Mineral Processing Wastewater Solutions

Through the application of cutting-edge technologies, WesTech helps to assure that the byproducts of minerals extraction processes are made inert, reliably contained, and kept from causing environmental damage. Implementation of WesTech tailings processing technology assures that groundwater, surface waters, wildlife, and communities will be protected from tailings dam leaks and levy failures.

WesTech offers innovative solutions for iron ore concentration, coal preparation, and sand aggregate applications. WesTech also provides package plants for water and wastewater treatment for remote locations and mine sites. These package plants allow for drinking water to be made onsite, as well as basic levels of sanitary safety to be met, all while ensuring continued production.

The process of refining bauxite to produce alumina (the Bayer process) can traditionally be considered in the following steps:

Extraction → Precipitation → Calcination

Extraction

Bauxite ore is crushed and milled to reduce the particle size, making the extraction step more efficient. The crushed and milled ore is then combined with spent liquor and makeup sodium hydroxide (caustic) and sent in slurry form to heated pressurized digesters where the aluminum-bearing minerals are dissolved.

Hydrocyclones are used to desand the digested slurry. The insoluble bauxite residue (red mud) is separated in hydrocyclones from the aluminumcontaining liquor. The overflow from the hydrocyclones is sent to paste thickeners to thicken the solids and recover the liquor. The liquor is sent to the liquor filters and then to the precipitation step.

The red mud is further thickened and washed with fresh water in multiple stages using a countercurrent decantation (CCD) process to recover the caustic and any remaining alumina content. The red mud from the final CCD stage is then collected as a paste and sent to a disposal site, thus eliminating tailings ponds.

Precipitation

The cooled and filtered pregnant supersaturated liquor from the settler is sent to a series of alumina hydrate precipitation tanks. To promote the alumina hydrate precipitation, the liquor is seeded with alumina hydroxide crystals.

The hydrate crystals are classified in hydroseparators to produce a coarse product fraction and a fine seed fraction. The resulting crystals are collected and sent to the next step in the recovery process, while the fine seed fraction is filtered and used as seed in the precipitation stage.

Calcination

The coarse aluminum hydrate crystals are filtered and washed over a horizontal belt vacuum filter to remove contaminating process liquor. It is then calcined to produce the final product, alumina.

Molybdenum (moly) is often produced as a byproduct of copper mining. Copper is used for electronics, construction, and metal alloys. Moly is mostly used to make metal alloys, and as a catalyst.

As markets need copper and moly, their ores are separated, concentrated, and sold separately. This separation and concentration of copper and moly is called the “copper-moly flotation circuit.”

Flotation – Sulfides

Copper and moly are often found together as sulfides*†. Sulfides in solution will float or sink with the right combination of chemicals and gas bubbles (froth flotation).

These sulfide ores are separated from gangue (waste) material, then from each other, by froth flotation.

Copper/Molybdenum Flotation Circuit Steps:

1. Grinding

Grinding mills liberate the ore from the gangue material (non-ores: silica, organics), and reduce it to an optimal size for flotation. Water mixes in to form slurry. This helps both in the transportation and separation of the solids.

2. Bulk (Copper-Moly) Flotation

Both the copper and moly sulfide ores together (bulk) float in rougher flotation cells, then cleaner column cells, and often scavenger cells. These groups of flotation cells work together to give a high total yield of the bulk ore, which is sent on to be thickened.

3. Tailings (Gangue) Thickening/Dewatering

The underflow gangue (waste) from the rougher and scavenger cells flows to a tailings thickener to recover water for the process. Filters dewater these tails further, or a tailings pond stores them.

4. Bulk (Copper-Moly) Thickener

A high-rate thickener dewaters the bulk of copper-moly concentrate before more separation.

5. Moly Flotation

The moly flotation circuit has similar groups of flotation cells with chemicals to float the molybdenite (moly concentrate), and settle out the copper sulfides (copper concentrate).

6. Copper Thickening/Dewatering

A high rate thickener thickens the copper concentrate. A vacuum drum (or other) filter dewaters the copper concentrate further before refining or storing until sale.

