Preparation Of A Leachate With High Mn And Low Fe Contents From Bioleaching Of Low-grade MnO₂ Ore: Process And Mechanism

China leads the world in the production and consumption of electrolytic metal manganese(EMM).The rapid depletion of manganese carbonate ore resources occurred due to high consumption of EMM production.It can be predicted that the use of low-grade manganese oxide ores as tailings to produce EMM is the trend.The bioleaching technology has unique advantages and potential applications in reductive dissolution of Mn²⁺from low-grade manganese oxide because of its features including green safety,cost-effective,energy conservation and environment protection.The autotrophic bioleaching system with sulfur and pyrite as the mixed energy substrate and the sulfur-oxidizing bacteria and ferric-oxide bacteria as the mixed bacteria has the advantages of high efficiency,high speed and short time in Mn dissolution;however,the residual concentration of total iron in the leachate was very high(?1.2 g/L),which significantly increased the cost of cleaning and affected the purity of EMM.In this paper,the bioleaching of Mn from low-grade manganese oxide ore and the green/economic preparation of leachate with rich Mn and low Iron contents were carried out by introducing the waste EMM anolyte(WEMA)with high ammonia nitrogen as both nitrogen source and iron scavenger into the traditional autotrophic aerobic bioleaching system,using a elemental sulfur as the single energy substrate for iron-oxide bacteria in the anoxic bio-reductive leaching system,and utilizating ammonia nitrogen as the single energy substrate for aerobic ammonium oxidizing bacteria in the aerobic bioleaching system.The main findings are as follows:(1)The manganese oxide ore used is a low-grade pyrolusite containing silicon and iron,accounting for 94.9%of Mn⁴⁺and 5.1%of Mn³⁺,respectively.The main phase is silica and pyrolusite,the minor phase is gismondine.(2)The manganese oxide ores mortified by the different pretreatment methods were performed within the traditional bioleaching system composed of mixed energy substrates and mixed autotrophic bacteria,and the effects of Mn²⁺dissolution was compared between the above pretreatment methods.The effectiveness of the five pretreatment processes are:mechanical crushing>high temperature calcination>microwave>hydrothermal decomposition>ultrasonic treatment.Wherein the mechanical crushing activates the manganese ore and enhances the contact and mass transfer of the activated manganese with the mixed bacteria(Acidithiobacillus ferrooxidans,Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans)and the active leachted products,resulting in a 90%for manganese dissolution efficiency at a high pulp density of 10%.The mechanical crushing is the most effective pretreatment method.(3)For the first time,the waste electrolytic manganese anolyte(WEMA)was used as both nitrogen source and iron scavenger for the mixed bacteria to extract Mn from low-grade manganese oxide ore,and to prepare a leachate with manganese-rich and low-iron contents at the high pulp density of 10%.Four significant factors influencing manganese extraction and iron removal were selected from eight factors affecting the bioleaching of manganese ore using Plackett-Burman,Steepest ascent and RSM-BBD,and the optimal conditions of manganese leaching and iron removal were predicted:6%(v/v)of WEMA addition,140 rpm of shaking speed,250 mesh of ore particle size,1.0 g/L of KH₂PO₄,p H value adjustment at 2.4,ratio of pyrite to sulfur at 14:10(g/L),incubation temperature at 31?,and bioleaching period of 14 days at a fixed pulp density of 10%.Under this optimized conditions,79.6%for the Mn extraction efficiency and 0.69 g/L for the total Fe residual concentration were obtained respectively.The interaction between the energy substrate and p H value adjustment has a significant effect on the extraction of manganese and the removal of iron.A 6%WEMA addition promoted the growth and metabolic activity of mixed leaching bacteria,which enhanced the reductive dissolution of Mn²⁺.Meanwhile,the high concentration of ammonium ion promoted the formation of more insoluble ammoniojarosite to greatly reduce the Fe residual concentration,resulting in a iron removed efficiency of 67%.