Reductive leaching of low-grade manganese ores from Way Kanan using corncob as reductant

OBJECTIVES The processing of mining products in Indonesia has not been utilized optimally. One of them is manganese ore. Way Kanan is one of Indonesia’s potential manganese ore sites. METHODS The manganese leaching study used a - 200 mesh Way Kanan manganese ore that was reacted with sulfuric acid at concentrations of 0.1 M, 0.5 M, and 1 M, as well as corncob as a reductant. To separate the dissolved iron in theleachingprocess,themanganesesulfatesolutionwaspu- rifiedwithsodiumhydroxideandthenfilteredtohaveaman-ganese sulfate solution free of iron. Manganese was precipitated using oxalic acid. The manganese oxalate precipitation was calcined at a temperature of 350 °C for two hours. RE-SULTS The best manganese extraction obtained at 92.3% was atasulfuricacidconcentrationof1M,atemperatureof80°C, and a solution volume of 400 ml. CONCLUSIONS Calcination of manganese oxalate converted the material into hausman-nite (Mn 3 O 4 ).


INTRODUCTION
Indonesia has a large variety of mineral resources, one of which is manganese ores (Sujoto et al. 2022). Manganese ores in Indonesia are distributed in many areas such as West Sumatera, Lampung, Yogyakarta, East Java, Kalimantan, and East Nusa Tenggara (Kisman and Edya 2016). However, despite having so many resources, manganese ore in Indonesia has not been utilized and processed optimally. Even to fulfill Indonesia's manganese demands, it needs imports. Manganese is a valuable metal widely utilized in metallurgy, the chemical industry, batteries, electronics, and various other industries (Sun et al. 2017).
Manganese ores can be categorized into two, namely FIGURE 1. XRD pattern of manganese ore from Way Kanan Regency, Lampung Province. high-grade manganese ore and low-grade manganese ore. The processing of manganese ores is dependent upon the Mn content of the ore where high-grade manganese ores are processed into suitable metallic alloy formed by pyrometallurgical processes, the hydrometallurgical processing of low-grade manganese ores is performed after conventional pyrometallurgical reductive roasting for the manufacturing of chemical manganese dioxide (CMD) or electrolytic manganese dioxide (EMD) (Jing-sheng 2007). Manganese ore which was found in Way Kanan Regency, Lampung Province, is a low-grade manganese ore (Mufakhir and Sumardi 2013). The manganese ores with low manganese levels can be processed by hydrometallurgy (Liu et al. 2014). To recover and extracts manganese from manganese ores, it can be extracted by adding a reducing agent and followed by acid leaching (Ismail et al. 2004;Sahoo and Rao 1989). Ismail et al. (2004) used sulfuric acid (H 2 SO 4 ) as the leaching agent or solvent and obtained 90.5% manganese from manganese ores.
Most manganese in nature is a pyrolusite mineral known as MnO 2 , which is stable in acid and alkali leaching. Therefore, manganese is leached from manganese ore in a reduced state. Previously, many reducing agents, such as coal, pyrite, iron sulfate, sulfur dioxide, and peroxide were utilized (Zhang and Cheng 2007). The reducing agents like iron sulfate, sulfur dioxide, and peroxide are harmful to the environment. Thus, many researchers currently use organic reductants as reducing agents. Instead of coal and pyrite, corncob is more effective as a reduction agent in this process because corncob has a lesser number of impurities than coal such as volatile matter (Putra et al. 2015).
Many researchers also use carbohydrates-based materials as reductants such as glucose, sucrose, lactose, and cellulose and utilize organic acid reductants such as oxalic acid, citric acid, and tannic acid (Astuti et al. 2019b). Carbohydrates are harmless, low-cost materials and easy to be obtained, both in pure and waste forms. Carbohydrates are reductants or reducing agents that effectively work at temperatures lower than 90°C.  Corn (Zea mays L.) is one of the agricultural products widely produced and consumed by the society of Indonesia. After East Java and Central Java, Lampung Province is the third-largest corn producer in Indonesia. Lampung's corn production in 2018 reached 2,582,224 tons (Badan Pusat Statistik Lampung 2019). Corn crops generate solid waste, Different mineral content from each location has different maximum operating conditions. The researchers chose to extract manganese ore from Way right because it has a fairly high manganese content, which reaches almost 40%. Thus, researchers are interested in the reductive leaching of manganese ore found in Way Kanan Regency (Lampung Province) by utilizing corncob as a reductant in sulfuric acid leaching solution and investigating the effect of temperature, acid concentration, and solution volume on the percent of manganese extraction in this study.

