Dept. of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran

Transcription

1 Hydrometallurgical Processes h m z n u. a c. i r Optimization of the Leaching Conditions of Cobalt and Manganese with Ferrous Sulfate from Filtercake of Zink Plants M. Rajaie Najafabadi a, B. Sedaghat b, M. Karimi c Received:4 March 2015/Accepted:20 March 2015 A R T I C L E I N F O A B S T R A C T Keywords: Filtercake Leaching Cobalt Manganese Orthogonal array The present research work is based on finding the optimum conditions for the leaching of cobalt and manganese from a washed purification filtercake and hence to achieve the highest recovery and the best robustness of the quantitation from the least number of trials in a laboratory scale. Leaching was performed using ferrous sulfate. The orthogonal array L 16 (4 5 ) that comprises five parameters at four levels was chosen. The parameters and their levels were as: ferrous sulfate quantity: 0.6, 0.9, 1.2 and 1.5 times of stoichiometric quantity of manganese dioxide; time: 15, 30, 45, and 60 min; temperature: 25, 40, 60, and 80 ºC; solid/liquid ratio: 1/4, 1/6, 1/8 and 1/10; and sulfuric acid concentration: 10, 50, 100, and 200 g/l. The ultimate optimum leaching conditions were found to be ferrous sulfate quantity: 1.2 times of stoichiometric quantity of manganese dioxide, temperature: 80 ºC, sulfuric acid concentration: 100 g/l, solid/liquid ratios: 1/8 and time: 30 min. Under these conditions, recovery for cobalt and manganese were 99.29% and 97.97%, respectively. a. Dept. of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran b. R&D Center, Research & Engineering Co. for Non-ferrous Metals (RECo) Zanjan, Iran. c. Dept. of Min. and Met. Eng., Yazd University, Yazd, Iran * majidrajaie66@gmail.com 1. Introduction In view of gradual exploitation of land based mineral resources and the lack of mineral resources in some countries, efforts are increasingly being made to explore alternate resources. The production of non-ferrous metals 8

2 from primary and secondary material results in the generation of a wide variety of wastes and residues. They are a result of metals separation that is necessary for the production of pure metals from the complex sources. These wastes and residues arise from the different stages of processing as well as from the off-gas and water treatment systems [1-4]. Cobalt filtercake, which mainly consists of cobalt, manganese, and zinc, is produced during the purification of zinc sulfate electrolyte, and is considered as a good resource of cobalt [5]. On average, 70% of the world s total cobalt production is obtained through hydrometallurgical routes (L-SX-EW) [6]. Manganese from the ores can be extracted selectively using hydrometallurgical techniques too. As manganese dioxide ores are stable in acid or alkaline oxidizing conditions, the extraction of manganese must be carried out in reducing condition. The selection of appropriate reducing agent for cobalt is critical for the acid leaching. Inorganic reagents such as coal lignite [7] (high reagent consumption, coal/ MnO 2 1:1), SO 2 gas (not easy to handle and to regenerate), H 2 O 2 [7] (needs very careful handling because of its explosive nature), cellulose [8] (high consumption), sucrose and glucose [7, 9] (high reagent consumption, slow leaching rate and regeneration issues), NaBH 4 and O 2 have been reported to be problematic in terms of filtration of pulp and higher reagent consumption. However, the use of aniline [10, 11] as reducing reagent seems to be advantageous considering lower reagent consumptions and easy regeneration by electrochemical means, especially phenol type organic acids that have a reduction oxidation couple [7]. Mwema et al. (2002) have reported the application of ferrous ions, copper powder, and sodium metabisulfite (Na 2 S 2 O 5 ) as the reducing agents for the leaching of trivalent cobalt oxide at Shituru plant (Congo) [12]. The above-mentioned discussions hold true for manganese as well, since manganese is also reductively leached from sea nodules and also cobalt filtercake. In order to obtain a high leaching recovery of manganese in acidic solution, reducing agents are required. Different reductants such as ferrous sulfate (Fe ) [13], sulfur dioxide (SO 2 ) [14], and organic alcohols [15, 16] have been used for the leaching of manganese from secondary sources and low grade manganese ores. Also in aqueous reduction, sucrose [17], charcoal [18], coal and lignite [19], pyrite [20], etc. can be used as reducing agents. Oxalic acid can also be used as a reducing agent for manganese extraction from manganese dioxide ore. Stone (1987) reported leaching of manganese ore using oxalic acid along with producing micro-organisms [21]. Carbohydrates are non-hazardous and low cost reducing agents that may be used under mild conditions [22, 23]. Trifoni et al. (2000) investigated the leaching responses of manganiferous ores using 20% glucose in 3N H 2 at 70 ºC [22]. Another study, sulphuric acid leaching of low grade high iron containing Mn-ore using sawdust and lactose has been reported [24]. A study by Das et al. (1982) showed that the reaction of MnO 2 in the low manganese ore with ferrous sulfate could occur in three ways [13]: With neutral ferrous sulfate solution MnO 2 +2Fe +2H 2 O Mn +Fe(OH) +Fe(OH) 3 )1( With ferrous sulfate solution and small amount of acid MnO 2 +2Fe +H 2 SO 4 Mn +2Fe(OH) )2( And with ferrous sulfate solution and excess amount of acid MnO 2 +2Fe +2H 2 SO 4 Mn +Fe( ) 3 +2H 2 O )3( Also in metal extraction processes, large amounts of various solid wastes including 9

