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Mining wastewater: towards precious metal recovery

The operation of a mine is intimately linked to a significant use of water. The water in the mining sector is used for several tasks, including process water, cooling water and cleaning water. In 2009, Statistics Canada revealed that process water represented the vast majority of water used in the mining sector, i.e., 72.5% of it. The large percentage associated with the use of process water is justified by the fact that it is used for various purposes such as separation technologies, metal removal technologies and dust control.

 

Different type of treatment

In mining, water treatment technologies are very extensive and often require several steps to achieve the desired quality. To begin water treatment, the first step is to remove as many solids as possible. To do this, the use of bar screens, different filtration technologies, sedimentation tanks or any other type of chemical or physical separation may be necessary. Chemical and physical separations include coagulation, flocculation, decantation, and many others.

 

Coagulation-Flocculation

These physical-chemical separation techniques are very effective in removing total suspended solids (TSS), total dissolved solids (TDS) and colloids.

coagulation-flocculation

Coagulation-flocculation is a process in which colloidal particles agglomerate with other suspended particles to increase their size and mass for subsequent separation by filtration or other separation methods. Coagulation-flocculation requires the addition of chemicals (coagulants or flocculants) to start the process.

Cationic coagulants provide an electrostatic positive charge to decrease the electrostatic negative charge and thus force the colloidal particles together to create more massive particles. Anionic coagulants provide the same electrostatic transfer, but from the opposite point of view to cationic coagulations. To maximize the impact of the added coagulants and flocculants, the water must be mixed vigorously to ensure uniform dispersion of the chemicals. However, the addition of too much anionic or cationic coagulants can lead to complete electrostatic reversal of the water mass and thus return the colloidal particles to their original state. On the other hand, too high a dosage of flocculant will not have a negative impact on the treatment.

Often presented as the same process, coagulation and flocculation have some differences. Although the processes are very similar, coagulation affects dissolved solids (TDS) and flocculation addresses suspended solids (TSS). In other words, the addition of a coagulant will agglomerate the TDS into larger particles that are then categorized as suspended solids. Once the dissolved matter has been transformed into TSS, the flocculant will agglomerate all the TSS present in the water, including the transformed TDS.

 

Sedimentation basins

Capture d’écran, le 2021-12-16 à 09.16.38Sedimentation is a step that allows the reduction of water turbidity. It is recognized that water with a low turbidity, less than 10 NTU, does not require sedimentation basins since the low presence of TSS can be removed by filtration. In short, sedimentation tanks are usually installed upstream of the coagulation/flocculation steps. However, sedimentation can be carried out in a natural way, without coagulation/flocculation.

These basins are separated in 4 sections: water inlet (influent), settling zone, sludge zone and water outlet (effluent). In order to decrease the turbidity of the incoming water, the sedimentation drastically decreases the velocity of the water, to the point where the suspended solids begin their descent to the bottom of the basin, where the sludge zone is located. The accumulation of floc in the sludge area is constantly removed from the tank and the treated water can leave the tank for the next treatment step.

 

Other treatments

Depending on the needs and expectations of the mine, several types of treatment can be added. Among these, we find different types of filtrations, water softeners, different filtering media or any type of disinfection. The implementation of these technologies can be imposed by governmental standards regarding the effluents discharged, the treatment of tailings ponds or specific processes depending on the type of mine operated.

Responsible management of mine tailings is a key issue for any organization since these tailings, if improperly managed, can pose a risk to the health and safety of the public, the environment and adjacent infrastructure. For more information on good tailings management practices, we invite you to refer to MAC's Tailings Management Protocol.

 

What can be found in mining waters?

Wastewater management in the mining sector presents several challenges since water parameters are extremely different from one location to another. One can think of the great diversity of contaminants, the different pH concentrations and all the variables that must be taken into account (temperature, costs, energy costs, geographical location, government regulations, etc.).

One of the most common scourges of mine wastewater is acid mine drainage (AMD). AMDs are acidic water flows that contain dissolved heavy metals. Although they can originate from natural sources, mining accelerates their creation and concentration. Generally characterized by high acidity, high conductivity, the presence of a mineral acid, high concentrations of iron, aluminum, and manganese, and finally, the presence of heavy metals such as copper and zinc, AMDs are a dangerous scourge for the fauna, flora and human health in the vicinity.

 

Tailings are all the water, sludge and minerals left behind after the extraction of economically viable minerals. These tailings are problematic from several angles. Tailings ponds can be composed of several contaminants. The presence of these contaminants is likely to make the wastewater particularly harmful to the environment, wildlife, and plants. Among the contaminants frequently found in mining wastewater are the following:

  • Copper ;
  • Zinc ;
  • Phosphore ;
  • Manganese ;
  • Cobalt ;
  • Nickel ;
  • Antimony ;
  • Arsenic ;
  • Lead ;
  • Selenium ;
  • Mercury ;
  • Cadmium.

This is a very rough list, for example, wastewater from oil sands mines is very often contaminated with naphthenic acids, polycyclic aromatic hydrocarbons, phenolic compounds, ammonia, and many other metals.

You would expect that some of these contaminants are valuable. So, do you think that contaminant extraction is possible and if so, is it profitable?

Today, the question is no longer whether it is possible to recover minerals with a good return on investment, but rather which technology is the most advantageous for each situation.