Materials
Manganese ore used in this study was obtained from Way Kanan Regency, Lampung Province (Indonesia). The raw material was characterized using X-ray Fluorescence (XRF, PANalytical, Epsilon 3XLE) and X-ray Diffraction (XRD, PANalytical, X'pert 3 Powder). Table 1 shows that the manganese content in the manganese ore used was around 36%. XRD pattern of raw ore ( Figure 1) show that the manganese compound in raw ore was in the form of MnO 2 (Pyrolusite type). Corncob as a reducing agent was obtained from a corn plantation in Lampung. The dried corncob was analyzed using X-ray fluorescence (XRF, Epsilon 3XLE, PANalaytical, Netherland) and Atomic absorption spectroscopy (AAS, AA 7000, Shimadzu, Japan). to calculate the cellulose content in the corncob (see Table 2).

Methods
The experiments used manganese ore from Way Kanan, Lampung. Manganese ore and corncob were prepared through sampling and milling. The sampling process was carried out directly from Way Kanan, Lampung. Meanwhile, the milling process goes through several stages. The first stage is the crushing process using a crusher. The next stage is the refining process using a ball mill. The last stage is the shieving process to get a more uniform particle size by using a size of -200 mesh. 20 g of manganese ores and 9.94 g of corncobs (The use of ore and reductant compositions is based on previous research conducted by Astuti et al. (2019a). were then reacted with sulfuric acid. Various sulfuric acid concentrations (0.1; 0.5; 1 M), various sulfuric acid volumes (100; 200; 400 mL) at different temperatures (30; 55; 80°C) stirring speed of 200 rpm was used. After four hours, the slurry was filtered. Then, the leaching filtrate was analyzed using an Atomic Absorption Spectrophotometer (AAS, AA 7000, Shimadzu, Japan). To precipitate the ferrous hydroxide, the leachate from the filtered leaching solution is heated at 60°C along with 10% NaOH until pH 6. Therefore, the solution was filtered to separate ferrous hydroxide. The remained solution was heated to 60°C and added with oxalic acid 1 M, ratio (1 filtrate from leaching (mL): 2 oxalic acids (mL)) for four hours with a stirring speed of 200 rpm to get precipitated manganese (II) oxalate dihydrate. Then, manganese (II) oxalate dihydrate was calcined in a furnace (Nabertherm Muffle Furnace LT 9/14) at 350°C for two hours.  In the present work, a 3 3 full factorial design was selected for conducting the leaching experiments. The variables selected were temperature, sulfuric acid concentration, and volume of sulfuric acid. The analysis was carried out using the response surface method (RSM) with the help of Minitab software. The levels of variables are given in Table 3.

RESULTS AND DISCUSSION
The use of corncob as a reductant is the novelty of this research. However, the reaction that occurs can be approximated by using the reaction of leaching of low-grade manganese ore in sulfuric acid media that can be explained by the following reaction (Unal and Mahmut 1988): After conducting a regression analysis on the effect of experimental variations on manganese ore extraction results using Minitab applications, the regression analysis results in this study are as Table 5.
Which, R squared (R-sq) is a number that ranges from 0 to 1 which indicates the magnitude of the combination of independent variables that together affect the value of the dependent variable. Adjusted R-Square (R-sq(adj)) only measures R-Square with significant independent variables. Predicted R-squared (R-sq(pred)) is used to determine how well FIGURE 5. XRD analysis of precipitated manganese oxalate. a regression model makes predictions. The R-sq in Table 5 showed that the effect values of the three variations in the experiment on the percentage of manganese ore leaching results.
By using Minitab, it can be drawn an equation that relates the % of Mn leaching results with the independent variables and the equation applies effectively to variables between lowlevel and high-level variables, as follows:

