磷酸盐沉淀法除铁过程热力学分析

Thermodynamic analysis on removing iron by phosphate precipitation

除铁是锌湿法冶金过程的重要步骤。磷酸盐沉淀法相比于传统的铁矾、针铁矿和赤铁矿沉淀法具有明显的优势。针对磷酸盐沉淀法的从硫酸锌溶液中净化除铁过程进行热力学分析,绘制了298K时Men+-P-H_2O(Me:Zn(Ⅱ),Cu(Ⅱ),Fe(Ⅱ),Fe(Ⅲ),Ni(Ⅱ))系组浓度对数-pH图,利用热力学平衡图对磷酸盐沉淀法的从硫酸锌溶液中净化除铁和磷酸铁碱分解过程进行热力学分析。结果表明:pH值为0~5.0时磷酸盐形成由易至难依次为Fe(Ⅲ)>>Cu(Ⅱ)>Fe(Ⅱ)>>Zn(Ⅱ)>Ni(Ⅱ);整个pH值范围可以分为难溶磷酸盐稳定区、Me(OH)_n稳定区;高p H区磷酸盐中的Me转变为稳定的Me(OH)_n,实现磷酸盐碱分解。验证实验表明,加入1.0倍理论量的磷酸钠,控制沉淀pH值为2.0,铁、锌、铜、镍沉淀率分别为98.9%、3.5%、2.8%、0.7%;FePO_4与其2.0倍物质量的Na OH反应,产物为Fe(OH)_3、Na_2HPO_4,磷浸出率为96.8%,分解液pH为11.3,实验与理论相符。

Removing iron is the key process of extraction zinc by hydrometallurgy. The removing iron from leach solution by means of phosphate precipitation may be a feasible alternative to conventional precipitation processes as jarosite, goethite or hematite. Aiming at removing iron by phosphate precipitation, the logarithm concentration-p H diagram of Me^（n＋）-P-FeSO_4-H_2O（Me：Zn（Ⅱ）, Cu（Ⅱ）, Fe（Ⅱ）, Fe（Ⅲ）, Ni（Ⅱ）） system was established on the basis of thermodynamic data at 298K, according to the laws of conservation of mass and simultaneous equilibria. Themodynamic analysis was carried out to discuss the process of removing iron from lixivium of zinc calcine by phosphate precipitation and decomposition of iron phosphate by sodium hydroxide. The results show that the ions can be removed respectively by phosphate precipitation in the following order： Fe（Ⅲ）〉Cu（Ⅱ）〉Fe（Ⅱ） 〉Zn（Ⅱ）〉Ni（Ⅱ）. The whole pH value of the system is divided into two areas. The one is the stable of sparingly soluble phosphate, the other is Me（OH）n. Me（OH）n is gradually superaturated in the solution with the increase of p H value, so the Me（OH）n is deposited as sediment, and phosphate goes into solution. The verification experiment results showed that the precipitation rate of Fe, Zn, Cu and Ni were 98.9%, 3.5%, 2.8% and 0.7%, respectively, by 100% theoretical amounts of sodium phosphate at pH2.0. When the dosage of Na OH is 2.0 times of the mole ratio of FeSO_4, FeSO_4 decomposed into Fe（OH）_3 precipitate and Na_2HPO_4, phosphorus leaching rate is 96.8%, the p H value of leaching liquid is 11.3, which is consistent with the thermodynamic analysis.