7. Moly Thickening/Dewatering

A high rate thickener thickens the moly concentrate. A vacuum drum (or other) filter dewaters the moly concentrate further before refining or storing until sale.

*Moly is found as Molybdenite (MoS₂), and copper is in various combinations with sulfur (e.g., Chalcocite (Cu₂S)).

†If copper is found as an oxide, it is not as easy to float. It is typically heap leached with sulfuric acid (dissolved), then removed from solution and concentrated by solvent extraction (SX) and electrowinning (EW).

Iron Ore Concentration Process

Magnetite is mined in large chunks and is crushed into small particles by a series of crushers. After primary crushing with jaw crushers, secondary crushing with gyratory crushers, and tertiary crushing with cone or high pressure grinding rolls (HPGR), the ore is screened on vibrating screens to size the particles. The portion of this process that is still too large is sent to a rod mill.

Magnetic Separators

From the rod mill, the material proceeds to the cobber magnetic separators. Any non-magnetic material that has been released by the rod mill is separated and sent to the tailings thickener. Magnetite iron ore particles are separated by the magnetite separator from the gangue (waste material) minerals in the cobber magnetic separators. This material flows to the ball mill for further size reduction.

Material from the ball mill flows to the cleaner magnetic separators. Again, non-magnetic material that has been released by the size reduction process is sent to the tailings thickener. The magnetic component is pumped to hydrocyclones for sizing.

The finer material in the hydrocyclone overflow is sent to the desliming hydroseparator, while the course material in the hydrocyclone underflow is returned to the ball mill for regrinding. Polymer is added to the desliming hydroseparator to aid in the settling and thickening of the solids.

The desliming prepares the ore for flotation, discarding the ultrafine particles. The underflow from the desliming thickener is sent to a series of finisher magnetic separators to further purify the solution. The overflow from this thickener reports to the tailings thickener.

Reverse Flotation Cells

The separated magnetic material from the finisher separators is sent to reverse flotation cells for further separation. The flotation phases employ conventional large-size mechanical cells, in addition to flotation columns. This process uses starch (depressant) and amine (quartz collector) as reagents to promote the separation of the contaminant mineral (quartz) from the iron-bearing mineral.

The floated material from the flotation process is also sent to the tailings thickener. The underflow from the flotation process is sent to the concentrate thickener.

In the concentrate thickener, the purified slurry is thickened prior to being pumped to a disc filter. Again, the addition of polymer aids in the separation and thickening process. The disc filter uses vacuum to dewater the magnetite iron ore concentrate and discharges a relatively dry cake, which is sent for pelletizing. This process allows ore of very low magnetite content to be processed into a high quality product.

Tailings

The various reject streams are sent to a tailings thickener. In this unit, the solids are allowed to settle and are then pumped to a tailings pond for further settling and water reclamation. The overflow from the tailings thickener is sent to the water reclaim pond and then recycled back into the process.

Mine backfill is defined as the material used to fill the cavities (i.e., stopes) created by underground mining. Backfilling can be a means to dispose of sludge and/or tailings that may contain hazardous materials and to reduce surface environmental impacts by storing tailings underground.

Alternately, backfilling with nonhazardous materials can allow for mining productivity improvements. To these materials are added a variety of fillers such as fly ash, course sand, or gravels along with a binder such as cement, which is added to provide structural strength.

Conventional Flow Sheet

A vacuum disc filter, preceded by a high-rate thickener to reduce the hydraulic loading, is typically used to produce the sludge portion of the mine backfill. The filter cake is discharged to a weigh hopper, then to a batch mixing hopper or a continuous mixer, where a measured amount of binder and other materials are added.

The cemented paste is then pumped via high pressure piston pumps below ground or distributed by gravity, depending on the specific site. Most backfill projects in the world use this conventional flow sheet with a vacuum disc filter because there is less water in the filter cake and, therefore, less cement binder (which is a major operating cost of a backfill operation) is required.