3 flotation tailings, slag, slimes, and flue dusts are generated. These wastes become activated due to the applied process such as grinding, leaching, roasting, smelting, quenching the same story. The exposure of these wastes to atmospheric oxygen and moisture results in solubilization of toxic metals which may seriously affect the water quality and biological life in surface waters. The potential releases of toxic heavy metals from such byproducts and waste materials to the surface and ground water are of particular concern [25-28]. In the zinc plant located in Zanjan, Iran, a leach-electrolysis process is practiced for zinc production. In this process, number of filtercakes is generated daily as by products. These wastes are stored for valuable elements recovery in the future and dumped in open stockpiles where they may cause heavy metal pollution problems. In these plants three types of wastes were produced: leaching filtercake, cobalt purification filtercake and Ni-Cd purification filtercake. All of the filtercakes have high levels of heavy metals [29]. The aim of the present article is optimization of the cobalt and manganese leaching from the filter-cake of zinc plant residues. 2. Experimental A. Materials After drying the filtercake, it was ground and homogenized. The chemical analysis was carried out by a Perkin-Elmer AA300 model atomic absorption spectrophotometer. The chemical analysis of the filtercake is given in Table 1. As can be seen from Table 1, the filtercake is mostly composed of zinc and manganese. In order to remove the zinc from the filtercake in the first stage of the experiment and also with the aim of increasing the recovery of Co and Mn in the second stage, washing tests were done. Based on the results from washing experiments in the previous study [30] for zinc removal, the following optimum washing conditions were chosen as: H 2 concentration 200 g/l, Temperature 25 C, the liquid to solid ratio (L/S) 8:1, and reaction time 120 min. Using the optimized conditions, the zinc recovery was nearly 96%. After completion of the washing experiment, the washing residue was filtered, washed, dried, and weighed. The analytic results of the washed filtercake are given in Table 2. Table 1 Chemical analysis of the leaching filtercake Table 2 Chemical analysis of the washed filtercake B. Procedure Experiments were carried out in a glass beaker of 2 liters volume equipped with a mechanical stirrer submerged in a thermostatic bath. Mechanical stirrer (Heidolf RZR 2020) had a controller unit and the bath temperature was controlled using digital controller (within ±0.5 C). For minimizing aqueous loss when the sys tem is heated, a reflux condenser mounted on top of the cell. After adding 1 liter of acid with a known concentration to the reaction vessel and setting the temperature at the desired value, a known weight of sample was added to the reactor while stirring the content of the reactor at a certain speed. At the end of the reaction period, the contents of the beaker were filtered and amounts of manganese and cobalt in the filtrate were analyzed. C. Taguchi Method To consider the optimum conditions for the leaching of cobalt and manganese, Taguchi 10

