This article compares the “cost” of lost yield to the “savings” from a cheaper plant. A partial list of money-saving design ideas includes undersizing fine coal circuits, eliminating intermediate circuits, using cheaper dewatering equipment, using cookie cutter designs, repeating past mistakes and using oversized classifying cyclones to reduce the footprint. Some of the money saving ideas in operations include: saving magnetite to the point of running low densities, overextending screen panel life, increasing deslime cut size to increase plant capacity, and failing to timely replace fine wire sieves.
Revenue and cost are not independent and can intimately affect each other. It is the proper balance of cost and revenue that will maximize profitability. In this case, profitability of the entire production chain, i.e., from mining to shipping of clean coal, should be pursued. Too often, the concern for profitability is lost in the pursuit of lower costs. Lower costs can result from lower capital costs in construction or lower operating costs. Lower costs are typically good, but the bottom-line value needs to be considered to ensure profitability is maximized. As mom used to say, “Just because something is cheaper does not mean it is better.”
Undersized Fine Coal Circuits
Fine coal circuits are often short-changed in the design process to lower the cost of building the prep plant. One familiar scenario is when the low bidder gets the job, but their bid is oftentimes lowest only because they have undersized the fine coal and small coal process equipment. The resulting poor performance is generally undiscovered until a plant efficiency test is conducted after the plant is built and operating.
As an example, the cut point in a spiral circuit goes up as the feed rate measured in tons per hour (tph) to the spiral increases. In undersized circuits, the feed rate is above the recommended 2.5 tph per start. Note: this occurs when the plant and screen panels are new. As the panels wear, the spiral tonnage increases. This increases the projected cut point and results in a spiral product ash that is higher than desired or expected. To compensate for high-ash fines, the density in the coarser circuits is reduced. Typically, the coarser circuit was already running at a lower separating gravity than the fine circuit and lowering the density further reduces plant yield.
This overload issue is particularly sensitive when making a low-gravity cut for metallurgical coal. In one location, the spiral circuit was overloaded to the point that the plant feed rate was reduced by 25% to achieve an acceptable yield. The yield when making product at design capacity was 10% lower than when running the plant at reduced tonnage. A 5% loss in yield for a 900-tph plant running 6,000 hours per year with a value of $50 per ton equates to $13.5 million per year in lost revenue. The “cost of saving money” was either a 10% loss in yield or a 25% reduction in plant capacity. Neither result was acceptable, but this was found only after the plant was built and operating.
Undersized spiral circuits can also be a big problem when the spiral middlings are recirculated. This recirculation adds additional tonnage to the spiral feed. The recirculating load can grow quickly and cause serious quality issues if not properly monitored and controlled. Undersized flotation circuits can be a serious issue when the flotation product is the lowest ash product. In this instance, the coarse densities have to be lowered to compensate for low flotation recovery. The plant yield suffers from low flotation yield and lowered yield in the media circuits.
Dense-media Cyclones and Flotation
In the 1970s, numerous cleaning plants were built with a two-circuit arrangement. The dense-media cyclone (DMC) was used to wash the plus-0.5-mm material and flotation was used to wash the minus-0.5-mm material. This arrangement saved money compared to using Deister tables on the small coal fraction. This was prior to advances and wide acceptance of the spiral technology.
But, the two-circuit DMC-flotation plant had several issues:
- Fine coal in the DMC was difficult to deslime, which increased magnetite usage;
- Coarse coal in flotation was difficult to float, which increased coal losses; and
- As the deslime screens wore, coarse coal in flotation increased and contributed to sanding and additional losses in the flotation cells.
Many of these plants have since been updated to include a spiral circuit to process the small coal in the 1-mm x 0.15-mm size fraction. This circuit simplifies deslime operations and takes the coarse material out of flotation. The spiral circuit addition in these plants typically increased plant yield by 6%-7% and increased plant capacity by removing coal from both of the original circuits. The payback was quick.
Dense-media Vessel and Spirals
In the early 1990s, Pittston Coal inherited a simple two-circuit plant that was built to wash surface-mined thermal coal with a dense-media vessel (DMV) circuit and spiral circuit. The plant was built for another company to meet a minimum product specification. The original design criteria included a specification to build it fast and build it cheap. Few other requirements were noted. Pittston acquired the plant while it was being moved to a new location.