Paste Flow Sheet

In underground mining, the WesTech Deep Bed™ paste thickener is an emerging option to the conventional solution of high-rate thickener/vacuum filter for paste backfill applications. There can be a number of factors that make paste thickening an attractive alternative. There are some backfill operations with shallow mines and long distance runs, making the pumping costs of a high-yield stress paste more attractive.

Because a paste is non-settling, the coarse particles do not have to be removed prior to thickening. Paste thickeners can eliminate the need for vacuum filters, which can be expensive to operate, and may not be feasible for high elevation mine sites. This also significantly reduces operator attention.

Alternatively, it is possible to use a Deep Bed paste thickener to feed a vacuum filter. This can reduce the size of the vacuum filter as the feed to the filter is more concentrated than that from a high-rate thickener. Another option would be to use a Deep Bed paste thickener in parallel with a filter. This option allows for the blending of the paste underflow with the filter cake.

The underflow from the paste thickener would be split, sending a portion to the vacuum filter. The paste thickener underflow and the filter cake would then be combined to obtain the desired moisture content for the backfill.

Oil Sands

Oil sands, tar sands or, more technically, bituminous sands, are a type of unconventional petroleum deposit. Oil and tar sands represent a vast wealth of energy reserves for the world, rivaling traditional sources in quality, availability, and the ability to meet the world’s needs for many years into the future. The technologies for extraction of this resource are increasingly efficient, and WesTech has worked with several companies on the leading edge of these efforts and is a ready partner for new projects and plant upgrades.

Open Pit Oil Sands Mining

The term “oil sands” is actually a bit of a misnomer. The deposits are saturated with a tarlike substance known as bitumen. A great deal of processing is required to separate this bitumen from the associated soil and other debris. One of the two most common ways to recover bitumen from oilsands is through open pit mining of deposits that lie near the surface. Oil sands deposits that lie within 75 meters of the surface are typically recovered via mining. This process is much the same as strip mining for coal or any other mineral.

Mining shovels remove the oil sand and load it into large mining trucks. These trucks carry the oil sands to mobile crushers. The crushed material is stockpiled for the next step.

Slurry

The oil sands broken up in these crushers are then fed to rotary breakers with the addition of hot water to remove rocks and other debris. The resulting slurry is pumped through a pipeline and chemicals are added as required. The slurry reports to a primary separator where it is classified into three distinct cuts – the overflow, the middle means, and the underflow.

The middle means are sent to flotation units where the floating material is recovered and returned to the head of the primary separator. The underflow from the flotation units is combined with the primary separator underflow and sent to a trash screen. The oversized material from the screen is washed and is returned to the mine via pipeline to fill in mined-out areas. The undersized material is sent to a further bank of flotation units. Floated material off the secondary flotation units is also recovered to the head of the primary separator, while the underflow is sent to the tailings thickener.

Bitumen

The overflow from the primary separator is sent for processing via steam heating of the bitumen. Bitumen is deficient in hydrogen. Bitumen must be upgraded to synthetic crude oil specification in order to be an acceptable feedstock for refineries. This is done by the addition of hydrogen or the rejection of carbon, or both. Upgrading uses natural gas as a source of heat and steam for processing and also as a source of hydrogen. Other hydrocarbons such as naphtha may also be used for upgrading.

In the tailings thickeners, the suspended solids are settled to a sludge that is sent to a horizontal vacuum belt filter for dewatering. The filtrate from the horizontal belt filter is returned to the head thickener for reprocessing. The dried cake from the horizontal belt filter is sent to tailings piles or landfills for disposal.

Overflow

The overflow of the tailings thickener is water that is recovered for recycling back into the circuit. This is not solely due to restrictions on water usage. It is therefore critical that treatment processes involving water recovery in reuse are employed in this application. The combination of tailings thickener(s) and vacuum dewatering equipment results in maximum water recovery.

Potash is an important family of potassium-based industrial chemicals. It is used in glass production and soap making, but its most prevalent use is as an agricultural fertilizer. The downloadable flow sheet for potash cold crystallization describes the production of potassium chloride from the decomposition of carnallite (KCl•MgCl•6H₂O + NaCl) and the subsequent re-crystallization of KCl (Sylvite) under ambient or “cold” conditions.