(4)Using sulfur as single energy substrate for mixed bacteria(A.t+A.f),a new bioleaching process of pyrolusite for preparation to prepare the leachate with rich Mn and low Iron contents was proposed.Through the single factor and orthogonal experiments,the main order of factors effected manganese extraction was established as sulfur concentration>heating temperature>shaking speed>leaching time,the corresponding process parameters were 25 g/L,34?,150 rpm and 33 day.Validation experiments showed that the maximum leaching efficiency of manganese at a pulp density of 1% was 98.6% and the residual iron concentration was 305.63 mg/L.During the first bioleaching stage(0?18days)with a higher concentration of dissolved oxygen(DO),the reducing intermediates from sulfur oxidation and derivative of Fe²⁺from Fe³⁺of manganese ore were involved in the reductive release of Mn⁴⁺/Mn³⁺;and in the later of bioleaching stage with a low DO or anoxic condition,Mn O₂was mainly reduced by electron from the coupling reaction of sulfur oxidation catalyzed by A.f and dissimilatory reduction of Fe³⁺.(5)A new bioleaching process of Mn²⁺from pyrolusite based on coupling technique of reductive reaction during the ammonium oxidation catalyzed by ammonia oxidation bacteria and acidic dissolution was studied firstly.By the single factor experiment,the optimal conditions of Mn²⁺extraction were established:1.26 g/L for ammonium nitrogen concentration,6.5 for culture medium p H,25?for culture temperature,shaking speed of160 rpm and reaction time of 30 days.the maximum dissolution efficiency of manganese at a pulp density of 1% was up to 33%.Compared with pure ammonium biooxidation system without manganese addition,the concentration of ammonium nitrogen decreased 11%,the maximum concentration of active hydroxylamine decreased 96%,the maximum decrease of nitrite nitrogen 21%,while the nitrate nitrogen increased 20%.The results show that the reduction of Mn O₂in pyrolusite is the result of the interaction of hydroxylamine and nitrite nitrogen,especially the contribution of hydroxylamine is large.Mn O₂was bio-reduced to Mn₃O₄and Mn O,resulting in a reduction of 47.4%for Mn⁴⁺and increases of 27.9%for Mn³⁺and 19.5%for Mn²⁺,respectively.The total rate of manganese bio-reduction is controlled by the product layer Ca SO₄and the inert layer Si O₂.Nearly 35%for manganese dissolution rate was also obtained using WEMA containing higher content of ammonium nitrogen as reaction substrate.(6)For the first time,a toxic polysaccharide-rich nitrocellulose acid wastewater(NAW)was used as a reductant to extract manganese from pyrolusite.The optimum conditions of 97.4%for maximum manganese dissolution efficiency were obtained by single factor optimization experiment:the dosage of NAW was 100 m L,the dosage of manganese was 100 g/L,the size of manganese ore was 200 mesh,the ratio of concentrated sulfuric acid to NAW was 0.12(v/v),the shaking speed was 160 rpm,the reaction temperature and time were 90?and 120 min.At the same time,98.5% of TOC(total organic carbon)was removed from raw wastewater.There is a significant negative linear relationship between the reduction of manganese and the removal of TOC.The removal of TOC conforms to the second-order reaction kinetics model,and the reduction and dissolution of manganese satisfies to the zero-order reaction kinetics model.The presence of pyrolusite causes the mineralization of toxins in NAW,resulting in a sharp drop in concentration and a significant reduction in toxicity.(7)The EMM was prepared using the leachate with high Mn and low Fe contents.The results showed that EMM could be electrolytically produced after the above four kinds of biological/chemical leachatespurified.The purity of EMM was above 99.6%,and even several have met the grade A for the EMM quality standard.The production of EMM from chemical leachate is slightly higher than that from bioleachate.The Measures to reduce or prevent the introduction of iron ions from the source of leachate preparation are effective,and can significantly reduce the iron impurity content in the EMM.