In 1970, Japanese scholars studied the phase diagram of iron oxide microcrystal formation, which laid a theoretical foundation for the preparation method of iron oxide citrine seeds. According to the research results, citrine seeds can be formed under acidic or alkaline conditions. Because iron yellow has a crystal structure, to make it crystallize into pigment particles, it must first form a crystal nucleus, which becomes a crystal seed, and then the crystal nucleus grows up to become iron yellow. Otherwise, only a thin, dull color paste without pigment properties will be obtained. The acid method can be divided into two processes: the iron sheet method and the dropping method.
1. Iron Sheet Method
The raw material used in the method is FeSO4, and in order to maintain the concentration of Fe3+ in the reaction medium within a specific range, a reducing agent iron sheet is added during the reaction. Iron yellow is synthesized under a certain pH condition by adding iron yellow seeds and blowing air. The method mainly has two steps: (1) First, FeSO4·7H2O is used as raw material, NaOH or NH3·H2O is used as precipitant or pH regulator, and air is used as oxidant to oxidize and prepare seed crystals; (2) Seed crystals, FeSO4, iron sheet, The air is subjected to two-step oxidation to produce iron yellow.
Here, NaOH or NH3·H2O is used as a precipitating agent or a pH regulator, and reacts with FeSO4 to generate ferrous hydroxide precipitation; air is used as an oxidant; needed ferrous ions and maintain the pH of the solution. The acid method consumes less alkali and the particles are easier to wash. The relative rates of the two processes, seed preparation and crystal growth, determine the particle size, particle size distribution, and particle morphology of iron yellow particles.
In the test, the growth of iron yellow particles can be inhibited by adding additives, so as to prepare iron oxide yellow crystals with small particle size and narrow distribution. In the crystal growth stage, neutralize the acid generated during the reaction by adding lye dropwise or use iron sheet to react with the acid generated during the reaction. The advantage of the acid method is that it can avoid the precipitation of metal impurities and consume less alkali. The disadvantage is that even after repeated washing, the pigment still contains anions, which will reduce its dispersibility, and a certain degree of transparency must be sacrificed to obtain a higher dispersibility.
2. Dropping Method
In the two-step oxidation, if the ferrous sulfate solution is added dropwise instead of the iron sheet, and ammonia water is added dropwise to neutralize the acid generated during the oxidation process, this method is called the drop method.
The reaction involved is: 4FeSO4+O2 (air)+6H2O=4α-FeOOH↓+4H2SO4
The dropping method needs to drop ferrous sulfate mother liquor and ammonia water into the three-necked bottle at a certain speed, measure the pH value of the reaction system with a pH meter at regular intervals, and analyze the concentration of Fe2+ and Fe3+ with the potassium dichromate method to control the reaction. System pH and Fe2+ concentration. Iron oxide yellow is an important inorganic pigment second only to titanium dioxide. Appropriate raw materials and processes should be used to produce high value-added transparent iron oxide yellow according to local conditions. However, because the reaction involves gas-solid-liquid three-phase, the production process is more complicated, the reaction and crystallization mechanism is not very clear, and the process is difficult to control accurately. Therefore, it is necessary to focus on the reaction and crystallization mechanism of iron oxide yellow in order to produce Iron oxide yellow with high quality and low cost.