Carnallite

Carnallite is a naturally occurring dual salt commonly found in the presence of other salt-type minerals such as halite. Under certain conditions, significant amounts of carnallite can be formed by means of solar evaporation in ponds filled with saturated brine solutions. Two major brine sources suitable for primary carnallite production are the Dead Sea in Israel and Jordan as well as brines found in the Qinghai province of China.

Thickener

Carnallite is harvested from evaporation ponds and delivered to a primary sizing screen, where oversized material can be separated, resized, and processed. Screened material is delivered to the carnallite thickener, where excess transportation brine is removed and the crystals are concentrated. The saturated overflow brine from this thickener is returned to the evaporation ponds.

Flotation Circuit

Thickener underflow is sent to a selective flotation circuit, where collector and frother chemicals are added and the gangue minerals and crystals are selectively separated. Concentrated carnallite from the flotation circuit is transferred to a flotation thickener where saturated flotation brine is removed and recycled to the flotation circuit or returned to the evaporation ponds.

Dewatering

Concentrated carnallite is then further dewatered on horizontal belt filters, producing a low moisture crystal product suitable for the decomposition/recrystallization process. In cold crystallization, carnallite is decomposed into free ions, and by carefully controlling the concentration at the appropriate ambient conditions, sylvite (KCl) will recrystallize while the MgCl remains in solution. However, any halite remaining in the solution also recrystallizes at these concentrations and conditions.

The halite crystals are generally much larger than the sylvite crystals and can be removed by screening prior to product thickening. The concentrated product sylvite moves from the product thickener to product horizontal belt filters, where the crystalline product can be countercurrent-washed and dewatered to remove wetting brine that contains MgCl. The filter cake is then leached for final cleaning in a centrifuge. Centrifuge cake is then dried, compacted, sized, and bagged for sale and use.

Two-Stage Fluoride/Phosphate Removal from Gypsum Stack

WesTech has experience in the phosphoric acid industry, including clarification, thickening, and dewatering. Calcium fluoride and calcium phosphate slurries present specific challenges. Another challenge is found in plants with hydrofluoric acid in their discharge.

Hydrofluoric acid is extremely corrosive and is toxic to humans. At these plants, solids contact clarifiers have been applied instead of external reaction tanks and flocculating clarifiers. These units are designed to prolong the period of precipitation. Solids growth is enhanced by the high level of precipitated solids present during the reaction. This has resulted in denser underflow and lower fluoride levels in the effluent than laboratory predictions. The underflow is sent to a gravity thickener and then to filter presses.

The waste from phosphoric acid plants usually consists of gypsum pile drainage. It is highly acidic with a pH of 1.0 to 1.8.

Two-Stage Neutralization

The extremely acidic drainage requires a two stage neutralization system. Clarification occurs between stages. The first stage uses lime to capture fluoride and elevate the pH to 4.5. The second stage pH is then raised to exceed 10.5. This approach reduces fluorine levels well below the mandated levels of 25 ppm. Phosphorus levels are similarly reduced below the 35 ppm limit. Alternative single stage processes fail to reduce fluoride levels below the required maximum.

Historically, calcium fluoride (CaF₂) precipitation required a 60 minute reaction time. Adding previously precipitated and thickened CaP₂ solids to the lime slurry improves the process. Resulting reaction time is reduced and the sludge precipitate is denser. WesTech testing suggests the recirculated solids should be three to five times the precipitated solids. Others have advocated higher recirculation rates.

The reaction slurry contains most of the fluoride plus a good portion of the phosphate. This thickens to produce an underflow with up to 40 percent suspended solids. Effluent fluorine is now less than 40 ppm and is fed to the second stage. The underflow is usually discarded in ponds, but this practice is being reviewed by the EPA.

