氢气氛下硫酸钠对红土镍矿晶相转变的促进作用

Promotion of sodium sulfate on the crystal phase transition of nickel laterite ore in a hydrogen atmosphere

实验选用氢气为还原剂,无水硫酸钠为辅助添加剂,在实验室自制搅拌式气-固反应装置中通过改变温度、时间、Na2SO4用量、H2/N2比率对低品位红土镍矿进行选择性还原焙烧实验,焙烧矿通过磁选管进行磁选分离制备高品位镍铁合金。原矿、焙烧矿和磁选精矿的矿物学性质通过热力学计算并结合TG-DSC、H2-TPR、XRD、光学显微镜等分析仪器进行表征。实验结果表明:添加硫酸钠对促进红土矿晶相结构转变和提升H2的还原能力起到了积极的作用。还原温度作为供热源能够显著改善硫酸钠的催化反应活性,提高磁性产品中镍铁含量。在温度为800℃,总气速为200 L/h(H2/N2=7/3),还原时间为220 min的最优条件下,含20%硫酸钠的红土矿经还原-磁选后能够获得镍品位6.43%,镍回收率97%的较好指标。从热力学角度分析,根据吉布斯自由能图和平衡气相图看出硫酸钠能与矿物中的硅酸镁(MgSiO3)在700℃左右即可自发反应释放出赋存于其中的镍,反应生成的SO2能够促使FeO转变为FeS,FeS则有助于Fe-S低熔点固溶体的形成,从而促进镍铁粒子的定向转移和聚集长大,便于后续磁选中磁性镍铁矿物与脉石矿物的分离。

Hydrogen was experimentally selected as a reducing agent and sodium sulfate as an auxiliary additive. The reduction of the low-grade nickel laterite ore was carried out in a self-made gas-solid reactor with provisions for agitation control by changing the temperature,time,Na2SO4 dosage and H2/N2 volume ratio,and the magnetic separation of the reduced ore was performed using a magnetic separator to prepare high-grade nickel-iron alloy. The mineralogical properties of the raw laterite ore,reduced ore and magnetic concentrate were characterized using thermodynamical calculation,differential thermogravimetry-differential scanning calorimeters （DTG-DSC）, hydrogen-temperature programmed reduction （H2-TPR）,X-ray diffractometry（XRD）,and optical microscopy. The experimental results show that the addition of sodium sulfate could accelerate the crystal phase transition of laterite ore and elevate the utilization of H2. The increase of reduction temperature , as a heat source , could enhance the catalytic reaction activity of sodium sulfate significantly and improve the content of nickel and iron of magnetic product. The optimal reduction condition was obtained as the selective reduction of laterite ore at 800℃ for 220 min in the presence of 20% Na2SO4 with an inlet gas mixture of 70% H2 in N2 at a total gas flow rate of 200 L/h. The maximum nickel content and recovery of magnetic product were 6.43%and 97%,respectively. From a thermodynamic point of view,it can be known according to the Gibbs free energy and equilibrium vapor phase diagrams that the sodium sulfate could react with MgSiO3 which exists in the minerals spontaneously at around 700 ℃ and release the nickel. Besides,the generated SO2 could promote FeO transform into FeS,which were conductive to the formation of Fe-S solid solution and thereby contributed to directional mass transfer,accelerated the coalescence of metallic ferronickel particles and facilitated the downstream magnetic separation.