Four Production Circuits In The Flotation Beneficiation Processing (Crushing, Grinding, Separating, Dewatering)
This is the first unit operation after the initial fragmentation provided by under-ground or open-pit blasting. The mechanisms of size reduction are based on the slow compression of large particles against rigid surfaces. Crushing is performed in one or more stages with smaller reduction ratios; i.e., the ratio feed top-size to product top-size. The first stage (primary crushing) is usually done at the mine, reducing the size of the blasted ore to a level adequate for direct feeding semi auto genous mills or convenient for further crushing, when the grinding mill is of a conventional type. In addition,screens are used before the crusher to mini size packing the crushing chamber by fines or downstream to separate already final-product size material.
The discharge of the secondary and tertiary crushers, normally on the surface, is usually classified on a screen to produce an oversize stream which is recirculated to the tertiary crushers and an under size product that goes to fine ore bins for feeding the concentrator. This final screening eliminates packing of fines in the tertiary crusher, reduces the production of finely sized material, and thus helps maximize circuit throughput.
The crushed material is then processed by wet grinding circuits. In the case of the most commonly used equipment (tumbling mills and tower mills) size reduction is accomplished through a combination of impact, chipping, and abrasion events caused by energy transferred from grinding media, such as balls, rods or large particles. The primary grinding stage can be carried out by rod mills, in which the grinding media consists of iron rods, by auto genous mills, which use large ore particles, and by semi-auto genous mills, which use a small load of balls. The final stages of grinding (secondary and rarely tertiary) are normally accomplished by ball mills, where the grinding media is made of balls or slugs. In a very few cases, ball mills are also used as a primary grinding stage. The discharge product of the ball mills requires size classification via hydrocyclones or mechanical (rake, spiral) classifiers, to produce a fine stream (cyclone overflow), which feeds the separation circuit, and a coarse stream (cyclone underflow) which is sent back to the ball mill for further grinding (liberation).
Only flotation separation processes will be considered in this article. A flotation circuit is usually composed of various sub-circuits (sections) with each having a specific objective. Each section is composed of a series of interconnected tank cells or banks of conventional cells, where flotation takes place. Tailings from the first cell become the feed of the next cell of the bank and so on; the tailing stream of the last cell in the bank is sent to a different bank or section for further processing, or discarded from the circuit (to the tailings pond). Usually, the concentrate stream of each cell in a bank or section are combined into an over all concentrate,which is then
directed to the next section for further processing, unless it is the final concentrate of the circuit in which case it is sent to the dewatering stage. Sometimes, regrinding or thickening units exist within the flotation circuit. Regrinding is necessary when floating two minerals with very different optimal particle size distribution or
when final concentrate grade cannot be achieved because of gangue contamination in non-liberated particles. Thickeners may be needed to increase the solid content of slurry. Before the actual flotation process, slurries need to be conditioned in an agitated tank, called a conditioner, where appropriate chemical reagents are added.
More additions might be needed further down the circuit. Three types of sections are normally encountered in a separation circuit: rougher, scavenger and cleaner.
The rougher section, the first after the grinding circuit, is fed with the classifier fine stream, the objective being the recovery of as much valuable material as possible with some disregard for the product quality (grade). Here, most of the fast floating valuable minerals are separated by flotation from the slurry and recovered from the froth of the concentrate. Tailings from the rougher stage are usually exhausted in the scavenger section, where maximum recovery is the target. The tailings of the scavenger section usually become the final circuit tailings, whereas the concentrate is normally recirculated to the rougher feed for reprocessing. The rougher concentrate is processed in one or more cleaner sections (re-cleaners) where an improved (optimal) concentrate grade is the aim. Since this typically implies getting rid of entrained gangue particles, cells having froth-washing capabilities (flotation columns) are preferred. Columns usually outperform conventional mechanical cells in cleaning stages (better product grade) due to their particular froth-washing features and thus, improved product grade. Whereas flotation columns do not have any mechanical agitator for slurry suspension (air bubbles injected at the bottom keep the particles in suspension), conventional cells achieve pulp dispersion through agitation by mechanically driven impellers.These cells employ one of two types of pulp flow and aeration systems: cell-to-cell flow with adjustable weirs between cells or open-flow without weirs and air intake via suction resulting from the operation of an impeller.
When dealing with multi-valuable species ores, the processing scheme can be of two types: selective flotation, where all valuable minerals are separated sequentially (one at a time in its own circuit, the tailings of the first mineral circuit becoming the feed for the next mineral circuit, gangue is mostly found in the last circuit tailings), or bulk flotation, where the separation of gangue from the all valuable mineral is accomplished in a single circuit, whose concentrate (bulk concentrate) is selectively processed in a separate circuit. Bulk flotation is typically used for Cu-Mo ores, whereas selective flotation is used for Cu-Zn ores.
Depending on the degree of water elimination required, dewatering circuits can be composed of one or more of the following processes: thickening(up to 65% solids), filtration (up to 93% solids) and drying (up to 99% solids). Thickeners are a specific type of sedimentation equipment which operates continuously, performing a wide variety of tasks in the mineral industry. Their most common application is the enhancing of the solid content of mineral concentrates (for transportation purposes and in respect of net smelter return contract clauses), as well as recovering water from flotation tailings (reclaimof the excess of water for process re-use).The objective of these units is to produce clean overflow water and maximum solid concentration in the underflow. Flocculants are normally used to agglomerate the solids to increase the settling rate and improve the overflow clarity. The use of synthetic flocculants has increased the efficiency of sedimentation rates allowing increased production.
Filtration is used when even smaller amounts of water are permitted (typically the case of concentrates); filters are then fed by the thickener underflow. Filters typically operate by passing the mineral slurry through a membrane (cloth, ceramics,etc.) whose pores retain the solid particles letting the liquid pass through. In some cases, the solid discharge is continuous (vacuum filters such as belt-, drum-and disk-filters) and in others the solid is discharged every few minutes (pressure plate-and-frame filters). Further drying of the solid product (filter cake) can be achieved in rotary driers using a heated (oil or gas) air stream. This operation is rarely used in base-metal processing. Niobium concentrates are rotary dried whereas graphite concentrates are filtered on a heated belt-filter.