Pollutants in Mining Wastewater And Treatment

With the continuous development of ore mining and mineral processing technology, wastewater treatment and recycling have become increasingly important in the field of mining processing. Wastewater treatment can not only reduce pollution to the environment but also realize the recycling of resources and improve the economic benefits of ore dressing. This article will introduce the pollutants contained in wastewater in ore dressing and the relevant technologies and methods of wastewater treatment.

 

Main Pollutants in Mining Wastewater

The composition of mining wastewater is complex. The main pollutants vary depending on the type of mine, ore composition, and mining process. Generally speaking, it can be summary into the following categories:

1. Heavy metal ions

• Typical pollutants: lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn), chromium (Cr), mercury (Hg), nickel (Ni), arsenic (As), etc.
• Source: These heavy metal ions mainly come from the metal components in the ore, as well as the reagents used in the beneficiation and smelting process.
• Hazards: Enriched through the food chain, damage to the human nervous system, liver and kidney function, and even cause cancer; damage to aquatic ecosystems.

2. Suspended matter and particulate matter

• Typical pollutants: ore powder, silt, coal powder, etc.
• Sources: solid particles generated by mining, crushing, ore washing, and other processes, as well as suspended matter carried by tailings pond overflow water.
• Hazards: blocking rivers, reducing water transparency, and destroying aquatic habitats.

3. Acidic substances

• Typical pollutants: sulfuric acid (H₂SO₄), nitric acid, and acidic wastewater (pH can be as low as 2.5-3.5).
• Source: Oxidation of sulfide ores and discharge of acidic wastewater. Sulfide ores (such as pyrite) oxidize to produce ferrous sulfate and sulfuric acid, forming acid mine drainage (AMD). For example, FeS₂ reacts with oxygen and water to produce sulfuric acid and iron salts.
• Hazards: Corrosion of equipment, acidification of soil and water, promotion of heavy metal dissolution, inhibition of microbial activity.

4. Organic pollutants

• Typical pollutants: phenols, cyanide, polycyclic aromatic hydrocarbons (PAHs), organic solvents, petroleum substances, etc.
• Source: Organic substances in mineral processing agents, coal washing process, and domestic sewage in mining areas. For example, the cyanide gold extraction process will produce cyanide-containing wastewater, and the coking plant will discharge phenol-containing wastewater.
• Hazards: Cyanide is highly toxic, 0.1g can be lethal; phenols destroy the self-purification ability of water bodies and affect aquatic organisms.

5. Radioactive materials

• Typical pollutants: radioactive elements such as uranium (U), thorium (Th), and radium (Ra).
• Sources: Leaching liquid from ore and tailings during the mining of radioactive minerals (such as uranium mines).
• Hazards: Long-term radiation exposure can lead to health problems such as cancer and gene mutations.

6. Other pollutants

• Sulfate (SO₄²⁻): Sulfate produced by oxidation of sulfide minerals, aggravates water acidification and salinization.
• Nutrients: Phosphates, nitrates, etc., which come from the decomposition of organic matter in mineral processing agents or tailings ponds, and are prone to cause eutrophication of water bodies.
• Oil pollutants: including waste oil, emulsified oil, etc., mainly come from equipment maintenance and transportation processes in mining areas. Mining machinery lubricates grease leaks, forming an oil film that hinders oxygen exchange and affects soil and water quality.

 

The direct discharge of mining wastewater has multi-dimensional and profound harmful effects on the environment and human health, such as causing water acidification, heavy metal enrichment in soil, biological toxicity, and threatening human health through the food chain. Therefore, it is particularly important to actively promote the treatment and recycling of mining wastewater.

 

Mining Wastewater Treatment Technology

Pretreatment Stage

Physical interception: Remove large suspended solids and oil film through screens, grit chambers, filtration, etc. to prevent equipment clogging (such as the floating objects that need to be removed before treating 60,000 tons of wastewater per day in the case of tin mines).

Neutralization and adjustment: For acidic or alkaline wastewater, lime (Ca(OH)₂) is often used to neutralize acidic wastewater. Mineral acids should not be used to treat alkaline wastewater to reduce sulfate/sodium chloride generation. For example, lime adjusts the pH to neutral in a gold mine case.

 

Heavy Metal Removal Technology

1. Chemical precipitation method

Technical principle

The chemical precipitation method is to add chemical agents (such as hydroxides, sulfides, carbonates, etc.) to the wastewater to react with the soluble pollutants in the wastewater to form solid precipitates that are insoluble in water, and then remove the pollutants from the wastewater through solid-liquid separation technology. Specific methods include:

• Hydroxide precipitation: Add lime to adjust the pH to 8-10, so that heavy metals form hydroxide precipitates.
• Sulfide precipitation: Add Na₂S to generate metal sulfides, which are suitable for low-concentration heavy metal wastewater.
• Cyanide treatment: FeSO₄ and CN⁻ generate ferrocyanide complex precipitates, and the CN⁻ concentration can be reduced from 3.258mg/L to 0.459mg/L.
• Carbonate precipitation method: Use alkaline reagents such as sodium carbonate to generate carbonate precipitates.
• Barium salt precipitation method: Use barium salts (such as barium chloride) to react with heavy metal ions to generate insoluble barium salt precipitates.

