Mine water mainly comes from underground exploitations and results from the drainage of rainwater, thaw water and groundwater, which, when passing through rocks and mining work, carry a high content of metals, sulfates and acidity variations that significantly affects lakes, rivers and other bodies of water into which the mine water discharges.
Water is one of the most-used resources in processes such as heating, cooling, products processing, cleaning and washing, among others.
Process water treatment optimizes production operations for a better product if it contains water, or simply allows the reuse of it in the same productive stages. This saves this resource and improves the impact of discharges on the environment.
In the mining sector, mine water and tailing water are characterized by a high sulfate content. Regulations limiting sulfates in discharges to watercourses as well as the importance of reusing process water creates the need to develop technologies that remove this chemical compound.
HDS (High Density Sludge)
This process is used for operations in which there are high flows of sulfate ions which are removed with high efficiency by forming calcium precipitates. The advantage of this system is that it generates a smaller volume of waste sludge with a high amount of solids.
Sulfates can be removed by membrane technology such as RO or NF. In particular, membrane technology is used to obtain water with lower sulfate content than that obtained through the calcium sulfate equilibrium precipitation.
The presence of metals in water, such as copper, mercury, chromium, nickel, zinc, lead and cadmium, is a significant environmental problem due to their high toxicity. To obtain a low metal concentrations in effluent, different methods and technologies can be used according to the particular removal features. Additionally, the metal which is recovered can be used as a byproduct in other processes.
The UNIPURE® process technology uses a unique mechanism for removing heavy metals. Coprecipitation on an iron matrix improves the efficiency of heavy metals removal, obtaining a simultaneous reduction of solids.
Heavy metals are captured in an insoluble iron matrix. This capture occurs because heavy metals are coprecipitated at the same time as iron, which is rapidly removed from the solution. In this process, the effectiveness of classical coprecipitation is optimized by prior association of heavy metals with the iron molecule. Heavy metals are related to iron in the solution by a mechanism of occlusion and adsorption, which creates an entropic effect or concentration that substantially increases metals removal, and it decreases iron requirements at the same time.
Ecopreneur is the exclusive representative of this technology to Chile, Peru and Colombia.
Metals which are soluble in water can be precipitated by the formation of metal hydroxide, reaching the pH range of lowest solubility of that hydroxide. This is widely used to separate metals from aqueous solutions, especially to remove metals as sole contaminants in the water. The presence of several different metals may require a serial process, working at the lowest solubility pH of each hydroxide formed.
The precipitated solids are coagulated, flocculated and separated in a settler, where Lamella® plate clarifiers are generally used.
Lamella® clarifiers require a very small fraction of the space needed by conventional settlers with the same settling capacity, which means a faster and more effective sedimentation.
When a liquid flow containing solids which have been coagulated and/or flocculated enters a tank and flows between a bundle of inclined plates, solids fall to the plate’s surface, sliding down by gravity into a sludge collection hopper. The clarified effluent will flow through weirs and then come out from the top of the clarifier.
Various membrane technologies are used in water treatment to filter very small solid particles and in water demineralization to separate salt molecules and other dissolved elements to obtain ultra-pure water.
Micro- and Ultrafiltration
The principle of micro- and ultrafiltration is physical separation. It is the pore size of the membrane which will determine when dissolved solids, turbidity and microorganisms are eliminated. Depending on the construction of a rejection layer on the membrane, substances larger than the membrane’s pores will be completely retained, and the smaller ones will be partially retained.
Normally, micro- and ultrafiltration systems are used as pretreatment for reverse osmosis (RO) and nanofiltration (NF) systems or as filtration systems for uses such as drinking water production, tertiary treatment of effluents, biological secondary treatment systems MBR (membrane bioreactor), and others.
Reverse Osmosis (RO) and Nanofiltration (NF)
In a reverse osmosis system, water is forced through a membrane that has a very low permeability to salt and other chemical contaminants. This system makes it possible to separate all contaminants from water, including monovalent ions and molecules, obtaining pure water.
Nanofiltration is a process which uses membranes that have permeability to most monovalent ions. In this way it has the same features as RO membranes permitting the monovalent ions going to the permeate water flow. The operating pressure of NF is lower than that of RO for water that has the same features.
In general terms, there are two kinds of reverse osmosis and nanofiltration membranes. For sea water (SW), membranes are used either for reverse osmosis (SWRO) or nanofiltration (SWNF). For brackish water (BW), BWRO or BWNF membranes are used.
RO and NF systems can be integrated locally or come assembled from factory. They have a wide range of uses and sizes, from a few liters/day to hundreds of liters/second. Information about water type, application and size will permit the selection of a system that will best suits the needs of each project.
When water from the mining-metallurgical industry is acidic, the acidity can be neutralized by the addition of alkaline agents.
The most-used alkaline agents are quicklime, hydrated lime, limestone, limestone powder, sodium hydroxide (caustic soda) and ammonium hydroxide, among others. The choice of any of these materials will be determined by their neutralizing power and cost.
Normally, the neutralization of acidic waters causes the precipitation of metal hydroxides and other compounds which are no longer soluble when pH increases. This property is used to separate contaminants from the system, such as metals, sulfates and others. Accordingly, it is usually necessary to use downstream sedimentation processes from neutralization units, which will be defined according to the type of precipitate formed.
Solids removal is usually performed at the early stages of global treatment. There is a fine screening to remove or retain smaller solids like vegetable matter. There are coarse screens where large solids, such as solids and stones waste or sand, are removed. These techniques prevent problems at plants or in water treatment processes, since if these solids are not removed they could block pipes or even damage the equipment.
The removal of smaller suspended solids than screening is done by usual techniques in this sort of process, such as coagulation, flocculation, clarification, filtration, and so on.
Drinking Water Plants
More than 20 compact treatment modules have been installed in industrial facilities and mining camps. The technology and total capacity are defined by available water quality and implemented systems, such as pressure filters, ultrafiltration, reverse osmosis systems or disinfection, which are integrated into transportable modules.
Sewage Treatment Plants
More than 50 compact treatment modules have been installed in mining camps with populations from 100 to 8,000 inhabitants. One or more transportable modules can supply the total treatment capacity that is required. The stages include aeration, sedimentation, digestion and disinfection.