4 technique was used. Primitive tests indicated that five parameters including: sulfuric acid concentration, amount of Fe O, solid/ liquid ratio, temperature and time were effective. Thus, to investigate the effects of above mentioned parameters in leaching of cobalt and manganese, four levels of them were selected. Recovery of Co and Mn were selected as responses of this process. The Orthogonal array L16, which clarifies 5 parameters at 4 levels, was chosen to perform the tests. The levels of the factors are given in Table 3. According to the Taguchi parameter design methodology, one experimental design should be selected for the controllable factors. The orthogonal array experimental design method was chosen to determine the experimental plan. The orthogonal array (OA), L 16 (4 5 ), which denotes five parameters each with four levels, were chosen since it is the most suitable for the conditions being investigated. The performance statistics were chosen as the optimization criterion. The order of experiments was obtained by inserting parameters into columns of orthogonal array, L 16 (4 5 ), chosen as the experimental plan given in Table 4. The interactive effect of parameters was not taken into consideration while some preliminary tests showed that they could be neglected. The validity of this assumption was checked by confirmation experiments conducted at the optimum conditions. Table 4 is an L 16 orthogonal array, a table of integers whose column elements represent the levels of the column factors. Each row of orthogonal array represents a run, which is a specific set of factor levels to be tested [31]. One of the advantages of Taguchi method over the conventional experimental design methods, in addition to keeping the experimental cost at minimum level, is that it minimizes the variability around the target when bringing the performance to the target value. Another advantage is that optimum working conditions determined from the laboratory work can also be reproduced in the real production environment. Taguchi method recommends the use of the loss function to measure the performance characteristics deviating from the desired value [32]. The value of the loss function is further transformed into a signal-to-noise (SN) ratio. Usually, there are three categories of performance characteristics in the analysis of the SN ratio: that is, the lower, the better; the higher, the better and the nominal, the better. The SN ratio for each level of the process parameters is computed based on the SN analysis. Table 3 Experimental parameters and their levels Table 4 Experimental procedure Using of the SN ratio of the results, instead of the average values, introduces some minor changes in the analysis as follows: Degrees of freedom of the entire experiments are reduced: DOF with SN ratio=number of trial conditions 1 (i.e., number of repetitions is reduced to 1). SN ratio must be converted back into meaningful terms. When the SN ratio is used, the results of the analysis such as esti- 11

5 mated performance from the main effects or confidence intervals are expressed in terms of SN. To express the analysis in terms of experimental results, the ratio must be converted back into the original units of measurements. 3. Results and discussion Finding the optimized conditions for leaching of both cobalt and manganese and determination of effective factors in this process were the intended aims of this research. The collected data were analyzed by Minitab14 software to evaluate the influence of each parameter on the process. Effects of each factor on the whole process will be elaborated in the following sections. A. Cobalt Recovery The effect of controllable factors on the recovery of Co is shown in Fig. 1. In Fig. 1 the vertical axis represents the mean recovery of Co, while the horizontal axes show the different levels of effective parameters. Based on Fig. 1; increasing the acid concentration from level one to level three increases the Co recovery. Yet, applying 200 g/l of acid causes the reducing in recovery. More than 100 g/l acid concentration causes to dissolve other elements which results in reducing recovery. Also, it is reported in the literature that at high acid concentration silica gel will produced from the solution resulting in lower leach recoveries [33] and hard filtration. Fe O acts like a reduction substance in the solution process, so it causes that Mn and Co travel to solution phase. A point out the increasing of the recoveries is the direct effect of addition of this material. Although increasing of Fe O from level three to four does not have significant role in the recovery of Co. This is because the amount of Fe O used in the reaction is sufficient to extract the cobalt from the solution and also more Fe O doesn t have positive effect on the cobalt recovery. Fig. 1 displays the cobalt recovery as a function of L/S ratio (weight of solid/volume of liquid). It was found that the cobalt Fig. 1. Effect of controllable factors and levels on the mean response for Co recovery 12

6 Fig. 2. Effect of controllable factors and levels on the mean response for Mn recovery recovery is increasing until the solid/liquid ratio of 1/8. It can be explained by the fact that having higher free surface of solid particle equals to comprehensive leaching process. Therefore, in the first level of this controllable factor Co was recovered from solution. In this research 4 levels of temperature including 25, 40, 60, and 80 C were investigated. Based on Fig. 1, lower degrees of temperature had lower effects on the recovery of Co. But increasing of this parameter from 60 to 80 C caused improvements in the recovery of cobalt from 78% to 95%. Increasing of the reaction time from 15 to 30 minutes enhances the recovery of Co. By giving additional time recoveries will be decreased and at the 60 min the recoveries reaches to lowest level, although this declination is smoother for Co. High leaching time causes to dissolve other elements and so to reduce the recovery of cobalt. B. Manganese Recovery Fig. 2 shows the graphs of the mean recovery of Mn against various effective factors. The H 2 concentration effects were studied in the range of g/l. The maximum recovery of manganese was achieved in 100 g/l of H 2 concentration. Applying more than 100 g/l of acid causes to leach other materials and therefore we would have decreasing recovery. It is clear from the graphs that the recovery of manganese is increased with the raise of Fe O quantity and increases from 80% to almost 100%. This behavior is due to the direct relation of Fe O with Mn recovery. The effects of L/S ratio on the recovery of manganese are shown in Fig. 2. The experiments show that the manganese recovery is increased by increasing L/S ratio. The maximum recovery of Mn is achieved in L/S : 8/1. It is clear that by increasing the L/S ratio, the free surfaces of solid particles increase and so the leaching process would be done better, because acid has more chance to attack the exposed. But this increasing of acid should be controllable because high free surface cause the other unfavorable elements to be leached and this fact cause to reduce the Mn recov- 13

