Chapter 8: Washing & Classifying | P&Q University Handbook

Recycling Wash Water

Maintaining clean wash water is essential to operations with a wet process.

Producers reusing dirty water in their wash system are essentially putting all the dirt, debris, clay and fines back onto the product they’re trying to clean. At the same time, this causes wear, tear, clogging and buildup on equipment, water pipes and spray bars.

Fortunately, there are three methods of recycling and reusing wash water that producers typically follow: traditional settling ponds, ultra-fines recovery systems (UFRs) and clarification systems.

Settling ponds

While pumping dirty water to a settling pond is a simple concept, particles settle out naturally with time and gravity, allowing fresh water to float to the next level of the pond to be reused. The disadvantages, however, are space requirements and permitting restrictions that make it harder to establish ponds.

There are also evaporation issues and maintenance costs involved. The costs can be hefty when having to dig out sediments to ensure the needed depth for required settling.

On the plus side, settling ponds can work well for operations that have a larger footprint and can successfully use flocculants to achieve greater settling efficiencies.

UFRs

A UFR system, meanwhile, offers the advantage of removing a good portion of material down to a #400 mesh, keeping more fines out of the pond. This results in less digging during pond maintenance. 

Additionally, a UFR provides a stackable material that can become a saleable product, resulting in minimized waste. Also, the water remains much cleaner and requires less processing before reusing it.

One of the downfalls of UFRs is an occasional situation where wastewater travels too far downstream in the pond, requiring additional flocculants to separate particles. In this event, enough larger particles are not present to stimulate the settling process.

The latter results in a “double dipping” during the process. Still, if a UFR can minimize the need to dig out the pond, a savings in operating costs can be achieved.

Clarification systems

Growing in use, clarification systems start with something as simple as a thickener or the use of a clarification tank. 

No matter the option, clarification systems have space and flocculant requirements. Clarification systems are becoming more popular, though, as they’re less likely to incur permitting issues. They also allow operations to eliminate settling ponds.

Clarification systems discharge a sludge waste product that’s typically pumped to a designated area, where it must sit and dry. Sometimes, this waste product requires the use of filter or belt presses that pull out extra moisture, creating a solid, stackable cake that’s potentially used as a fill product. 

The biggest drawback of a clarification system is the capital investment required. But, depending on the cycle time of the complete clarification system – which can create a bottleneck – it delivers extremely clean water back into the wash circuit.

Finding the right solution

Efficiency improves when optimizing all the equipment in the wash plant. 

For example, if sand screws are not running properly, an operation may be washing valuable product and sending it to the pond. Conduct a thorough plant audit to evaluate the performance of each piece of equipment in the plant.

Again, each wet processing operation is different, and the right solutions depend on parameters such as material type, moisture content, specification requirements, size of footprint, permitting restrictions, financial limitations, water rates, water availability and more. 

Proper analysis by an experienced applications engineer is advised. Avoid cookie-cutter or one-size-fits-all equipment approaches. The best practice is to rely on highly targeted, customized solutions.

Winter Washing

Winter’s icy grip presents a unique set of challenges for wash plant operators in northern climates. 

With effective planning, though, producers can protect their investments and ensure operations are ready for an efficient return to production come spring.

When it comes to plant maintenance, it is essential that any potential complication is identified and remedied quickly to protect an operator’s investment and ensure optimal performance – particularly during the coldest months of the year, when washing and classifying equipment is especially susceptible to the elements.

With planning and preparedness, operators can ensure the safe and efficient shutdown of a wash plant in winter and facilitate an efficient restart of operations in the spring.

Five steps to follow

1. Water management. Effective water management as the cold weather sets in should be paramount for operators. It’s advisable to drain all standing water from tanks and pumps when plants are mothballed for the winter season. Otherwise, operators risk lines freezing and cracking, leading to a delayed and costly restart in the spring.

2. Insulate. The winter months can present unforgiving conditions, so consider insulation to protect pipes, valves and pumps. Ensure your control cabin is closed and, if possible, keep the internal heating on to protect the beating heart of your plant from plummeting temperatures.

3. Open valves. In addition to draining the system and, where possible, insulating pipes, valves and pumps, it’s advisable to keep valves open during winter shutdown. This will prevent any residual water from accumulating in the system.

4. Spray bars. Clean out spray bar nozzle heads, and ensure smaller pipework is free of debris such as silt and sands. Also, ensure spray bars receive adequate water pressure, and that the spray from nozzle fans is at the correct angle. This will allow operators to get more water onto the screen to ensure the product is getting the best washing treatment.

5. Safety protocols. Winter also poses safety concerns for plant operators. Consider refreshing and rehearsing your training procedures to ensure your teams are well versed in the appropriate safety protocols when working in the vicinity of a wash plant to avoid slips, trips and falls.

Proactive and preventive approaches to plant maintenance during winter months are essential for long-term efficiency and profitability, but unforeseen challenges can still arise despite the best-laid plans.

Water Management

Mine operators know how critical optimal mine water management is. 

