Mineral Beneficiation and Upgradation

• By: Mitra S.K ADMIN
• December 03 2024
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Mineral beneficiation[1] is the process of separating economically essential minerals from their respective ores. The main motive of ore dressing is to remove unwanted particles such as dust and other minerals, gangue or matrix from the ore. It is the first process that most ores undergo after mining in order to provide a more concentrated material for the procedures of extractive metallurgy.
 

Steps in Mineral Beneficiation

The ore dressing or mineral processing is complicated, with several steps depending upon the metal to be extracted. The typical stages of mineral beneficiation are as follows:

Comminution 

Comminution[2] is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. Heavy forces such as compression, impact (mainly for crushing) and attrition (mainly for grinding) are responsible for crushing or grinding. Crushing and drying are slightly different, as crushing involves a drying process. On the other hand, grinding is a wet process.

• Crushing: In order to produce a crushed material suitable for use as mill feed (100 percent of the pieces must be less than 10 to 14 millimetres, or 0.4 to 0.6 inch, in diameter), crushing is done in stages. In the primary stage, the devices used are mostly jaw crushers with openings as wide as two metres. These crush the ore to less than 150 millimetres, which is a suitable size to serve as feed for the secondary crushing stage. In this stage, the ore is crushed in cone crushers to less than 10 to 15 millimetres. This material is the feed for the grinding mill.
   
• Grinding: In this process stage, the crushed material can be further disintegrated in a cylinder mill, which is a cylindrical container built to varying length-to-diameter ratios, mounted with the axis substantially horizontal, and partially filled with grinding bodies (e.g., flint stones, iron or steel balls) that are caused to tumble, under the influence of gravity, by revolving the container.

Sizing 

After crushing and grinding, the next step is sizing these materials. As the name suggests, sizing[3-5] is the process of the separation of particles based on their size. The equipment involved in sizing is Bar Screens, Wedge Fire Screens, Vibratory Screens, Flip Flop Screens etc. The sizing consists of two procedures, screening and classification. Screening involves introducing materials to the screen surface and allowing the finer particles to fall through the screen. The greater particles remain behind, and the smaller ones get out, just like flour sieving. The basic principle of classification is that particles with different sizes, but the same density will settle within a given fluid at different rates. Therefore, classification involves the separation of particles based on their settling velocities. 

Concentration 

Concentration involves the separation of valuable minerals from the other raw materials received from the grinding mill. In large-scale operations this is accomplished by taking advantage of the different properties of the minerals to be separated. These properties can be colour (optical sorting), density (gravity separation), magnetic or electric (magnetic and electrostatic separation), and physicochemical (flotation separation). Concentration is increasing the percentage of the desired mineral in a given ore. 

• Optical separation: It's a cost-effective method that can produce highly purified products. This process is used for the concentration of particles that have sufficiently different colours (the best contrast being black and white) to be detected by the naked eye. In addition, electro-optic detectors collect data on the responses of minerals when exposed to infrared, visible, and ultraviolet light.[6,7]
   
• Gravity Concentration: Gravity concentration[8] is one of the oldest techniques (dates back to 3000 BC) that involves the separation of minerals with different gravity based on their relative movement in response towards the gravitational force. It may affect other forces such as centrifugal force, magnetic force, buoyant force etc. we must know the suitability of the gravitational concentration of specific ore before continuing with the process. 
  
  In heavy-media separation (also called sink-and-float separation), the medium used is a suspension in water of a finely ground heavy mineral (such as magnetite or arsenopyrite) or technical product (such as ferrosilicon). Such a suspension can simulate a fluid with a higher density than water. When ground ores are fed into the suspension, the gangue particles, having a lower density, tend to float and are removed as tailings, whereas the particles of valuable minerals, having higher density, sink and are also removed. The magnetite or ferrosilicon can be removed from the tailings by magnetic separation and recycled.
  
  In the process called jigging, a water stream is pulsed, or moved by pistons upward and downward, through the material bed. Under the influence of this oscillating motion, the bed is separated into layers of different densities, the heaviest concentrate forming the lowest layer and the lightest product the highest. Important to this process is a thorough classification of the feed, since particles less than one millimetre in size cannot be separated by jigging. 
  
  Finer-grained particles (from 1 millimetre to 50 micrometres) can be effectively separated in a flowing stream of water on horizontal or inclined planes. Most systems employ additional forces—for example, centrifugal force on spirals or impact forces on shaking tables. Spirals consist of a vertical spiral channel with an oval cross section. As the pulp flows from the top to the bottom of the channel, heavier particles concentrate on the inner side of the stream, where they can be removed through special openings. Owing to their low energy costs and simplicity of operation, the use of spirals has increased rapidly. They are especially effective at concentrating heavy mineral sands and gold ores.
  
  Gravity concentration on inclined planes is carried out on shaking tables, which can be smoothed or grooved and which are vibrated back and forth at right angles to the flow of water. As the pulp flows down the incline, the ground material is stratified into heavy and light layers in the water; in addition, under the influence of the vibration, the particles are separated in the impact direction. Shaking tables are often used for concentrating finely grained ores of tin, tungsten, niobium, and tantalum.
   
