Point-of-Need Waste Management Technology
Mines and mining camps are moving away from permanent environmental legacies such as landfills or using a third-party to move waste off-site, a potentially risky practice to modern incineration technology.
By Jean Lucas
The mining industry is always under the microscope in terms of its approach to the protection of the environment. Mining wastes such as waste rock and tailings tend to get all of the attention; however, there are many waste products that seem relatively innocuous but can pose a risk of serious environmental damage if they are mishandled. For example: waste products generated by the human population that live and/or work on-site, as well as from maintenance shops must also be managed properly. Many of these waste streams can and should be treated on-site rather than re-handled for transportation and off-site disposal.
Waste Management Challenges
Many mines and exploration sites are some of the most remote locations in the world. Shipping waste off-site is extremely costly in that it involves trucking over long distances to a private or publicly owned landfill. In some cases, where there is no road access this requires air transport. Some mines are able to establish a contract to have solid waste picked up by a local hauling company and taken to a local landfill.
In many countries, the landfills are not modern, engineered facilities; rather they are ill-planned, improperly constructed and poorly maintained dumps. By using these facilities, mining companies are actually contributing to local environmental problems and exposing themselves to future liability. Also, these mining companies will have relinquished their control over the handling and proper disposal of the waste by trusting a local waste hauler to act responsibly and deliver the waste to the dump and pay applicable fees—rather than short-cut the process by illegally dumping the waste somewhere else. There have been some rather notable incidences of this occurring in recent years. If discovered, the waste generator—the mining company—is usually found guilty for the violation and may be held accountable for the very significant cost involved with environmental remediation if it is deemed necessary.
To avoid these issues, many mining companies opt for an on-site landfill, long perceived as the simplest and most inexpensive solution. However, meeting today’s standards of an ecologically sound, engineered landfill is neither simple nor cost-effective in remote locations. Regardless of the size and location of the property, it may not be possible to find a suitable area to create a landfill. Landfill sites are not compatible with many terrains, hydrogeologic and climatic conditions including but not limited to permafrost, marshland, spongy soils, flooding, high rainfall, and high water tables. Even when these factors are not present, the long-term legacy of the landfill must be considered. Mine closure plans are centered on returning the land as closely as possible to the original state before the mine was built. A landfill is not conducive to mine closure plans as it is a permanent feature that will forever alter the landscape in which it is sited. Over time the landfill will become more active as the waste decomposes; which can produce airborne pollutants such as methane gas - a powerful greenhouse gas. In order to properly care for the environment, and to avoid a possible explosion, these gases should be collected and flared off, or recovered. It is also possible for the landfill to generate toxic liquid by-products over the longer-term such as leachate and/or run-off; which should be collected and treated to avoid contaminating the local receiving environment. These are long-term legacies that mine developers, regulators, and other stakeholders are becoming aware of and looking to avoid.
Food waste is a huge attractant to a range of insects, birds, rodents and animals. This attraction upsets their normal behavior and may change migration routes, and breeding patterns. The consumption of waste can directly impact the health of the animal. Also, to workers living onsite, unwelcome wildlife can become a nuisance and even create human health issues as some of these are disease-carrying vectors. Large animals can become dangerous predators, posing a safety hazard to everyone on-site.
A modern, advanced technology incinerator can be the basis of a pollution prevention approach to waste management. The incinerator allows the company full control of the disposal of waste which is dealt with immediately rather than buried in a landfill for future generations to deal with. Food waste should be processed on a daily basis in order to prevent the attraction of wildlife to the site. With proper incineration, waste is turned into non-toxic, non-leaching ash residue that will represent less than 10% of the original volume prior to incineration. The residue can be deposited safely with other inert materials. Metals and glass, if comingled, are preserved and can be recycled, if practical.
