When we talk about mining, we all imagine the same thing: ore being excavated, hauled, treated, and then sold at profit. Mining is innovative and exciting, and certainly can become quite complicated. But there’s one thing we don’t really think about – and that’s what comes after the mine is finished producing at profit, and has to be shut down. Naturally, we can’t just leave an open mine sitting there. The land has to be remediated and reclaimed, returned to as much of a natural state as can be managed.
There are plenty of chemicals involved in a mine. Diesel, sulfides, asbestos, and a host of other chemicals both released from mining equipment and as a product of the earth itself. Metal mines make up the bulk of these chemical releases.
There are many processes for treating and reclaiming mine sites. These methods vary depending on the company and geochemical makeup of the site itself. In this blog, we discuss a particular method: BioGenesis™ Soil Washing Technology. This is one of the many methods that can be used safely and effectively to bring the earth back to its pre-mining state.
Soil washing technologies are designed to transfer contaminants from soil to a liquid phase. The BioGenesis™ technology uses a proprietary surfactant solution to transfer organic contaminants from soil to wastewater. This surfactant enhances the biodegradation to reduce residual contaminant levels. Laboratory tests indicate that soil washing and biodegradation act together to reduce 85-88% of contaminants in 120 days.
Soil washing is conducted with water in a slurry phase and generally relies on mechanical processes to separate particles that contain contaminants. Soil-washing can be used to treat heavy metals, but more generally can be prescribed to sites contaminated with halogenated solvents, aromatics, fuel oils, and a host of other contaminants. The process is based on the fact that contaminants adhere to fine-grained soil, like silts and clays, and organic carbon instead of coarse-grained soil.
The BioGenesis™ process uses high-energy mixing of excavated contaminated soils in a mobile washing unit to transfer organic compounds from soil to liquid phases. Because the BioGenesis™ solution is rapidly degraded by soil microbes, the contaminant-rich wastewater generated during the mixing can be biodegraded in an aerated reactor. Additionally, residual BioGenesis™ solution remaining on soil particles stimulates biodegradation of soil contaminants not entirely removed by the washing process.
Upon receipt of contaminated soil, substances are first delivered to a large vibrating screen on top of the BioGenesis™ unit in order to separate the oversized materials from finer particles. After this, chemicals based on contaminant makeup and degree of contamination are mixed with water, which is sprayed at high pressures to force particles through the screen, soaking them completely in the process.
The soil and contaminants mixed with the solution then fall into a sediment washing and aeration chamber. Here, air is pumped into the water at high pressures in order to mix and aerate the solution. Foam is naturally a side-product of this step – foam is transported to its own side stream, a skimming tank for the floatable organic contaminants.
The bulk of the contaminants which are not a part of the foam are then transported to the sediment washer. This is where the BioGenesis™ solution is added, along with more high pressure water to thoroughly wash the contaminated soil. This unit is attached to a cavitation unit used for destruction of organic contaminants.
From the cavitation unit, the solution is separated into cleaned solids and contaminated water. The cleaned solids may be acceptably decontaminated after a single cycle, or may require additional washing cycles before release. However, the water is sent to a pretreatment facility, where the contaminated sludges are allowed to settle out before being sent to a landfill. Finally, the water is recycled back through the system or to a water treatment facility for the final cleaning stage.
The process generates three waste streams: treated solids, wastewater, and sediments in wastewater. Treated solids may require disposal at permitted facilities. Wastewater will usually require further treatment; at most sites, BioGenesis™ proposes to recycle wastewater and treat it with its oil and water separators and a bioreactor. Additionally, BioGenesis™ wash units come equipped with carbon filters to treat volatile air emissions, should any form.
Applicability and Limitations
This technology can be used as a stand-alone technology because it includes biodegradation to reduce contaminant levels and toxicity of washed soils. In general, soils containing sand and other coarse materials are most ideal for treatment by soil washing. Soil containing large amounts of silt, clay, and humic substances, or soils with high total organic carbon content, are not as effectively treated by soil washing. This is because such soils are usually strongly sorbed to their contaminants, and are very difficult to separate.
This process is capable of extracting volatile and nonvolatile hydrocarbons, including petroleum hydrocarbons, chlorinated hydrocarbons, pesticides, PCBs, and others. However, for soil contaminated with heavy petroleum hydrocarbons, multiple washing cycles are needed to reduce contaminant levels in treated soils.
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