The original plant was a well-built 300-tph plant with a raw sizer, a dense-media vessel circuit, a spiral circuit and a thickener. The plant was built to process the plus-1/8-in. material in the DMV circuit and the 1/8-in. x 100-mesh material in the spiral circuit. The minus-100-mesh coal was sent to rejects. In regards to the design criteria, the plant was easy to relocate, reconstruct and restart. The process efficiency was another matter. Similar to the DMC-flotation example, an intermediate circuit was needed to bridge the gap.
With the new location and the new feed coal, circuit testing showed that the plant had some significant shortcomings. The plant was still cleaning a surface-mined thermal coal with no need for flotation. However, the plant efficiency testing showed substantial yield losses when the first organic efficiency test was conducted. The source of the losses was twofold:
- The coarse DMV circuit did not efficiently process the 3/8- x 1/8-in. coal (This material was subject to rafting and entrapment); and
- The spiral circuit did not process the 1/8-in. x 1-mm coal effectively.
It should be noted that the design criteria for the equipment specified that the DMV should process plus-3/8-in. material and the spirals should process minus-1-mm material. However, the process ranges were stretched to simplify the plant into two circuits. Therefore, the plant circuitry was simplified and capital dollars were saved, but the “cost of saving money” was considerable.
The solution to the plant inefficiency was to add an intermediate circuit. A DMC circuit was inserted to process the ½-in. x 1-mm coal. This circuit removed tonnage from both existing circuits and boosted the plant feed to more than 450 tph. With the DMC circuit, the fine coal was removed from the coarse circuit and the coarse material was removed from the spiral circuit. The resulting plant efficiency test showed a 6% increase in organic efficiency. Because of this circuit, the plant yield increased from 60% to more than 64%.
Based on $50/ton and only 5,000 operating hours per year, this 4% yield increase equated to a revenue increase of $3.1 million per year. The capital savings for eliminating the DMC circuit was less than $2 million. The “cost of saving money” was substantial: $3.1 million per year versus a one-time cost of $2 million in capital. Also, additional operating savings were realized since the plant capacity was increased by 50% with the new circuit. Payback was quick.
Proper Dewatering Equipment
The dewatering of fine coal can be critical to maximizing the plant yield and profitability. If the fines are not fully dewatered, money will likely be left on the table in terms of lost plant yield or lost revenue on the contract, or both. The moisture has a direct effect on the shipped heating value and can be a limit to fines recovery if the moisture specification is too low.
The circuits and systems are for a four-circuit plant with three clean coal dewatering circuits. The three dewatering circuits are well applied and show:
- A coarse stoker centrifuge;
- A horizontal centrifuge for the DMC product; and
- A screen bowl centrifuge for the spiral and flotation products.
Using the screen bowl to dewater the 1-mm x 100-mesh spiral product is an area of particular interest. In some plants the spiral product is dewatered with a screen scroll type of centrifuge. This will typically give a product that is about 13% surface moisture with a solids recovery of 88-90%. The screen scroll unit is cheaper to install than the screen bowl and requires less connected horsepower.
However, the typical screen bowl moisture on spiral material is about 8% surface moisture and the recovery is greater than 96%. The 5% improvement in surface moisture and the 8% increased recovery are significant improvements. The “cost of saving money” by using the cheaper dewatering equipment can be significant in terms of moisture gain and recovery loss. A full analysis is warranted for both the spiral product dewatering and the DMC product centrifuge.
Cookie Cutter Designs and Repeating Mistakes
An issue often seen is a standard configuration provided by one or more designers that is used for many different applications. The cookie cutter design is convenient for the designer and reduces engineering costs, but may not be the best circuit for the coals to be processed. This is particularly true if the design has known issues that are then repeated without question or correction.
One issue seen repeated is the use of single stage spirals. It’s generally understood that the two-stage or rougher, cleaner spirals outperform single-stage spirals under all conditions. Additionally, the multistage spiral configuration provides more process flexibility for handling different coals and reduces quality fluctuations by reducing reject loading prior to making the final cut. The additional cost and space required for the RC spirals is minor compared to the increased process efficiency and increased yield over the life of a project. The “cost of saving money” here is substantial.
