Mesa Minerals Limited
Electrolytic Manganese Dioxide

Overview

The first step in a conventional production flow sheet for converting manganese oxide ore to alkaline grade electrolytic manganese dioxide (EMD), which is a high purity product that possesses the ‘recipe specific’ electrical characteristics desired by batterymakers, is a high temperature pyrometallurgical roast process, wherein the manganese ore is heated to between 800°C and 900°C to reduce it so that it can then be dissolved in hot sulfuric acid. This process has many drawbacks that will inhibit its future use including:

  • As the roasting process reduces all oxides present in the ore, EMD producers must compete with steel and chemical industry buyers for the highest grades of manganese oxide fines if they are to ameliorate the production and product quality problems caused by non-manganese metal ions in the sulfate electrolyte;
  • In today’s world, the steel industry alone can consume all of the high grade manganese oxide ores that are available leaving existing EMD plants, which are dependent on sourcing high grade ores, at risk of closure even in times such as these when the market for their EMD production is particularly strong;
  • The roasting process is highly energy inefficient with a conventional EMD plant requiring more than double the energy of a same size plant employing the Mesa modified sulfur dioxide leach technology described below; and
  • The roasting process is highly polluting, emitting high carbon dioxide loads to atmosphere, particulate pollution that can pollute the surrounding environment and very insidious metal ion pollution that can find its way into streams and aquifers, travelling large distances before reporting in drinking water or crops grown for human consumption.

Mesa has developed and patented an alternative hydrometallurgical route for extracting manganese from low-grade manganese oxide ores, or ore tailings, and converting this manganese into a very pure high quality electrolytic manganese dioxide (EMD) for use in alkaline and lithium-ion battery production. The following schematic process flowsheet entitled depicts the steps in the Mesa process for production of EMD that, with two important exceptions, are identical to the conventional process. The exceptions are in the manganese leach step and in the availability of the plant residues, or tailings, for manufacture of a multi-nutrient fertilizer (MNF).

Process Flow

The principal sub-processes depicted above are as follows:

Milling Milling of manganese ore and limestone.
Manganese Leach Reduction leach using SO2 to form a manganese sulfate solution.
Jarositing Removal of impurities, predominantly potassium, from the solution.
Goethiting Removal of impurities, predominantly iron, from the solution.
Sulfiding Removal of heavy metal impurities from the solution.
Electrowinning EMD is plated from the purified solution onto titanium anodes.
Product Preparation EMD is washed, milled, dried and packaged as a fine powder.

Advantages of Mesa’s Patented Processes

Process Control

The main benefit of the hydrometallurgical route is the simplicity of the leach process and the ease of measuring the condition of the reaction to enable its control. Adding SO2 gas to an acidic slurry of fine ground manganese oxide ore at below 100°C is considerably easier to monitor and control than the pyrometallurgical route. Process control becomes an increasingly important factor when plants are sited in remote areas, closer to where their ore source, because constant high calibre technical supervision is sometimes more difficult to achieve.

The pyrometallurgical roasting and acid leach process is less reliable and potentially wasteful in that it is more difficult to control and will always produce some poorly reduced ore. The product from the reaction, which occurs between 800°C and 900°C, aided by the addition of coal and gas, then requires cooling to enable milling before its addition to the acid leach. Clearly this is not an energy efficient process.

Even when best conditions are assumed for the pyrometallurgical roast process, some manganese ore will remain in its oxidised form and will not release its manganese content in the acid leach step, resulting in that manganese oxide ore going to waste. In contrast, the Mesa sulfur dioxide leach is a hydrometallurgical route which does not depend on the valent state of the manganese in the ore feed and thus will always maximise the leaching of the manganese from the ore.

The roasting of a manganese ore, together with all of its mineral components other than manganese, enables these other components that would not normally leach in acid, to be converted to a form which makes them leachable in acid. (NB:This is not generally the case in the Mesa sulfur dioxide leach route.) This adds cost to the pyrometallurgical route in removing this increased contamination load, and, in many instances, in interruptions to production. Importantly, it can also result in lower product quality that will in turn lower a plant’s revenue potential.