7 Fig. 3. Contour plot for the Co recovery vs Mn recovery and controllable factors ery. The effects of temperature on the recovery of Mn were studied in the range of C. As seen in Fig. 2, increasing the temperature up to 40ºC makes a significant change in the recovery. The maximum recovery of Mn was found in th e temperature of 80 ºC. High temperature cause to more effective leaching because the internal energy of particles increase due to high temperature and the number of collisions raise up and so the leaching process would enhance. The Mn recovery for 15, 30, 45, and 60 minutes of leaching time by H 2 solution are given in Fig. 2. Results indicate that the recovery of manganese decreases with increasing leaching time to 60 min. This is because the manganese leaches at the first 30 minutes and by increasing the time other elements would leach and decrease the recovery. Therefore the maximum recovery of Mn was achieved in time of 30 min. C. Contour Plot of Co Recovery against Mn Recovery The contour plots could evaluate the concurrent effect of one parameter on the responses under study. Fig. 3 shows the ef- 14

8 fects of both manganese recovery and cobalt recovery on the controllable factors. The optimum regions for high Mn and Co recovery are shown in Fig. 3. It is evident that the maximum Mn and Co contents (>90%) is achieved by appropriate control of factors, as shown in the region represented by the darkest color in the graphs. D. Analysis of Variance To determine the most influential factors on each response, the statistical analysis of variance was carried out to see whether the selected parameters are statistically significant. The F-value for each parameter clarifies the effectiveness of certain parameter on the recovery of Co and Mn. Normally, the larger F-value means that this factor has greater influence on the recovery. Besides, the P-value is another indicator. Values of less than 0.05 shows the model terms are significant. The optimal combination of the process parameters can be predicted using ANOVA and performance characteristics. According to Table 5, F-value for Fe O factor is greater than extracted F- value from the table for 95% confidence level (F=4.76). Therefore, variation of this parameter has significant role on the performance of the process. Besides, P-Value confirms this finding. Table 5 Analysis of variance table for recovery of co Table 6 displays the analysis of variance for the recovery of Mn in the leaching process. In this table two terms, including H 2 concentration and time, were pooled. As Table 6 indicates F-value for Fe O factor is greater than extracted F-value from the table for 95% confidence level (F=4.76). So this parameter also has noticeable effect on the Mn recovery. In other words, variations of Fe O concentrations could change the recovery of Mn in a meaningful manner in this set of experiments. Table 6 Analysis of variance table for recovery of mn As the ANOVA tables show Fe O is such a significant term that both responses could be varied through its changes. To evaluate the concurrent effect of this term on recoveries, surface plot could be an applicable option. Fig. 4 displays the effect of Fe O on the Mn and Co recoveries. The optimum region for the high Mn and Co recoveries is shown in this figure. It is evident that the maximum recoveries (>90%) are achievable. And if the level of Fe O is higher than 1.2 times of stoichiometric amounts, the recoveries higher than 90% will be guaranteed. Finally, using these findings about influential parameters on the process, the optimum working conditions could be predicted. The proposed optimum conditions are: acid on level three (100 g/l), Fe O on level 3 (1.2 stoichiometric amounts), solid / liquid ratio on level 3 (1/8), time on level 2 (30 min) and temperature should be on level 4 (80 ºC). The predicted recoveries for both elements are 100%. To investigate the accessibility of this outcome, a test on the optimum conditions was done. The experimental recoveries for Co and Mn were 99.29% and 97.97%, respectively. 4. Conclusion The effect of operating conditions such as 15

