What we know about processing nitrogen compounds in mining effluents

Depending on the type of nitrogen compound, its concentrations, speciations and other physico-chemical parameters (pH, temperature, etc.), it may be necessary to plan for a water treatment system to ensure compliance before discharging water to the environment. Québec’s Directive 019 for the mining industry sets out the requirements that must be met at the final effluent discharge point. To date, no criteria directly related to nitrogen compounds have been adopted.

However, mining effluents are subject to toxicity tests, notably acute toxicity tests, based on the survival rate of control organisms—rainbow trout (Oncorhynchus mykiss) and daphnia (Daphnia magna)—after being exposed to a mining effluent for a certain period of time. These tests establish the toxicity of the effluent to the receiving environment. Some nitrogen compounds, alone or in combination, can induce toxicity. A methodical investigation is needed to target them and define the conditions that lead to their toxicity.

Before going any further, it's important to establish the definition of certain nitrogen compounds mentioned in this article. Ammonium refers to its aqueous form (NH₄⁺); ammonia refers to its gaseous form (NH₃); and ammoniacal nitrogen refers to the sum of aqueous and gaseous forms (NH₄⁺/NH₃).

Ammonium nitrate-fuel oil (ANFO) explosives made from ammonium nitrate (NH₄NO₃) are the most widely used in the mining industry. Explosive detonation is rarely a complete reaction. In fact, it can be estimated that between 5% and 30% of explosives are unreacted and remain on the surface of the ore (Water Conscious Mining, 2017).

Ammonium (NH4⁺) and nitrates (NO₃^-) from unreacted explosives leach into dewatering waters. These explosive residues can also be found in ore and sterile material storage areas and leach out during precipitation. Most contaminants end up in tailings pond water. Depending on concentration and speciation, ammoniacal nitrogen can cause effluent toxicity. Nitrates, on the other hand, do not usually cause toxicity at the concentrations encountered in mine effluents. However, nitrates are nutrients and could accelerate eutrophication of the receiving environment.

For each mining operation, the extent of contamination can be minimized. Storage practices for explosives and blasting methods can be major contributors to water contamination. Handling and use procedures established by explosives suppliers should always be followed. A good practice to adopt after blasting operations is to dispose of explosive residue properly before it leaches into water in galleries or pits. The type of explosive used will also influence the presence of these contaminants. If contamination becomes critical, ANFO can be substituted with emulsion explosives, which are known to release less ammonium nitrate. These explosives are generally more expensive and require a risk-benefit assessment before they can be used. Moreover, operational group personnel must be made aware of the impact of their activities on water contamination, to encourage them to adopt these good practices.

Cyanide is mainly used for extracting gold. Because of its high toxicity, cyanide is generally destroyed within the process. Oxidizing agents, like sulfur dioxide, peroxide and Caro acid, are brought into contact with the pulp containing residual cyanide and other cyanide complexes that were formed during the extraction process. Depending on the composition of the ore, these will be oxidized to produce compounds that are less toxic and hazardous on human health, such as thiocyanates, cyanates and ammoniacal nitrogen. Although less dangerous, these compounds can induce toxicity in the effluent, depending on their concentrations. In such cases, water treatment systems must be installed.

There are several types of nitrogen compound treatments: chemical, physico-chemical, passive and biological.

• Chemical treatment often involves using oxidants like peroxide, ozone and chlorine, which in most cases form nitrates that generally do not cause toxicity in the effluent. However, this type of treatment can be costly given the price of chemicals.
• Membrane separation (by reverse osmosis), adsorption on zeolite, the use of ion exchange resins, acidification and volatilization of un-ionized ammonia (NH₃) are examples of physico-chemical treatments, which can sometimes be less costly than chemical treatments. Contaminants, on the other hand, are not degraded, but rather concentrated, and eventually may require residue management.
• Passive treatment involves natural biodegradation processes. It requires little or no mechanical equipment or chemicals. One of the most widely used passive systems is degradation in artificial wetlands. The effluent is sent to a pond planted with vegetation selected to promote biodegradation reactions with the micro-organisms present in the soil. Nitrogen compounds are ultimately transformed into nitrogen gas, completing the natural nitrogen cycle. Although inexpensive and labour-saving, passive systems are not suitable for treating large volumes of water from mining operations and would require a large footprint. They are most often used to treat water during mine site closure.
• Biological treatments as moving bed biofilm reactors (MBBR) are considered the best available technology to remove ammonium (Mine Environment Neutral Drainage Program, 2014), and other nitrogen compounds such as thiocyanates (SCN^-), cyanates (CNO^-), nitrites (NO₂^-) and nitrates (NO₃^-).
  
  This process is based on using micro-organisms to degrade contaminants contained in the mine effluent. These micro-organisms use contaminants as a source of energy, degrading them into less harmful substances, thereby eliminating toxicity from the effluent. Air can be added depending on the type of bacteria to be grown and whether the water is nitrified (ammonium oxidation) or denitrified (nitrate reduction).
  
  In an MBBR reactor, the effluent is brought into contact with moving supports, often made of plastic, known as media. These provide a large contact surface for bacterial biofilm formation. As the effluent passes through the bioreactor, bacteria dying on the surface of the media are replaced by new ones until equilibrium is reached.
  
  The challenge with this type of treatment is to maintain this delicate balance to ensure its performance. The pH, temperature, alkalinity and presence of inhibitors, such as metals, are all factors that can affect treatment. Compared with chemical or physical treatment, biological treatment is often less costly and does not require using corrosive chemicals. It may, however, require auxiliary heating to increase microbial activity, which will enable the removal of the load of contaminants that can be treated, especially when water temperatures are cold. It also requires a few months of inoculation and adaptation before reaching its full capacity.

Although biological treatment stands out because of its many advantages, that does not mean it is always the best option to consider. Every mining operation is different and has its own reality. A comprehensive analysis that takes this reality into account, as well as objectives, is needed to determine the optimum solution.

BBA's integrated approach, which involves the client at every stage of the project, is applied by its water treatment professionals who include engineers, chemists, biologists and geochemists. Having operated and sized many ammoniacal nitrogen treatment systems in the mining and industrial sectors, our experts have extensive experience in treating nitrogen compounds and can provide you with the right support.

• Margareta Wahlström, Tommi Kaartinen, Henna Punkkinen, Antti Häkkinen, Maria Mamelkina, Ritva Tuunila, Pertti Lamberg and Maria Sinche Gonzales (2017). Water Conscious Minning (Wascious).
• Mine Environment Neutral Drainage Program (2014). Study to Identify BATEA for Management and Control of Effluent Quality from Mines.