Effect of temperature
Sulfuric acid concentration at 1 M, 400 mL volume of sulfuric acid, and leaching time of four hours. Figure 2 presents the effect of temperature on manganese and iron leaching efficiency. As a result, when temperatures were increased from 30°C to 80°C, the efficiency of manganese increased from 21.44% to 92.3%. Accordingly, the extraction of iron increased from 4.47% to 46.49%. Based on the analysis data, it can be concluded that the higher the temperature, the faster the leaching reaction will occur. This is in line with the theory of the effect of temperature on the reaction rate (Levenspiel 1999).

Effect of sulfuric acid concentration
Leaching experiments were performed at different sulfuric acid concentrations of 0.1 -1 M, and the other conditions were fixed at a corncob of 9.94 g (stoichiometric ratio). The volume of sulfuric acid was 400 mL, with a stirring speed of 200 rpm, and a temperature of 80°C. Figure 3 showed that the leaching efficiency of manganese increased with the sulfuric acid concentration. With an acid concentration of 1 M and a leaching time of four hours, approximately 92.3% of manganese was extracted. However, iron dissolution rose from 4.94% to 46.49%. The higher the concentration of the reactants, the closer the distance between the particles, as a result, the probability of the particles colliding will be greater. The reaction rate will be faster if the effective collisions that occur are also getting bigger (Levenspiel 1999).

Effect of sulfuric acid volume
The effect of sulfuric acid volume on manganese's leaching efficiency was investigated in the range of 100 mL -400 mL. The other leaching conditions were fixed at 1 M of sulfuric acid concentration, a corncob of 9.94 g (stoichiometric ratio), a stirring speed of 200 rpm, and a temperature of 80°C . Leaching experiments at the different volumes of sulfuric acid were carried out, as shown in Figure 4. The result demonstrates that the leaching efficiency of manganese and iron increases as the volume of sulfuric acid increases. The leaching efficiency of manganese increased from 49.54% to 92.3% for four hours, while iron increased from 17.62% to 46.49%. Leaching using several other reducing agents has been carried out as was done with sawdust as a reducing agent where the efficiency is only 90.50% (Ismail et al. 2004). In addition, using molasses as a reducing agent only produces an efficiency of 86.26% (Sumardi 2014). This shows that corncob is a good reducing agent.

Precipitation and calcination
The best Mn extraction used 1 M of sulfuric acid concentration was 400 mL of sulfuric acid volume and 80˚C of temperature. The leachate was purified from a ferrous ion with sodium hydroxide (NaOH) and precipitated ferrous. The remained solution was heated to 60˚C and added with oxalic acid 1 M for four hours with a stirring speed of 200 rpm to get 6.789 grams of precipitated manganese oxalate. Figure  5 showed an XRD analysis of the precipitated mineral composition. Analysis of XRD affirmed the precipitate was manganese (II) oxalate dihydrate. Chemical composition of manganese oxalate is presented in Table 7.
The manganese (II) oxalate dehydrate was calcined in a furnace at 350˚C for two hours. In this process, manganese (II) oxalate dehydrate was oxidized to hausmannite (Mn 3 O 4 ). The product is shown in Figure 6.

CONCLUSIONS
Corncob has a good ability as a reducing agent when compared to some other reducing agents such as molasses and sawdust. In this leaching experiment, the maximum manganese extracted was 92.3% for 1 M of sulfuric acid concentration and 400 mL of sulfuric acid volume at 80˚C of temperature for four hours. Precipitation of Mn from manganese sulfate leachate can be conducted using oxalic acid. Calcination at 350˚C for two hours can oxidize manganese (II) oxalate dehydrate to haussmannite (Mn 3 O 4 ).