 

Government intervention

Whether we are thinking of AMD or tailings, in Canada, standards are set and must be respected under penalty of very severe sanctions. For example, Syncrude had to pay $3 million for the death of 1600 ducks in one of its wastewater ponds in 2008.

The federal government, through its environmental departments, monitors mining activities when its tailings come into contact with :

In addition to federal government departments, some provinces have powers with respect to mining. For example, Quebec has enacted Directive 19 to define the permitting requirements for any mining project.

 

The effectiveness of different technologies

Since flocculation/coagulation and sedimentation have already been discussed, it bears noting that these separation techniques are effective in reducing water turbidity.

 

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Filtration

Whether we talk about microfiltration (MF), ultrafiltration (UF) or nanofiltration (NF), the principle remains the same : The water must pass through a membrane that will filter the different particles according to their size.

 

Microfiltration membranes have pores that allow the filtration of particles ranging from 2 μm to 100nm and require a pressure varying from 0.1 to 2.5bar. With this combination, this type of filtration can remove undissolved particles present in the water as well as the presence of certain bacteria. Since it offers a very basic filtration, microfiltration is often used during the prefiltration process. It is important to remember that the importance of quality prefiltration lies in the efficiency and consistency of operation of the various treatments following prefiltration.

For ultrafiltration, the membranes are made of pores varying between 10 to 100nm. To operate, a constant pressure between 0.5 and 2.5 bar is required. Once the ultrafiltration is initiate, undissolved particles and substances with a high molecular weight will be filtered by the membrane. Common metals filtered are:  copper, nickel, and cobalt.

Secondly, nanofiltration has pore sizes smaller than 10nm to 1nm and requires a pressure of 5 to 15bar to be effective. Often used as an alternative to reverse osmosis since it offers a higher percentage of flux, nanofiltration is effective in removing micro contaminants and polyvalent ions. In other words, NF membranes can remove sulfates, chlorine, calcium, magnesium, and many other contaminants from water.

By far the most versatile technology, reverse osmosis works on the same principle as filtration, but with the presence of a semi-permeable membrane. With pore sizes smaller than 1nm, Reverse Osmosis is able to separate salts, ammonia, nitrate, arsenic, lithium and many other contaminants from water! For more information on reverse osmosis, we invite you to consult this article.

 

Other technologies

Ion exchange, explained in more detail in this article, is a technology that uses a resin charged with a cation and anion. Also known as a water softener, ion exchange can act as an oxidation reducer with different types of resin. It is thanks to this capacity that ion exchangers are able to reduce the presence of metal ions in solution (Fe, Ni, Co, Cu, etc.).

In terms of types of disinfection, one can think of chlorination, which has become an important part of non-ferrous metal extraction. The different chlorine agents that can be used are very varied. Chlorine gas, hydrochloric acid, carbon tetrachloride and some alkaline chlorines are widely used in the mining industry.

Secondly, ozonation has unique chemical properties that allow it to accelerate the oxidation of refractory minerals and to eliminate the presence of cyanide in the effluent.

Finally, activated carbon is known to remove organic impurities from water and chemical residues. In addition to a very high adsorption capacity, activated carbon allows the removal of several contaminants such as trihalomethane, a contaminant resulting from chlorine disinfection.

It is clear that each of the technologies discussed above has different benefits with respect to different contaminants. Unfortunately, there is no magic formula. However, this wide variety of technologies and a good combination of them can ensures that the objectives are met, regardless of the situation.

 

Is there a hiding treasure in your water?

What do you think can be found in your water that can be of benefit when removed? Obviously, gold is a mineral that is very interesting, but rare earth elements are also very interesting! REEs are 15 elements of the lanthanide family with atomic numbers between 57 and 71 of the periodic table, including scandium and yttrium. The value of these minerals is justified by their use in the manufacture of many electronic devices, or the manufacture of magnets, catalysts, and batteries. Currently, China is the largest exporter of REE, but Canada has some of the largest reserves of REE in the world. Is there any REEs in your tailings? If so, how much?

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It’s hard to quantify but logically, one would think that the quantities of minerals that can be extracted from wastewater are relatively small. To put things in perspective, here are the quantities removed from the wastewater of a gold & copper mine located at Mount Polley owned by the Imperial Metals Corps. Excluding the thousands of tons of copper, zinc, phosphorus and manganese, here are some quantities:

  • 138 Tons of Cobalt;
  • 71 tons of Nickel;
  • 3.6 tons of antimony;
  •  84,831kg of arsenic;
  • 38,218kg of lead;
  •  8,695kg of selenium;
  • 562kg of mercury;
  • 995kg of cadmium.

Ideal for the environment, the recovery of these contaminants from mining wastewater can offer a very interesting return on investment for the company. Unfortunately, since there’s no data is available on the quantities of gold that can be extracted from mining waters, we’ll let you dream about that!

 

Capture d’écran, le 2021-11-23 à 10.54.15Here you can see our mobile testing unit located in a gold mine in Ontario. With a wide variety of technology inside, our test unit is used to identify the different minerals or metals that can be removed from the wastewater. Thanks to the tests carried out, we are able to estimate in a relatively precise way the quantities of minerals or metals that can be removed and thus, estimate the return on investment.

Promoting the environment & your business

In short, whether we are talking about acid mine drainage or mine wastewater, variations in contaminants, temperature, acidity, and other parameters affect the types of treatments that can be effective in recovering elements present in the water. Briefly, the important thing is to know that the solution exists and that it only takes a few tests to identify the right technology to meet the varying needs.

 

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