Liquids from vehicle and equipment maintenance is another common waste material that is expensive and inherently risky to deal with. In urban areas waste oils and glycols can be sent for recycling at minimal cost. However, in remote locations recycling facilities do not exist nearby and transporting the material to such a place is logistically challenging. In some cases, this can only be done by air, requiring special handling and permits. Consideration of the complete environmental impact including the burning of fossil fuels for the back-shipment, and the carbon footprint of this practice, negates the benefit of recycling. When road transport to a local recycler is available, the drawbacks can be worse. There have been some cases of large volumes of liquid industrial wastes improperly handled. If there is a spill, contamination can become widespread very rapidly. By their physical nature, liquid wastes can pose a significant threat to the environment and must be handled with care. On-site re-use in the incinerator eliminates the risk and high cost associated with transportation. Further, using waste oils can replace all or most of the fuel required in the process. This approach not only reduces operating costs but off-sets carbon emissions through re-use.
Packaging from chemical reagents and explosives form a surprisingly large portion of the waste stream for many mines. Materials such as super sacs, crates, and pallets that are contaminated with trace amounts of chemicals must be treated as potentially hazardous. The most important fact to remember is that these materials should not be re-used by someone who is unaware of the origin of the material. This means that a worker handling this packaging should always be wearing the proper personal protective gear to prevent exposure.
It should be noted that mines often allow local populations to re-use high-quality materials such as clean hardwood; which is a great practice to reduce waste and conserve resources. However, packaging materials that have been in contact with toxic chemicals such as pelletized cyanide could pose a serious health threat and should never be re-used. The International Cyanide Management Code Standards of Practice recommends that users “Identify potential cyanide exposure scenarios and take measures as necessary to eliminate, reduce and control them”. The immediate destruction of these packaging materials can help to avoid inadvertent exposure and potential harm.
Incineration is one solution that can manage all of these waste materials on-site. Incinerators today have evolved tremendously from those of the past with modern incinerators being very clean burning and producing no visible emissions. A new, properly engineered incinerator is a negligible point source of air pollutants, meeting very strict environmental standards. How is this possible?
It has long been established that clean burning requires a properly engineered and controlled dual-chambered incinerator. Old style, crude incinerators or burn barrels merely contain the waste for burning and although the waste may have been thoroughly reduced to ash, the burning still produced a lot of smoke and pollution. Unfortunately many remote locations still employ burn barrels. Replacement of the burn barrel with a properly designed incinerator is a significant step forward in terms of the resulting air emissions. The dual chambered incinerator evolves the burning of waste through the addition of a second stage to contain the products of the waste combustion (smoke) and expose them to the “Three T’s” (time, temperature and turbulence). Most of the products of combustion are hydrocarbon-based and act similarly to other hydrocarbon fuels. When these products of combustion are exposed to oxygen and mixed at high temperature they are able to fully combust. Complete combustion eliminates smoke and produces no visible emission. Although not visible, many combustion processes create unwanted air pollution such as dioxins and furans. To avoid the creation of these compounds, the dual-chambered design must also include automatic controls to ensure that the gases endure very high temperatures, ideally 1000°C (1,832°F) for a minimum retention time of one second. Advancements such as instrumentation and computerization with PLCs (programmable logic controllers) ensure that the correct operating conditions are constantly maintained for optimal combustion without need for operator intervention.
Advanced incinerators with superior designs produce far less pollution than their predecessors. For example, with proper design, particulate matter can be largely avoided and regulations met without the need for costly dust collection devices. However, other by-products of combustion may require scrubbing to meet the emissions standards in some jurisdictions.
Primary Chamber: The waste is loaded into and remains in the primary chamber; which maintains a temperature of 650°C to 850°C. This stage involves very little oxygen and is where the waste is converted into a hydrocarbon-based gas. It should be noted that this stage reduces the volume of the waste by over 90%, leaving only ash and non-combustible materials such as metal cans and glass bottles. These materials can be recycled post-combustion.
Secondary Chamber: The gases then enter the secondary chamber; which is heated to 1000°C and is oxygen-rich. Here, the gases are burned again and held for a minimum of one second before they are released or enter the third stage (scrubber).
Main Control Panel: The entire process is computer controlled and key process operating parameters are monitored and recorded.
Air Pollution Control: If required, this stage involves the use of an air pollution control device (scrubber). It works to capture pollutants such as particulates (tiny solid particles) and heavy metals. The scrubber also neutralizes acid gases before these gases exit the stack to the atmosphere. However, scrubbers are not always required. The Eco Waste Oxidizer can produce a very clean emission without any scrubbing. The need for a scrubber is determined by the regulatory standards and the waste to be processed.
