| dc.description.abstract | Variation of crystallization parameters allows for the control of important particle properties, such as morphology, size and size distribution, which influence down-stream processes like solid-liquid separation, powder flow and drying. Polycrystalline particles with a roughly spherical shape have been identified in the industrial production of a secondary aromatic amine derivative, where filtrations problems are considered to be the major bottle neck. The striking similarity to polycrystalline particles L-glutamic acid has motivated a comparative study of both of these organic substances with respect to crystallization from aqueous solutions and their filtration behaviour. This work is an attempt at bridging the gap between the two well-studied fields of crystallization and filtration. In order to fulfil this task in a broader context a third, inorganic substance was to be selected which could be precipitated as polycrystalline spheres. Calcium carbonate was chosen as an inorganic counterpart because of its importance in oil and gas industry where both precipitation phenomena and solid-liquid separation is crucial for efficient solid liquid-separation. The importance of calcium carbonate furthermore resides in its wide occurrence, in nature, as an important industrial product and problematic scaling mineral. The formation mechanism of polycrystalline particles produced in some crystallization processes is currently not firmly established in the literature. Their formation is usually explained by aggregation although there is evidence that the morphology might be the result of complex growth processes such as spherulitic growth. A further subject of this thesis was thus to investigate the mechanism of formation of the observed polycrystalline particles.
In this work, the crystallization parameters supersaturation, temperature, time and stirring speed were varied in reaction crystallization experiments performed in aqueous solutions for some industrially relevant systems, an aromatic amine derivative, L-glutamic acid and calcium carbonate. Together with different seeding strategies it was possible to affect nucleation and crystal growth. As a consequence the particle morphology, size and ageing behaviour could be manipulated. Filtration tests were performed at constant pressure to shed light on the relationship between particle morphology, size, compressibility and the average specific filtration cake resistance. The filtration cake resistance was used as a means to characterize the filterability of crystal suspensions. Thereby, high values for the cake resistance express poor suspension filterability. In addition, the powder flowability was investigated by means of failure strength experiments in a uniaxial tester device.
The effect of initial supersaturation and temperature on the particle morphology of L-glutamic acid and an aromatic amine derivative was compared in batch experiments by pH-shift precipitation in water at temperatures of 5-80 °C. At higher initial supersaturation ratios both systems give rise to polycrystalline particles which were identified to grow by a spherulitic growth mechanism. On the other hand, monocrystals were obtained at low values of the initial supersaturation, plate-like crystals in the aromatic amine system and needle-like particles of the β-polymorph of L-glutamic acid.
The interconnection between crystal morphology, FBRM (focused beam reflectance measurement) chord length distribution and porosity, cake resistance and compressibility was investigated for L-glutamic acid and the aromatic amine derivative. The best particle morphology with respect to filtration behaviour is supposedly that of spherical particles. However, experimental investigations revealed that higher cake resistance values are associated with the polycrystalline, spherical particles of both systems as compared to monocrystalline needles and polyhedra (the α-polymorph) of L-glutamic acid. High cake resistance values for the spherulites are accounted for by lower mean chord length values and cake compression during filtration, which is assumed to occur predominantly directly above the filter cloth. The cake resistance of polyhedral crystals is in turn found to be 3 times lower than for needle-shaped crystals although the needle-like crystals exhibit 30% higher cake porosity and the mean chord length is approximately the same for both crystal morphologies. Analogous to the performed filtration experiments the different observed morphologies have been compared with respect to their flowability. The failure strength values were found to be lower for polyhedral particles than for needle-shaped crystals. Results for the investigated polycrystalline particles also show that they exhibit worse flowability compared to the monocrystalline morphologies, due to their small mean chord length and their unstable nature.
The crystallization behaviour of polycrystalline particles of the aromatic amine and L-glutamic acid was further studied by determining time-dependent crystallization and filtration characteristics. It was shown that the FBRM mean chord length of the investigated spherulites decreases in the course of time. Concomitant with the decrease in crystal size the average specific cake resistance increases. A faster decrease of the mean chord length with increased temperature and the fact that the solubility of resuspended polycrystalline particles in distilled water drops with time imply an ageing mechanism and not particle breakage to be responsible for the observed changes in the particle size. Unlike for the aromatic amine system, the measured cake resistance for suspensions of L-glutamic acid spherulites decreases after a certain stage of spherulite ageing.
The effect of initial supersaturation and temperature on the particle morphology of calcium carbonate has been compared by reaction precipitation in water at temperatures of 5-90 °C. For the aragonite polymorph it has been observed that higher initial supersaturation ratios give rise to polycrystalline particles. This agrees well with the findings for the aromatic amine and L-glutamic acid which illustrates the role of the supersaturation in the switching from monocrystalline to polycrystalline growth. Moreover lower temperatures were found to favour polycrystalline growth of aragonite. Also for the calcite polymorph the switching from monocrystalline to polycrystalline growth was observed as the temperature was decreased. However, in this case the onset of polycrystalline growth might also be the effect of a higher level of supersaturation. In case of the vaterite polymorph spherulitic growth is expected and can hardly be avoided. Polycrystalline particles of calcium carbonate have been shown to grow by a spherulitic mechanism and not by aggregation of nano-sized precursor crystals as often claimed in the literature. Vaterite spheres, for example, nucleate and grow at a moderate supersaturation ratio of approximately S=7. A supersaturation ratio as low as 7, however does not provide high enough nucleation rates to yield the necessary number of nanosized crystals to account for a nano-aggregation process. Seeded experiments show that the onset of nucleation occurs in accordance with a classical concept of nucleation and crystal growth when varying the supersaturation, seed amount and size, which is not compatible with excessive aggregation in the system. The fact that the degree of branching of aragonite spherulites is higher at lower temperatures further disqualifies aggregation as a possible enlargement mechanism of calcium carbonate spherulites since nucleation rates are expected to be lower at lower temperatures.
Then, to be able to link morphological characteristics of CaCO3 to their filtration behaviour spherical polycrystalline particles of the vaterite polymorph, needle-like crystals of aragonite, cube-like, and novel plate-like crystals of calcite were crystallized. The shape of the crystals was changed mainly by changing the supersaturation, temperature, crystallization time and chemical additives. The mean diameter and the size distribution of vaterite spherulites were varied by applying a seeding strategy. Unlike spherical particles of the aromatic amine and L-glutamic acid, the vaterite spherulites do not exhibit compressible behaviour during filtration testing. The experiments show that spherical crystals with a larger size exhibit lower values for the average specific cake resistance whereas populations with a larger size distribution width, as a consequence of nucleation, exhibit worse filtration behaviour. It was shown that plate-like crystals and especially needle-like crystals of calcium carbonate give higher values for the average specific cake resistance than spherical particles and cubelike particles. | nb_NO |