9 ferrous sulfate quantity, time, solid/liquid ratio, temperature, and sulfuric acid concentration on the recovery of manganese and cobalt leaching was studied with the Taguchi method. As a result, the ultimate optimum leaching conditions were found to be ferrous sulfate quantity: 1.2 time of stoichiometric quantity of manganese dioxide, Temperature: 80 ºC, sulfuric acid concentration: 100 g/l, solid/liquid ratio: 1/8 and time: 30 min. Under these conditions, recovery for cobalt and manganese were 99.29% and 97.97%, respectively. The most effective parameter for maximum dissolution of manganese and cobalt is found to be ferrous sulfate quantity. Fig. 4. Surface plot Mn Recovery vs. Co Recovery vs. Fe O Acknowledgment The authors are thankful to the Research and Engineering Co. for Non-Ferrous Metals for financial and technical support and the permission to publish this paper. References [1] P. J. Florijn, M. L. Van Beusichem, Plant Soil 150 (1993) [2] Hatch, Review of Environmental Releases for the Base Metals Smelting Sector, PR , [3] S. M. Ross, Toxic metals in soil plant systems, 1994, John Wiley & Sons, Chichester, UK. [4] C. A. Wentz, Hazardous Waste Management, 1989, McGraw-Hill, NewYork. [5] D. Stanojević, B. Nikolić, M. Todorović, Hydrometallurgy 54 (2000) [6] N. Pradhan, P. Singh, B. C. Tripathi, S. C. Das,. Miner. Eng 14 (2001) [7] Y. Zhang, Q. Liu, C. Sun, Miner. Eng 14 (2001) [8] F. Veglio, L. Toro, Int. J. Miner. Process 40 (1994) [9] R. Das, S. Anand, S. Das, P. Jena, Hydrometallurgy 16 (1986) [10] Y. Zhang, Q. Liu, C. Sun, Miner. Eng 14 (2001) [11] Y. Zhang, Q. Liu, C. Sun, Int. J. Miner. Process 65 (2002) [12] M. D. Mwema, M. Mpoyo, K. Kafumbila, J. S. Afr. Inst. Min. Metall (2002) 1-4. [13] S. C. Das, P. K. Sahoo, P. K. Rao, Hydrometallurgy 8 (1982) [14] P. K. Naik, L. B. Sukla, S. C. Das, Hydrometallurgy 54 (2000) [15] F. W. Y. Momade, Z. G. Momade, Hydrometallurgy 51 (1999) [16] M. Trifoni, L. Toro, F. Veglio, Hydrometallurgy 59 (2001) [17] F. Veglio, L. Toro, Hydrometallurgy 36 (1994) [18] S. C. Das, S. Anand, P. R. Das, P. K. Jena, P.K. Aust. IMM Bull. Proc 294 (1989) [19] H. A. Hancock, D. O. Fray, Trans. Inst. Min. Metall., Sect. C 95 (1986) [20] K. M. Parida, B. B. Nayak, K. K. Rao, S. B. Rao, Kinetics and mechanism of the reactive leaching of manganese ores by iron pyrites in mild acidic solution, In: Gaslall, D.R. (Ed.), EPD Congress. The Minerals Metals and Materials Society, 1990, 217. [21] A.T. Stone, Geochim. Cosmochim. Acta 51 (1987) [22] M. Trifoni, F. Veglio, G. Taglieri, L. Toro, Miner. Eng 13 (2000) [23] F. Veglio, I. Volpe, M. Trifoni, L. Toro, Ind. Eng. Chem. Res 39 (2000) [24] A. A. Ismail, E. A. Ali, I. A. Ibrahim, M.S. Ahmed, Can. J. Chem. Eng 82 (2004) [25] F. Tumen, J. Chem 12 (1988) [26] F. Tumen, M. Boybay, B. Solmaz, M. Cici, M. Bildik, Doga Tr. J. Eng. Environ. Sci 15 (1991) [27] F. Tumen, M. Bildik, M. Boybay, M. Cici, B. Solmaz, J. Eng. Environ.Sci 16 (1992) [28] H. S. Altundogan, A. Ozer, F. Tumen, Kimya ve Kimya Muhendisligi Sempozyumu 4 (1992) [29] A. Hakami, IZMDC 224 (2005) [30] A. R. Eivazi Hollagh, E. Keshavarz Alamdari, D. Moradkhani, Kinetic analysis of isothermal leaching of zinc from zinc plant residue, Hydrometallurgy Con- 16

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