Environmental Performance

Roasting is also not environmentally friendly as the off-gases of CO2 and CO from the roasting kiln have to be burnt to eradicate the CO component, forming more CO2 in the process before scrubbing to remove the dust carried from the kiln. This significant quantity of CO2 gas is then released, adding to the greenhouse gas load in the atmosphere.

By contrast, the off-gases from the Mesa sulfur dioxide leach are predominantly steam, nitrogen (present as a dilutant in the sulphur dioxide and being returned to the atmosphere) and unreacted SO2 gas. Importantly there is no dust. The unreacted SO2 gas is scrubbed using limestone or lime to meet environmental standards and recirculated. Logically, the operation needs to use efficiently the SO2 gas, and hence minimise any loss in the off-gases. This minimisation of SO2 gas loss will be readily manageable as part of the standard metallurgical operation procedures for operating an efficient process plant.

Product Quality

The Mesa sulfur dioxide leach process is highly selective for manganese in preference to other metals both because of its inherent chemistry and its ready controllability. As a consequence the loading of other metals in the electrolyte prior to purification is very low relative to the conventional roast method. In practice this has the effect of making the same electrolyte purification steps used in each process far more efficient in the Mesa leach case. EMD produced in Mesa’s demonstration plant was rated as superior in terms of minor element content and ideally suited for use as a feedstock to produce lithiated manganese dioxide for consumption in lithium ion batteries due to its exceedingly low iron levels.

EMD structure EMD is a complex composite of various crystals of manganese and oxygen that is produced through electrowinning. It is used primarily as the active constituent of alkaline batteries and increasingly as the feedstock for the cathodic material in lithium-ion batteries. The structure of EMD is highly disordered, but predominantly made up of the manganese dioxide crystal ramsdellite, depicted here, with the red balls signifying the oxygen atoms in the green manganese dioxide crystal lattice.

Cost Minimisation

The Mesa sulfur dioxide leach process was conceived to process efficiently the medium grade, high iron manganese oxide ore found on the company’s own mining leases located in the Pilbara region of Western Australia. Subsequently, the concept has been refined to deal with grades of ore down to 25% Mn, whether these ores are of ferruginous or silicious origin, and even high tenor manganese wastes from the electrolytic zinc and nickle laterite industries. As such, it is a process that utilises low cost feeds that in many cases are otherwise unsaleable or are a ‘cost of production’ for their producers in that they have to be removed from their processes and impounded in tailings dams indefinitely.

This is clearly a far superior cost structure starting point than that of the conventional process for the production of high performance EMD’s which requires the highest grades of manganese oxide available. These high grade ores must be bought in competition with the steel and chemical industries, whose requirements and financial resources vastly overshadow those of the EMD producers, and as these higher grades of ore are usually mined at greater distances from the EMD plants than low grade alternatives, then higher freight and handling costs are often also incurred.

Resource Maximisation

Finally, there is a further very important difference between the two processes and this is in the chemical state of the plant residues or tailings. In the case of the conventional roast route, the tailings contain largely higher forms of oxides of the metals, which would quickly disperse into the watertable and streams, and are thus unsuitable for further processing into products for agricultural use as a fertilizer. These tailings must be impounded for the long term, and at significant cost, to ensure that the ‘readily available’ metal ions contained therein do not leach naturally into the watertable and migrate away from the plant site. (This migration is sometimes referred to as an EMD or EMM plant’s “manganese ion plume”).

By contrast, the Mesa sulfur dioxide leach preconditions the tailings and leaves them in a state that is ideally suitable for further processing into a valuable micro nutrient fertilizer product. This is a factor that not only maximises the profitable utilisation of the ores bought by an EMD producer, but also reduces plant costs by avoiding the need for long term tailings impoundment whilst simultaneously creating a second valuable revenue stream.

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