• Magnetic Concentration: Magnetic separation[9] is based on the differing degrees of attraction exerted on various minerals by magnetic fields. Success requires that the feed particles fall within a special size spectrum (0.1 to 1 millimetre). With good results, strongly magnetic minerals such as magnetite, franklinite, and pyrrhotite can be removed from gangue minerals by low-intensity magnetic separators. High-intensity devices can separate oxide iron ores such as limonite and siderite as well as iron-bearing manganese, titanium, and tungsten ores and iron-bearing silicates.
   
• Electrostatic Separation: The electrostatic method[10] separates particles of different electrical charges and, when possible, of different sizes. When particles of different polarity are brought into an electrical field, they follow different motion trajectories and can be caught separately. Electrostatic separation is used in all plants that process heavy mineral sands bearing zircon, rutile, and monazite. In addition, the cleaning of special iron ore and cassiterite concentrates as well as the separation of cassiterite-scheelite ores are conducted by electrostatic methods.
   
• Froth Flotation: In the froth flotation process,[11,12] the desired minerals are separated by inducing them to accumulate over the froth layer. The froth flotation process involves the separation of hydrophobic minerals (minerals that repel water) from hydrophilic (attracts towards the water) gangue. 
  
  Flotation is the most widely used method for the concentration of fine-grained minerals. It takes advantage of the different physicochemical surface properties of minerals—in particular, their wettability, which can be a natural property or one artificially changed by chemical reagents. By altering the hydrophobic (water-repelling) or hydrophilic (water-attracting) conditions of their surfaces, mineral particles suspended in water can be induced to adhere to air bubbles passing through a flotation cell or to remain in the pulp. The air bubbles pass to the upper surface of the pulp and form a froth, which, together with the attached hydrophobic minerals, can be removed. The tailings, containing the hydrophilic minerals, can be removed from the bottom of the cell. This process has been described as the vital operation employed for recovery and upgrading of sulfide ores containing copper, lead and zinc.

Benefits of Mineral Beneficiation

The benefits of mineral beneficiation or ore dressing are as follows:

• It makes mineral resources profitable.
• It increases the value of the ore by removing gangue.
• It allows for increased mining production. 
• It supports the extractive metallurgy industry by minimizing metallurgical losses.
• It’s less costly (when compared to the direct purification of mineral ore).
• If done mechanically (by crushing, grinding, agitation, screening, gravity separation, magnetic separation, or electrostatic separation) it’s eco-friendly.
• It recycles old mine tailings products.
• It produces contaminant-free, clean, industrial materials.

References:

[1] Rao DS. Mineral beneficiation: a concise basic course. CRC Press; 2011 May 13.
[2] Gauti Asbjörnsson, Luís Marcelo Tavares, Aubrey Mainza, Mohsen Yahyaei, Different perspectives of dynamics in comminution processes, Minerals Engineering, 176, 2022, 107326.
[3] Holbrook EA, Fraser T. Screen sizing of coal, ores and other minerals. US Government Printing Office; 1925.
[4] Moncada M M, Rodríguez CG. Dynamic modeling of a vibrating screen considering the ore inertia and force of the ore over the screen calculated with discrete element method. Shock and Vibration. 2018;2018(1):1714738.
[5] Yu C, Wang X, Pang K, Zhao G, Sun W. Dynamic Characteristics of a Vibrating Flip‐Flow Screen and Analysis for Screening 3 mm Iron Ore. Shock and Vibration. 2020;2020(1):1031659.
[6] Ergin Gülcan and Özcan Yıldırım Gülsoy, Optical sorting of lignite and its effects on process economics, International Journal of Coal Preparation and Utilization, 38, 2018, 107-126.
[7] Ergin Gülcan and Özcan Y. Gülsoy, Performance evaluation of optical sorting in mineral processing – A case study with quartz, magnesite, hematite, lignite, copper and gold ores, International Journal of Mineral Processing, 169, 2017, 129-141.
[8] Barry A. Wills, James A. Finch, Chapter 10 - Gravity Concentration, Editor(s): Barry A. Wills, James A. Finch, Wills' Mineral Processing Technology (Eighth Edition), Butterworth-Heinemann, 2016, 223-244.
[9] Oberteuffer J. Magnetic separation: A review of principles, devices, and applications. IEEE Transactions on Magnetics. 1974 Jun;10(2):223-38.
[10] Haga K, Chang JS, Kelly AJ, Crowley JM. Applications of the electrostatic separation technique. InHandbook of electrostatic processes 1995 Feb 8 (pp. 365-386). New York: Marcel Dekker.
[11] Mondal S, Acharjee A, Mandal U, Saha B. Froth flotation process and its application. Vietnam Journal of Chemistry. 2021 Aug;59(4):417-25.
[12] Fuerstenau MC, Jameson GJ, Yoon RH, editors. Froth flotation: a century of innovation. SME; 2007.