Advantages and limitations

Advantages: low cost (0.25-0.43 yuan/ton), easy operation, suitable for high-concentration heavy metal ion wastewater.

Disadvantages: a large amount of sludge is generated and needs secondary treatment, which is easily interfered with by coexisting ions.

 

2. Adsorption and ion exchange

Heavy metal treatment agents (such as crystal forming agents) or special resins selectively adsorb heavy metals, suitable for low-concentration wastewater and resource recovery (such as recycling copper and nickel in the semiconductor industry).

 

3. Membrane separation technology

Reverse osmosis, ultrafiltration, etc. are used to deeply remove soluble salts and trace heavy metals, but attention should be paid to membrane pollution problems.

 

Biological Treatment Method

Artificial wetland: Use plants and microorganisms to synergistically degrade organic matter, suitable for end-of-pipe treatment and ecological restoration, but the efficiency is low and the area occupied is large.

Activated sludge/biofilm method: used to degrade organic pollutants, but has poor tolerance to heavy metals and requires pretreatment. The activated sludge method is a traditional biological method for degrading COD, which needs to be combined with oxidants (such as hydrogen peroxide) to improve treatment efficiency. Biofilm technology separates pollutants through semi-permeable membranes, which is highly efficient and energy-saving, but the equipment cost is relatively high (such as biological rotary disc technology).

 

Membrane Separation Technology

• Reverse osmosis (RO): The single-stage desalination rate can reach 99%, and the double-stage system desalination rate exceeds 98%. It can remove heavy metals, bacteria, viruses, and organic matter with a molecular weight >100 Da. The retention rate of divalent ions (such as Ca²⁺, Mg²⁺) exceeds 99%, and the retention rate of monovalent ions (Na⁺, Cl⁻) is about 98%.
• DTRO (disc tube reverse osmosis): suitable for highly polluted wastewater, with a recovery rate of more than 75%.
• Nanofiltration (NF): Selectively intercepts heavy metals, and the permeate can be reused.

The initial investment in membrane separation technology is high, but the long-term operating cost is lower than that of traditional processes (such as membrane treatment fees are 30-50% lower than chemical methods).

 

Composite Treatment Technology

For complex wastewater, the combination of “chemistry + biology + physics” is often used:

• Coagulation-precipitation-membrane filtration: In the treatment of Guangsheng Nonferrous Rare Earth wastewater, nanofiltration and electrodialysis are combined to achieve an ammonium chloride recovery rate of 90%.
• Oxidation-biodegradation: When treating cyanide-containing wastewater, chemical oxidation is first used to decompose cyanide, and then biological methods are used to degrade residual organic matter.

 

Mining Wastewater Treatment Technologies Comparison and Selection

Pollutants in mining wastewater are complex, highly toxic, and difficult to degrade. Its treatment requires a combination of processes for different components (such as neutralization precipitation + adsorption + membrane separation). For example, the sulfidation method can efficiently remove heavy metals; the biological method is suitable for low-concentration organic wastewater; membrane technology is preferred in arid areas to ensure reuse; the SRB biological method combined with chemical neutralization is used in high-acid mines; chemical precipitation is mainly used in economically restricted areas. Please refer to the table below to select mining wastewater treatment methods.

Technology Type	Investment Cost (yuan)	Operation Cost (yuan/ton)	Applicable Scenarios	Limitations
Chemical precipitation method	Low (150,000-200,000 yuan)	0.25-0.42	High-concentration heavy metal wastewater	Difficulty in sludge disposal
Biological treatment method	Medium	0.5-1.2	Low-toxicity organic wastewater	Poor environmental adaptability
Membrane separation technology	High (more than 1 million yuan)	2.7-4.0	High-salt/high-pollution wastewater	High membrane pollution and replacement costs
Combined treatment technology	High	1.5-3.0	Complex component wastewater	Difficulty in process integration

 

Summary

Mineral processing wastewater is one of the main factors that pollute the ecological environment. It is necessary to strengthen the treatment of mineral processing wastewater. In the specific work, it is required to comprehensively analyze the basic characteristics of mineral processing wastewater and determine the best treatment method based on the actual situation. At the same time, the mineral processing wastewater treatment technology is optimized, the application scope of mineral processing wastewater recycling technology is expanded, and its utilization effect is maximized.