Common Manganese Ore Beneficiation Processes

Manganese ore presents a specific set of difficulties. Most manganese deposits are low grade, and much of the ore is finely or very finely disseminated, with a substantial proportion carrying high phosphorus, high iron, or associated valuable metals. Beneficiation is therefore hard. The methods commonly applied fall into two groups: mechanical beneficiation — washing, screening, gravity separation, high-intensity magnetic separation and flotation — and special methods, namely pyrometallurgical enrichment and chemical beneficiation.

Washing and screening

Manganese ore generally contains slime. The slime not only affects the grade of the manganese minerals but, more importantly, seriously degrades the performance of the downstream separation stages — flotation, gravity and magnetic separation alike. Where the ore is open-pit mined or carries a high slime content, washing to remove the slime is therefore a necessary first step.

Washing uses hydraulic flushing with mechanical scrubbing to separate ore from clay, and is normally a pre-treatment operation ahead of beneficiation. For carbonate manganese ore, a vibrating screen with water spraying is usual: the oversize is washed ore, while the undersize is classified, with the spiral classifier sand rejoining the oversize and the overflow either discarded or further treated to recover manganese minerals. Oxide manganese ore is generally washed in a double-spiral trough washer with stronger scrubbing action.

Gravity separation

Gravity separation sorts minerals by differences in density and particle size. Oxide manganese minerals — pyrolusite, psilomelane, braunite — have a density of around 4 g/cm³, a marked contrast to silicate gangue, so most can be treated by gravity methods. Carbonate manganese ore is a different case: its density contrast with the gangue is relatively small, so jigs and shaking tables are generally ineffective and only dense-medium separation provides enrichment. In production, carbonate manganese ore is mostly treated by a dense-medium plus high-intensity magnetic separation combined flowsheet, discarding wall rock in the front section to restore the geological grade and carrying out deep separation in the rear section.

High-intensity magnetic separation

For carbonate and oxide manganese ores that are simple in composition and coarsely disseminated, a single high-intensity magnetic separation flowsheet can achieve good separation indices. Separating carbonate manganese ore requires a magnetic field intensity above 480 kA/m, while oxide manganese ore needs above 960 kA/m. Magnetic separation is simple to operate, easy to control and highly adaptable.

Flotation

Manganese flotation mainly uses collectors such as oleic acid, tall oil and oxidized paraffin soap, and suits minerals such as rhodochrosite and pyrolusite as well as fine-grained manganese. Magnetic separation plus flotation, or gravity separation plus flotation, combined flowsheets can also be used: the magnetic or gravity stage discards part of the gangue and slime first, then flotation performs the cleaning.

Pyrometallurgical enrichment

Also called the manganese-rich slag process, this treats high-phosphorus, high-iron, refractory, low-grade manganese ore. It exploits the different reduction temperatures of manganese, phosphorus and iron, controlling the temperature in a blast or electric furnace to separate them selectively. The process is simple and the production stable, and it effectively separates iron and phosphorus to yield a manganese-rich slag low in both.

Chemical beneficiation

Chemical treatment of complex low-grade manganese ore begins with leaching. The leach solution is purified to give a clean manganese sulphate solution, from which a range of products can be made: electrolytic manganese, manganese alloys, high-purity manganese carbonate, manganese sulphate and manganese nitrate. Pyrolusite can also be treated with potassium hydroxide to make potassium manganate, then electrolysed to potassium permanganate. The commonly used chemical routes include the disulphate method, the hausmannite method — roasting followed by selective dilute-acid phosphorus leaching — and bacterial manganese leaching.