of the sucrose solution and the residence time within the applied field. The lactose study under static conditions provided similar results concluding that applied fields resulted in a more controlled crystallisation resulting in increased crystal size, increased crystallinity and changes in morphology. Crystallisation of cocoa butter from the melt, under normal production conditions in applied fields, resulted in changes in morphology and the time taken to reach optimum tempering which were dependant on the type of applied field and the residence time in the applied field.

Advances in molecular-simulation methods allow for ever larger systems of particles to be studied and on longer timescales. Calculations are reaching such a scale that they can be used to address a vast range of key questions across chemistry, physics, and engineering. In this work, molecular dynamics and Monte Carlo simulations are employed to address two key areas: the structure and dynamics of simple aqueous ionic salt solutions at high concentrations; and the structure, dynamics, and phase behaviour of dipolar fluids (such as colloidal ferrofluids). The first part of the work begins with a study of the structure and dynamics in metastable, supersaturated, aqueous solutions of potassium chloride, and the possible relevance of these to the phenomenon of non-photochemical laser-induced nucleation (NPLIN). It is thought that the potassium and chloride ions form long-lived, amorphous clusters that may, under the influence of nanosecond laser pulses, undergo structural reorganisation to form post-critical crystal nuclei. It is found that spontaneous nucleation does not occur on the simulation timescale, but that amorphous clusters do form with cluster lifetimes comparable to those of the shortest laser pulses that can be used in NPLIN ( 100 picoseconds). Next, an alternative scenario for NPLIN involving rapid laser heating of impurity particles is examined by simulating heated carbon nanoparticles in saturated aqueous solutions of sodium chloride. The concentration at which an aqueous sodium chloride solution first crystallises on the simulation timescale is determined. A spherical carbon impurity is then added to a system with concentration close to, but lower than, the concentration at which crystallisation occurs on the simulation timescale. The effects that adding, and heating, this impurity has on the structure of this near-crystallising system are then observed. The second part of the work discusses model dipolar fluids, of direct relevance to colloidal ferrofluids (suspensions of magnetised nanoparticles in simple carrier liquids). The two-body, dipole-dipole interaction is long-ranged and anisotropic, and it is computationally expensive to handle in molecular simulations. Here a new method is proposed that relies on a formal mapping between the partition function of a dipolar fluid and that of a hypothetical fluid with many-body, short-ranged, isotropic interactions. Only the leading-order two-body interactions (akin to the van der Waals attraction) and three-body interactions (corresponding to the Axilrod-Teller potential) are retained. It is shown that this simple model is sufficient to reproduce the characteristic particle chaining and the associated disappearance of the vapour-liquid phase transition of dipolar fluids. Finally, the dynamical response of ferrofluids to oscillating magnetic fields (the dynamic magnetic susceptibility [DMS]) is studied. The DMS of ferrofluids, predicted by a new theory that takes into account the leading-order effects of dipole-dipole interactions, are critically compared to those found using Brownian-dynamics simulations of monodisperse systems of dipolar particles. This new theory is found to provide more accurate predictions of the DMS than previous theories, with the DMS predicted to a high degree of accuracy for systems with dipolar coupling strength in the experimentally achievable region.