A similar issue exists with the larger diameter cyclones used for desliming applications. These cyclones are used to make the split between spirals and flotation. The large diameter cyclones, up to 1 m diameter, produce a much coarser separation than the smaller, 450-mm cyclones. This larger cyclone directs more coarse coal out the top to the flotation circuit where coarse material can easily be lost. If there is any oxidized coal, flotation recovery is typically extremely low and all misplaced coarse coal sent to flotation is lost.
An additional concern is the increased cut size exhibited in a cyclone as the particle gravity is decreased. Particles are separated based on their terminal velocity inside the cyclone. Terminal velocity depends on the size of the particle, but also its specific gravity and the specific gravity of the pulp inside the cyclone. Unfortunately, the coarsest material in the cyclone overflow is the lightest gravity material and typically the lowest ash. Using large diameter cyclones for this application increases the losses of this valuable coal.
More cyclones of a smaller diameter are required to make a finer cut. This requires more expensive distributors and pumps to process the same feed rate and likely a larger footprint for the cyclones. However, the “cost of saving money” here should not be underestimated. A full analysis is warranted.
Many operators try to save money by reducing operating costs. This is good practice, but can be carried to an extreme. In one plant, the efforts to reduce magnetite costs would result in low process densities. The operator was instructed, “Do not add fresh magnetite to the circuit.” He would typically finish his shift with a process density of 1.45 when the plant should have operated at 1.55 to maximize yield at the target quality. The lost yield was substantial, but the magnetite consumption was low. The “cost of saving money” was high due to the lost yield.
Screen Wear and Deslime Cut Size
Similar issues can be seen with screen wear on the deslime screens and basket wear in the centrifuges. When the screens are new they make a cut at the desired size and put a certain loading to the small and fine circuits. As the screens wear, the load in the small coal and fine coal circuits is increased and the potential for overloading these circuits increases.
As noted previously, overloaded small coal and fine coal circuits cost the plant in terms of increased fines product ash and decreased plant yield. As a result, the most effective time to replace a worn screen panel is likely well before it gets a gaping hole in the screen. The balance of screen life versus process efficiency should be considered. Implications must also be considered before deciding to increase the cut size of the deslime screen. Desliming might be more efficient and solve the magnetite consumption issue forever. However, how does the fine circuit handle the increased tonnage and coarser size material?
Fine Sieve Wear
A separate issue in operations savings is neglecting the effect of fine wire sieves and fine product desliming. Depending on the fine material, the fine wire sieves can lower the product ash of the fines/small coal by up to 10% ash. With high-clay coals, the spiral product can contain significant amounts of high-ash clays, 65% ash and higher. When new, the fine sieves can take a 17%-20% ash spiral product and reduce it to 9%-12% ash depending on the coal and ash distribution. As the sieve screen wears, the performance drops off considerably.
Regular testing of the fines circuit, through to the centrifuge product, is recommended to track this performance and quantify the effect. The difference between a 9% ash spiral product and a 17% ash spiral product can be the 65% ash fines that are carried along for the ride. Removing this high-ash material allows for an increased DMC cut point where some lower ash material can be added in greater amounts and at a lower product moisture. Obviously, sufficient capacity must be available not only for solid-solid separation but also for solid-liquid separation and following water treatment.
Desliming and Thickening Cyclones
Low gravity and low-ash material can be displaced to flotation, where it can be lost. Sometimes capital expenditure is reduced by simply increasing solids content to desliming cyclones, which reduces residence time in separation and water treatment requirements. This solution is almost always adopted during operation to reduce energy and operating costs.
Increasing solids content in cyclones feed above a certain point, which depends on the material processed, will exacerbate density separation. This causes even coarser material to be misplaced and yield losses to increase because the cyclone is now working as a density separator.
The goal in coal preparation is to maximize profitability. The means to maximize profitability is to find the balance between cost and revenue. Often, too much focus is put on lowering costs when doing so might reduce yield and profitability. Higher profits might come by a thorough review of the alternatives and making an informed decision as to which way might maximize profitability. Companies should not let the “cost of saving money” keep them from maximizing their profits.
T. Anthony Toney and Paolo Bozzato are with Norwest Corp. This article was adapted from a presentation Toney made at Coal Prep 2014, which took place during May in Lexington, Kentucky. The full paper is available at: www.coalprepshow.com.