Continuous Emissions Monitoring System (CEMS): If required, this optional device provides a constant real-time measurement of the stack gases that are being emitted to the atmosphere. A sample probe in the stack is connected to an inline analyzer that analyzes the information and sends out a signal to a data recorder (in case of the Eco Waste Oxidizer, the PLC and data monitoring package). The specific pollutants that need to be measured are dictated by the environmental authority.
The Eco Waste Oxidizer is available in two configurations: the ECO model (shown above) and the CA model (shown below). Both designs utilize the same concepts and patented process control; which results in an advanced, state-of-the-art incineration technology, based on Controlled-Air Batch Incineration concepts.
The system is operated in a "batch mode" over a 24 hour period, which entails a single charge or load at the beginning of the cycle, followed by combustion, ash burnout, cool down, and finally ash removal. The smaller format CA model can operate intermittently processing several batches in a 24-hour period; however, it is still classed as a true batch incinerator because waste is not loaded during the combustion cycle.
One important thing to note about this technology is that there is no need for pre-sorting of the waste if source separation is practiced to keep inappropriate materials out of the waste feed. Loading waste into the unit is kept as simple as possible, often requiring no special equipment. Where climatically feasible the equipment operates outdoors; and it is often sited near a natural gradient or ramp to allow for top-loading of waste from above, by emptying waste trucks or waste containers directly into the unit. The system is loaded to capacity over the course of the day or all at once.
Once the loading of waste is complete, the doors of the primary chamber are sealed shut and the secondary chamber fired. The system is interlocked so that the waste in the primary chamber waste is not allowed to combust until the secondary chamber is at operating temperature (1000 ºC). Once this occurs; which is usually within the first 30 minutes of the cycle, the primary chamber cycle is initiated. In this phase, gaseous products of combustion resulting from the burning of waste products in the primary chamber are allowed to fully combust in the highly oxygenated, turbulent environment of the secondary chamber.
The secondary chamber is also the location for processing flammable liquid waste. The liquid waste is blended according to factory recommended procedures for safety and efficiency, and mixed using the supplied electric stir mixer mounted on the liquid waste tank. The injection of liquid waste is metered using integrated pumps and atomized in the combustion cone with the combustion air supply; which is also controlled by the PLC in the main control panel. The secondary chamber burners will modulate down to low-fire and priority will be given to utilization of waste oils, minimizing virgin fuel to the main burner as required to maintain temperature for efficient combustion or when waste fuels are not available for firing.
Operation of the incinerator does not require full-time attendance. The operator only remains present to load waste products and supervise the beginning of the incineration process; usually the first hour of the burn cycle, and then is no longer is required to be at the incinerator. The incinerator will complete the burn cycle and cool down phases automatically; this is a major advantage over older, traditional style incinerators with continuous feeding devices that require the operator to load waste into a hopper on a constant basis.
The burn cycle occurs over 8 to 10 hours in the ECO model or over 3 to 6 hours in the CA model, depending on the batch size and characteristics of the waste. Upon completion of the burn cycle, the cooldown phase initiates automatically and generally takes between 6 to 8 hours. Once the cooldown process is completed, the operator will be able to open the primary chamber doors and clean out the ash. The cleanout of the incinerator is generally performed the next day prior to loading the primary chamber with the next batch of waste.
All key operating parameters will be controlled to factory pre-programmed settings using the integrated PLC. For added simplicity in operating the incinerator, the system comes with an LCD screen that provides a graphical view of entire operation displaying the status of all of critical components including alarms for any malfunctions.
It is not necessary to constantly monitor the process. However, the system has the capability to be monitored remotely by the client’s own DCS using a range of communication protocols.
Advanced incineration has progressed to the level of other critical process equipment at the mine. Technology such as Eco Waste Solutions’ Eco Waste Oxidizer offers a secure, environmentally sound means of dealing with waste commonly produced in remote mining operations.
Jean Lucas ([email protected]) is the Director of Business Development with Eco Waste Solutions. Eco Waste solutions manufactures patented thermal waste treatment systems and is based in Burlington, Ontario Canada.