Thiourea crystal growth kinetics, mechanism and process optimization during cooling crystallization

Abstract

In the cooling crystallization process of thiourea, a significant issue is the excessively wide crystal size distribution (CSD) and the abundance of fine crystals. This investigation delves into the growth kinetics and mechanisms governing thiourea crystals during the cooling crystallization process. The fitting results indicate that the crystal growth rate coefficient falls within the range of 10^-7 to 10^-8 m·s^-1. Moreover, with decreasing crystallization temperature, the growth process undergoes a transition from diffusion-controlled to surface reaction-controlled, with temperature primarily influencing the surface reaction process and having a limited impact on the diffusion process. Comparing the crystal growth rate and the diffusion-limited growth rate at different temperatures, it is observed that the crystal growth process can be broadly divided into two stages. At temperatures above 25 °C, 1/qd (qd is diffusion control index) approaches 1, indicating the predominance of diffusion control. Conversely, at temperatures below 25 °C, 1/qd increases rapidly, signifying the dominance of surface reaction control. To address these findings, process optimization was conducted. During the high-temperature phase (35-25 °C), agitation was increased to reduce the limitations posed by bulk-phase diffusion in the crystallization process. In the low-temperature phase (25-15 °C), agitation was reduced to minimize crystal breakage. The optimized process resulted in a thiourea crystal product with a particle size distribution predominantly ranging from 0.7 to 0.9 mm, accounting for 84% of the total. This study provides valuable insights into resolving the issue of excessive fine crystals in the thiourea crystallization process.

1. Introduction

Thiourea is a sulfur-containing organic compound widely used as raw materials in the synthesis of pharmaceuticals, dyes, and resins [1,2], as well as a vulcanization accelerator for rubber [3], a flotation agent for metal minerals [4,5], and an organic catalyst [6]. The common production of thiourea crystals is to generate thiourea solutions through the reaction of lime nitrogen with hydrogen sulfide and calcium hydroxide, subsequently, a batch crystallization process is employed to cool the solutions from 35 °C to 15 °C, finally yield the aimed products by solid-liquid separation [7]. The resultant products frequently exhibit a broad crystal size distribution (CSD), primarily spanning from 0.15 mm to 0.9 mm, with a substantial presence of fine crystals, which causes moisture absorption and agglomeration [8,9], thereby affecting the production of downstream industries.

To tackle the above problems, the crystallization process of thiourea has been investigated. For example, employing classical nucleation theory to scrutinize the nucleation process of thiourea crystallization reveals that at high supersaturation, thiourea is susceptible to homogeneous nucleation, whereas at low supersaturation, it tends to undergo heterogeneous nucleation [10]. It was also found that the addition of sodium polystyrene sulfonate can inhibit the nucleation, with the apparent order of nucleation m increasing from 0.99-1.65 to 1.54-3.44, and the width of the metastable zone significantly increasing [11].

Compared with the crystal nucleation process, the crystal growth has a more important impact on regulating crystal size, morphology, etc. [12,13]. In general, crystal growth encompasses two fundamental processes: bulk diffusion and surface reaction [14]. For instance, the crystal growth of pentaerythritol is controlled by the complex mechanisms of surface reaction and diffusion limitation [15], while the controlling step for the growth of mesalazine and allopurinol is the surface reaction [16]. Hence, an investigation into the kinetics and mechanisms of crystal growth can elucidate the governing steps in growth, thereby strengthen the crystallization process and achieve CSD and morphology control [17-19]. However, there are few relevant research reports on the growth process of thiourea crystals.