The practical application of non-photochemical laser-induced nucleation (NPLIN) to continuous flow was investigated. Supersaturated aqueous solutions were screened with a 5 ns pulsed laser (532 nm 44 MW cm-2) for NPLIN activity. Upon irradiation succinic acid nucleated at S20 = 4.3 and adipic acid at S20 = 2.0 - 3.0. NPLIN activity is reported for the first time in nicotinic acid (S20 = 2.6 - 3.0). No overall pattern was observed of chemical structure on NPLIN activity. From inorganic compounds similarly screened, ammonium chloride (S20 = 1.04 - 1.20) was identified as most suitable for further tests. It was shown to have an increase of NPLIN crystals with higher supersaturation from 13 at S = 1.038 to 252 at S = 1.135. A quadratic increase in number of crystals with increased laser power. The effects of NPLIN upon ammonium chloride are diminished upon filtration through a 0.2 μm poly (ether sulfone) filter, reducing the number of crystals from 350 to 10 per 70 mJ pulse (25 MW cm-2). The use of NPLIN in continuous flow was demonstrated from the first time. A S23 = 1.1 solution of aqueous ammonium chloride in flow produced crystals when irradiated by 10 pulses s-1 of a 1064 nm 6 ns laser. When the laser was stopped, crystals were no longer produced and the system returned to flowing supersaturated solution. Lab scale apparatus for continuous NPLIN experiments was developed. A design involving a re-dissolution step and loop flow was constructed for both laminar and slug-flow regimes. Nucleation of ammonium chloride (S = 1.1) was demonstrated in both systems. Repeatable NPLIN experiments were hindered by spontaneous nucleation. Spontaneous nucleation in flow was observed around areas where supersaturated solution passed from one component to another. Spontaneous nucleation was also observed upon cooling (25 to 10 °C). Filtration was observed to both suppress NPLIN and spontaneous nucleation in flow.

Scale formation is one of the persistent problems in mineral processing and related industries. One of the main components of the scale is frequently gypsum or calcium sulphate dihydrate (= CaS04.2H20). Gypsum is formed through the process of crystallisation, and it is well known that crystallisation process is significantly affected by the presence of admixtures. Industrially, scale formation occurs in an environment which is very rarely free from the presence of admixtures. In a typical mineral processing industry, certain types of admixtures are present, which may include metallic ions (e.g. originated from corrosion products) and certain types of the flotation agents used. The effect of admixtures on crystallisation kinetics and cyclical morphology can be very significant, even if they are present in trace amounts. It is important to emphasise that the effects are generally specific, that there is no unified theory that applies to all and every situation. The present study has investigated the effect of certain admixtures on gypsum crystallisation, and was accomplished in three phases of experiments: (1) seeded batch crystallisation; (2) seeded continuous crystallisation, and (3) once through flow system under isothermal condition. The three phases of the work used equimolar solutions of CaC12 and Na2SO4 to produce CaS04 which is the precipitating species. The seeded batch crystallisation experiment explored the effect of two flotation agents commonly used in mineral processing plants: (1) sodium isopropyl xanthate (= SIPX) and, (2) isopropyl thionocarbamate. The experiments were performed at 25, 35, and 45°C, respectively. The initial concentration of the crystallising solution was 2,000 ppm of Ca 21 and it reached the equilibrium concentration values of between 1,000 and 8,00 ppm of Ca 2+ in 90 minutes. / The effect of the two selected admixtures on crystallisation was measured by continuous monitoring of the desupersaturation of the crystallising solution with time, which subsequently resulted in the determination of the crystallisation rate constant. The results arc as follows. Firstly, the admixtures selected (either individually or in combination) were able to retard the growth rate of gypsum. In the absence of any admixture, the second order rate constant was between 1,405 x 10-6 and 1,561 x 10-6 ppm-1 min-1. Addition of SIPX at a typical plant dosing level 0.200 g/L) reduced the rate constant to 475 x 10-6 PPM-1 min', while isopropyl thionocarbamate at a typical plant dosing level (= 0.070 g/L) decreased the rate constant to 254 x 10-6 ppm-1 min-'. However, addition of a combination of the two admixtures, each at a typical plant concentration level, reduced the rate constant to 244 x 10-6 ppm-1 min-1, which was only slightly below that in the presence of isopropyl thionocarbamate. Thus, in these batch crystallisation studies, isopropyl thionocarbamate seemed to be dominant over SIPX. Secondly, the batch crystallisation system in the current work did not show any induction