Study On Crystallization Process In Different Chromate Systems

Sodium chromate,sodium dichromate and chromic anhydride,which are important compounds in chromate chemical industry,can be used as oxidizing agent,tanning agent and colorant.They can be also used to product other chromates.Restricted by the traditional production process,the majority quality of domestic chromates was inferior to the abroad products.High quality chromate production needs advanced production technology.As a green technique,electrosynthesis method was an effective means to obtain the high quality chromates,of which the separation and purification need the crystallization kinetics data.At present,there are few reported methods and standards for the quantitative detection of high purity chromates.In order to solve these problems,the laser turbidity technique was used to study the metastable zone width(MSZW)in different solutions in this paper.The trace impurities of chromate in chromates were determined by inductively coupled plasma atomic emission spectrometry(ICP-OES).The main research contents are shown as follows:1.The solubility of sodium chromate tetrahydrate and sodium dichromate dihydrate in different systems such as Na2CrO4·4H2O-H2 O,Na2Cr2O7·2H2O-H2 O,Na2CrO4-Na2Cr2O7·2H2O-H2 O,H2CrO4-Na2Cr2O7·2H2O-H2 O were determined experimentally by the polythermal method with the temperature range from 293K~323K.Based on the solubility data,the dissolution enthalpy and entropy of sodium chromate tetrahydrate and sodium dichromate dihydrate were calculated by van’t Hoff equation.2.The metastable zone width(MSZW)of sodium chromate tetrahydrate and sodium dichromate dihydrate in different systems such as Na2CrO4·4H2O-H2 O,Na2Cr2O7·2H2O-H2 O,Na2CrO4-Na2Cr2O7·2H2O-H2 O,H2CrO4-Na2Cr2O7·2H2O-H2 O were determined experimentally by the turbidity monitoring technique with the temperature range from 293K~323K.The effects of cooling rate,saturated temperature,stirring rate and presence of seed particles,sodium chromate,chromic acid on the MSZW have been evaluated in this paper.3.The nucleation equation and apparent nucleation order m of sodium chromate tetrahydrate and sodium chromate tetrahydrate in different systems were studied by the classical Nyvlt equation.The nucleation constant K and the nucleation order m of sodium chromate tetrahydrate and sodium chromate tetrahydrate in different systems were investigated by the self-consistent Nyvlt-like equation.The nucleation parameter A and the interfacial energy γ of sodium chromate tetrahydrate and sodium chromate tetrahydrate in different systems were evaluated by the novel equation of three-dimensional nucleation theory.Result shows that the changes of MSZW in different systems can be explained by the changes of the nucleation constant K and the interfacial energy γ.4.The content of trace elements in chromates was determined by means of inductively coupled plasma optical emission spectrometry(ICP-OES).For the strong oxidizing property and complex matrices of chromates,it is essential to propose a suitable detection method for the determination of trace elements.In this paper,the methods of standard curve,standard addition curve,matrix matched standards and barium chloride precipitation approach were used to determine the trace impurities of sodium chromate.The detection limits,precision and sample recoveries were studied.By contrast,the methods of standard addition curve and matrix matched standards were selected to determine the trace impurities of indusial sodium chromate.The method of standard addition curve and matrix matched standards was applied for the determination of trace elements in three kinds of chromates.Provincial standards were standardized and established strictly accordance with the requirements for industrial sodium chromate tetrahydrate,sodium dichromate dehydrate and chromium trioxide,respectively.