Study On Increasing The Bulk Density Of Thiourea

Thiourea is a fine chemical product which is widely used in many fields such as pharmacy, printing and dyeing, synthetic resin, the flotation of metal mineral, electroplating and so on. The bulk density, as a property of thiourea, is not attracted attention since it is not prescribed definitely in product quality standard. But increasing thiourea bulk density may decrease the fare of transport and stock. So, it is significant and necessary for studying on the increase of thiourea bulk density. The undesirable habit, unreasonable size distribution and the severe aggregation of thiourea crystal will lead to the decrease of its bulk density. Usually, thiourea product has a bulk density of 0.60~0.65g/ml. To meet the need of container transport, thiourea bulk density is required to increase to 0.75ml/l. Based on the packing theory, the bulk density is controlled by the crystal shape, and size distribution. Therefore, in this thesis, the factors of affecting the habit and crystal size distribution of thiourea are investigated. Firstly, the effect of three additives, polyethylene glycol, cetyl pyridineium and dodecyi benzene sulfonic acid sodium, on thiourea crystal shape are studied. The experiment shows that the shape of thiourea crystal are changed to diamond shape and slice-like rectangle after adding polyethylene glycol and cetyl pyridineium respectively. When adding dodecyi benzene sulfonic acid sodium, columned thiourea crystal is obtained. An optimum amount of dodecyi benzene sulfonic acid sodium can be found. When its dosage is 5mg/l, the crystal with the ratio of height to radius 1:2 is prepared, and has the maximal bulk density. Next, the metastable zone width of thiourea was measured by laser light scattering method. The experimental results show that the metastable zone width of thiourea widen with the increase of the cooling rate and narrow with the temperature rising. The job provides the basis for the further experiments to select the suited supersaturation. In the end, the effect of the flow situation, the addition amount of seed and the cooling rate on the bulk density of thiourea were studied. The results of the experiments indicated that setting baffles in the crystallizer can optimize the flow situation of the solution and eliminate the primary nucleation. As a result, the crystal dominant size increased. When the addition amount of seed is 3.5%, the crystallization process can be well controlled. In addition, the study shown that the agitation rate affects the crystal aggregation, thus affects the bulk density of thiourea. With the increase of the agitation rate, the aggregative degree was slighter and slighter. But, all the crystals were break up when the agitation rate is increased to 300rpm. When the agitation rate is 220rpm, the product has the maximal bulk density because the crystal aggregation is slight and its size distribution is consistent with the value of the particle packing theory. After the growth rate of thiourea is measured, the optimal cooling curve is calculated. Three cooling ways, optimal cooling curve, linear cooling, and the combination of the above two method, are studied. The experimental results show that the crystal size of thiourea is large and uniform by the optimal cooling way and very fine by the linear cooling. By the way of the combination of the optimal cooling and linear cooling, the bulk density of thiourea is maximal,and reaches to 0.79g/ml. The above-mentioned experimental results have been applied to the industrial practice. The results indicate that the experimental data are accurate and reliable.