As finite water resources become ever scarcer, investors, regulators and local communities are increasingly focused on how the industry is managing water. In the U.S., where some parts are facing a historic drought, an estimated 4 million gallons of water per day are withdrawn for mining purposes.

Holistic mine water management – including smart technologies – presents a powerful opportunity to unlock compelling economic benefits for mine operators while protecting the surrounding environment and local community.

Experience shows that optimal water management begins with a holistic view of how water is used across the entire mining operation – from sourcing to dewatering to treatment and reuse. Every step is part of a process that can increase productivity, reduce costs and, ultimately, turn mining water management from an expense to a strategic advantage.

Managing efficiently

Every mine has a distinct set of operating conditions, and customized water management solutions can deliver better results. Still, challenges facing many mine operators are increasing productivity, reliability and sustainability. 

Efficient mine water management, and tapping into the power of digital technologies, can support the effort by minimizing production downtime, operating costs and the environmental impact while enhancing performance and safety. Let’s examine what this means in reality:

• Water quality and scarcity. Remote monitoring and control with real-time, continuous monitoring systems give operators a better understanding of water use and ensure water can be extracted from multiple sources, transported and treated at the desired pressure and quantity. 
• Operational continuity and efficiency. Smart technologies like remote monitoring and control minimize costly, unplanned maintenance and unwanted downtime while reducing safety risks. Visibility and understanding the health of assets enables efficient scheduled maintenance, smart inventory management and reduced energy consumption.
• Regulatory compliance. Real-time, continuous monitoring systems and pipeline integrity management monitor water quantity and quality to ensure the extraction and transport of water and management of tailings meet regulatory standards.
• The golden rule of pumping systems. Some operators may find it difficult to know where to begin to achieve efficient water management on-site. Experience, however, shows that aiming to operate as close to the best efficiency point for a site’s pumping system is a great place to start. 

Straying too far from a pump’s best efficiency point reduces overall efficiency, which leads to premature wear, higher energy consumption, increased maintenance requirements and, as a result, more downtime. An audit of an operation’s water management system is the first step in a process to boost efficiency, reduce costs and enhance profit margins.

Once the appropriate selection is made based on proximity to the best efficiency point, integrating smart technologies is a logical next step. Let’s consider a typical, phased scenario: An open pit mining operator is looking for an efficient dewatering solution. The first step is to develop a customized solution using the most efficient dewatering pump to meet the head/flow requirements of the application. The pump can be paired with a variable frequency drive to ensure optimal performance.

Additional pumps may be needed as the mine expands and dewatering requirements increase. This second phase of integrating smart technologies can involve a PLC intelligent controller being incorporated into the system, enabling the operator to easily control multiple pumps via the PLC or intelligent controller. 

A third phase would involve enhancing the dewatering system further by adding a remote monitoring and control solution. For isolated or inaccessible mine sites or operations where manpower is limited, remote monitoring and control provides visibility of assets and peace of mind.

Manufactured Sand

Natural sand deposits are a depleting resource in many parts of the United States, prompting quarry owners and operators to take a second look at a waste product many aggregate producers already have on site in massive stockpiles.

The process of dry crushing and screening produces a screenings byproduct that contains high levels of fine material, silts and clays. This product has a very limited market but, as a result, producers can accumulate hundreds of thousands of tons of this “waste” over time.

These stockpiles take up valuable space on the quarry floor and cost money to maintain rather than generate a revenue. Using advanced washing systems, screenings can now be reprocessed to create spec manufactured asphalt and concrete sands that can be used in place of natural sand.

Introducing washing to a completely dry process in a quarry was traditionally a challenge. The permitting required to get wet processing permits was costly and time consuming – if it was possible at all. Settling ponds used to recirculate water are also costly to maintain and take up valuable real estate.

These issues have pushed some quarry operators to look at closed-loop water and silt management systems. They have many benefits, including the elimination of tailings ponds and the associated cleanout costs, recycling up to 95 percent of the process system water and reducing waste by 75 percent.

These benefits make the permitting process much easier, with no concerns over groundwater contamination and much less fresh top-up water required.

The process

The first stage of the process is the washing of the screenings to reduce the percentage of silts to meet asphalt and concrete sand specifications. One efficient method of doing this is using hydrocyclone technology that maximizes the recovery of in-spec sand and minimizes the amount of waste material going to the silt management system.

The addition of a dewatering screen after a hydrocyclone reduces the moisture percentage in the finished product and maximizes the recovery of water to be reused in the system.

The second stage of the process is a closed-loop water and silt management system to clean the wastewater and recycle it back to the wash plant. The water management system will maximize the process water return, reducing fresh top-up water demand to a minimum.

The system comprises a thickener tank that scalps off most of the water over a weir system and produces a slurry of milkshake consistency. A high-pressure pump then transfers the material to a plate press that filters the mud through the plate chambers, producing a perfectly dry, dehydrated, low-moisture mud cake.

Because the plate press is fully automated and does not require a flocculant/coagulation additive to achieve the dry cake results, running costs are kept to a minimum.

The environmental and economic benefits of the process are a reduced demand for natural sand while turning a waste product into revenue and profit for the quarry owner.

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