time. It was concluded that the seeds added into the batch system could be capable of eliminating the induction time. Thirdly, the reduced growth rate of the gypsum crystals as affected by the admixtures was probably caused by the adsorption of admixtures onto the crystal surface. The second phase of the project involved a seeded continuous (MSMPR) crystalliser. Some parameters used in this experiment (mean residence time, agitation speed and type of one admixture) were taken from the batch experiment carried out in the first phase of the project. / Three admixtures were chosen for the seeded continuous crystallisation: (1) SIPX, (2) Fe3+, (3) Zn2-, and they were used either individually on in combination with each other. SIPX was chosen as it is one of the most common flotation agents used in mineral processing. Metallic ions: Fe3+ and Zn2+ were selected, since they were found in substantial amounts in both scale samples and process water in certain minerals processing industries. In general, the admixtures tested were found to be able to inhibit the crystal growth rates, but to enhance the nucleation rates. In addition, the growth rate was found to be dependent on crystal size, and hence, a correlation between these two parameters and the admixture concentration was formulated. For a fixed level of concentration (f 700 ppm of Ca z+ at steady state) and crystal surface area, it was proved that for each crystallisation temperature: 25 and 40°C, the correlation function can be represented as G = k Lα (1 +C)β where: G = linear growth rate, micron/hour; k, α, and β = dimensionless constants; L = (sphere equivalent) crystal size, micron; C = concentration of the admixtures used, ppm. For both the crystallisation temperatures used, the correlation function shows that the growth rate is significantly dependent on crystal size, but a weak function of admixture concentrations. The mechanism of crystal growth inhibition was assumed to be that of adsorption of admixtures onto the active growth sites, thereby decreasing or stopping the growth. Similar to the first phase of the present study, this seeded continuous crystallisation also showed no induction time. The third phase of the project investigated the gypsum scale formation in a oncethrough pipe flow system under isothermal condition and in the presence of admixtures. / Four types of pipe materials were tested: PVC, brass, copper and stainless steel. Two admixtures were selected: SIPX and Fe3+. The behaviour of the gypsum scale formation was measured as the mass of the gypsum scale deposited on the substrate per unit area of the pipe surface. Within the range of the experimental conditions applied in this scale formation study, the following results were obtained. Firstly, the mass of the gypsum scale increased with concentration (in the range: 2,000 to 6,000 ppm of Ca t+) and that the correlation between the mass and the concentration can be represented by quadratic functions. Secondly, the mass of the gypsum scale decreased with increasing concentration of the admixtures used. Thirdly, the flow rate of the scaling solutions (in the range: 0.4 to 1.3 cm/sec) did not significantly affect the mass of the gypsum scale. PVC produced the highest mass of gypsum scale, followed by brass, copper, and stainless steel, respectively. Fourthly, the presence of admixtures caused the surface of the scale deposit to become rougher than was the case in a pure system, and longer scaling experimental times resulted in denser scale deposits. In this scale formation project, the induction time was investigated. In contrast with the first and the second phase of the projects, the induction time in the scale gypsum formation experiment was significant. At a concentration of 2,000 ppm of Ca 2+' pure gypsum solutions had induction times of about 105 minutes at 18.3°C and 97 minutes at 20.3°C. In the presence of 10 ppm of SIPX, the scaling solution at 2,000 ppm of Ca2+ and 19.2°C had an induction time of 1,400 minutes. The present study produced three important findings. / Firstly, the presence of Fe 3+ or sodium isopropyl xanthate (SIPX) reduced the growth rate of gypsum crystallised either in a vessel (= a continuous crystalliser) or in a pipe flow system. Secondly, the rate of growth of gypsum crystals was found to be consistently higher in the vessel than in the pipe flow system. The rate of growth of the pure gypsum in the crystalliser at 25°C was 0.0389 kg/ m2 hour while those in the pipe flow system were between 0.0289 and 0.0202 kg/m2 hour, depending on the pipe material and the scaling solution flow rate. Thirdly, with respect to gypsum scaling, PVC was the least favourable material, followed by brass and copper, while the most favourable was stainless steel. It is believed that the present study has significantly contributed to the understanding of the effect of admixtures on crystallisation of gypsum, especially in relation to the scale formation.