The second stage recirculates precipitated solids, reducing reaction time. Solids, phosphate quantity, and settling characteristics make it impractical to recirculate solids more than once per pass. The solids are thickened to 10 percent suspended solids. Clarified effluent is discharged to waterways and the underflow is discarded.

Second stage feed liquor and underflow contain phosphate that is essentially free of fluoride. Some of this may be recycled through the process if the plant water balance permits.

Uranium Ores

Uranium ores are generally classified as acidic, alkaline, or phosphate rock based. In all cases, the ore is milled and classified prior to leaching. Since the final milling is done in a ball or rod mill as a slurry, it is necessary to concentrate the milled ore slurry in a pre-leach thickener prior to the leaching process in order to reduce dilution. Acidic and phosphate ores are leached with acid, while alkaline ores are generally leached with sodium carbonate and/or sodium bicarbonate.

Solvent Extraction

Once leached, the ore slurry is washed in a countercurrent decantation circuit where the valuable dissolved uranium-bearing materials are separated and washed from the gangue ore. This pregnant solution is then passed through a buoyant media clarifier where any finely divided gangue material can be further separated and the valuable pregnant solution is polished.

This separation is important to minimize the development of crud in the subsequent solvent extraction process. The solvent extraction process allows for ion exchange of uranium ions between two aqueous phases. It is also common to use conventional direct resin-based ion exchange in lieu of solvent extraction.

Yellow Cake

Following ion exchange or solvent extraction, a precipitation reaction produces finely divided yellow cake particles. This yellow cake material is thickened and then filtered to produce a solid cake material suitable for further conversion into UO₂ or UO₃.

WesTech specializes in liquid/solids separations that are critical in the production of the uranium intermediate product called yellow cake. These processes include: sedimentation for concentrating ore slurry, counter current decantation for washing and recovery of pregnant solution from leached ore slurry, polishing of pregnant solution prior to solvent extraction, and final concentration and filter dewatering of yellow cake product.

Zinc (Zn) is the fourth-most widely used metal, following iron, aluminum, and copper. It is mined mostly in Canada, the former USSR, Australia, Peru, Mexico, and the US. The US is the world’s largest consumer of zinc.

Zinc is a metallic element that has only moderate hardness and can be made ductile and easily worked at moderate temperatures. Its most important use, as a protective coating for iron known as galvanizing, derives from two of its outstanding characteristics: it is highly resistant to corrosion, and, in contact with iron, it provides sacrificial protection by corroding in place of the iron.

Most zinc is used in the galvanizing steel process. Other uses include the automotive, construction, electrical, and machinery industries. Zinc compounds include agricultural chemicals, paints, pharmaceuticals, and rubber.

Refining

Zinc concentration is usually done at the mine site prior to reaching the zinc processing plant (refinery). The concentration includes crushing, flotation, and thickening. The most common process in the refining is electrowinning, which uses an electrolytic cell to reduce the zinc. An electric current is run from a lead-silver anode through a zinc solution. The zinc deposits on an aluminum cathode and is harvested. The zinc is then melted and cast into ingots.

In a typical refinery flowsheet, zinc concentrate is converted to zinc calcine (ZnO) by burning the concentrate in fluid bed roasters. Zinc calcine is the soluble zinc form that is the primary feed for the leaching plant. After leaching, the acid leach slurry is thickened and dewatered to recover residues, which go to the lead smelter for further processing. After neutralization, clear zinc sulfate solution flows from thickeners to the zinc dust purification circuit.

Removing Impurities

Trace impurities are removed from the solution by adding slurried zinc dust through three purification stages. The solution is filtered in the first two stages with filter presses. In the third stage, the solution is cooled and then directed to a gypsum clarifier. The purified clarifier effluent is then pumped to the electrolytic plant.

Other byproducts produced in this process can include fertilizers, lead, cadmium, indium, germanium, and elemental sulfides. Wastewater from the lead and zinc operations is treated by liming in the effluent treatment plant. Heavy metals are precipitated in reaction tanks and separated from the clean water by thickening and dewatering. The solids are recycled back to the plant and the clean water is discharged to the river.