When One Screener is Better Than Two

Posted on December 15th, 2016 by vapro

Adding production capacity to an already crowded processing line can feel like asking for your cake and eating it too. Most often, increasing production means building a plant addition. But one potash producer was able to  increase production by finding a more efficient screener.

The term potash describes common salts containing potassium. The term is applied to any mineral used for its potassium content, including potassium chloride, potassium sulfate, and potassium carbonate. Potash is used around the world as an agricultural fertilizer, playing a critical role in achieving healthy yields of crops such as corn, soybeans, coffee, rice, and potatoes. More than 95 percent of the potash produced in the world is used as fertilizer.

Potash deposits occur as beds of solid salts beneath the ground or as brines in dying lakes and seas. Potash mining methods include conventional underground mines, continuous mining methods that use special machines to remove ore from veins without blasting, surface mining from natural surface brines such as in the Great Salt Lake or the Dead Sea, and solution mining, which uses a hot brine solution to dissolve potassium deposits below the ground.

IMC Kalium is among the world’s leading producers of potash. A division of IMC Global, the company operates four mines in Saskatchewan, Canada, and three in the US. Canadian potash deposits are estimated to be among the largest in the world, containing about 5 billion tons in a band up to 50 miles wide that stretches 450 miles across Saskatchewan.

At its plant at Belle Plaine, Saskatchewan, IMC Kalium mines and processes potash for use in industrial and agricultural applications. The plant produces nearly 2.5 million tons of potash each year, primarily for  agricultural applications. Dave Dusterbeck, manager of dry process design, oversees capital and maintenance work for dry process plant operations.

The Belle Plaine plant uses patented solution mining methods. At the mine, potash is taken from naturally occurring ore deposits located more than 5,000 feet below the surface. To capture the ore, a hot undersaturated brine solution is pumped through drilled holes to the ore to dissolve the potassium. The concentrated potash brine is then returned to the surface and taken to the processing plant, which uses an evaporation-crystallization process. In the evaporation stage, moisture is removed and the brine is brought to its saturation point. The crystallization stage then crystallizes out the solid materials, a mix of potassium chloride and sodium chloride salts that must be separated.

“What allows the two materials to be separated is that they have different solubility curves as a function of temperature,” Dusterbeck said, “SO we can crystallize out one and keep one in solution.”

The sodium chloride is kept in the solution and pumped back underground. The potassium chloride is used to make potash. Evaporation of the liquid from the potassium chloride occurs in two “quadruple-effect evaporators.” The evaporators are heated with steam generated from an on-site power plant to remove the moisture.

The material then goes through vacuum crystallizers, which produce solids that are then centrifuged in screen-bowl centrifuges. Once the potassium chloride is in solid form, the plant takes advantage of gravity to process it into usable industrial and agricultural grades of potash.

“Our plant is mostly designed for gravity flow,” Dusterbeck said. “We’ve arranged and designed the plant so there’s a minimum number of conveyors. So our bucket elevators tend to be quite high and we use gravity for distribution and collection.”

From the centrifuges,the potash is fed by gravity through one of two rotary dryers or one of two fluid-bed dryers, depending on the amount of moisture and particle size of the material being produced. The dried potassium chloride, or potash, has a bulk density of 70 lb/ft3.At this point, most of the moisture has been removed. Then it is fed by gravity to the screeners to produce various cuts or fractions of potash.

“We actually produce six different grades,” Dusterbeck said. “Fine, standard, and coarse are all screened out of the dryers. And then fine and standard sizes that don’t fit size specifications report to our compaction plant.” In the compacting plant, crushing, screening, and compacting equipment is used to increase the particle sizes. Potash produced at the plant is primarily used for agricultural applications. A fine grade of potash -approximately -48/+100 mesh – is used mainly for liquid fertilizer applications. Standard grades of potash for both industrial and agricultural applications are approximately -20/+65-mesh. Coarse grades of -6/+14 mesh and granular of -5/+12 mesh are used in the blending grades of fertilizers.

For sizing the potash, coarse screening is handled by 11 inclined vibratory screeners and fine screening by 24 gyratory screeners. “The screeners right off the dryer run in an open-circuit screening system,” Dusterbeck said, “and those in our compaction circuits operate in closed-circuit systems, where the oversized material is crushed and reports back to the screener in the same circuit.

‘The most typical and demanding application in our plant is using the screen in the closed-circuit compaction  system, where the undersize reports back to the compactor, the oversize reports to a crusher and then is circulated on the screener, and the midsize product reports to product dispatch. The product dispatch goes directly to a warehouse.”

Processing operations at the potash plant continue around the clock, with little time for error. Dusterbeck said the potash plant is designed to produce the maximum amount of product possible, and production depends primarily on the screeners.

“Our plant actually runs binless, if you will,” he said. “It’s a continuous process. Our dryers operate, product is screened, compaction systems are fed, and product is dispatched without bins. Our plant really runs with zero surplus screening area or machinery. All of our equipment runs 365 days a year, 24 hours a day. There are no planned outages, and reliability is absolutely crucial. Our equipment is typically loaded at full capacity.”

Needing More Screening

In 1990, demand for potash began to increase. Demand for industrial grades, such as those used to make such products as caustic potassium hydroxide, was particularly high. Plant managers saw a need to expand the plant’s production. Meeting demand would require increasing the plant’s screening capacity.

“This plant was originally designed for making agricultural-grade products only,” Dusterbeck said. “And the advent of the industrial-grade products is somewhat recent. The need for additional screening capacity was driven by two issues: to produce new products and to expand our existing capacity.”

In the past, both the gyratory and inclined vertical screeners made the required cuts of potash. But the machines weren’t without problems -one of the most prevalent being capacity.

“We have increasing production demands and shipping rates,” Dusterbeck said. “And in all cases, [when we increase the screener loads] carryover of fines from the product into the undersize is our problem. Total aggregate screening capacity in a plant is typically given by the amount [of screening] required to clean fines out of a product. So what it really boils down to is staying on our size specification at higher and higher production rates.

“We have some applications where we develop oversize, midsize 1, midsize 2, and undersize. The [inclined vibratory and gyratory screeners] can do it, but they do it with significant compromise in screening capacity.”

The plant operates continuously,with temperatures inside processing equipment reaching between 300” F and 400” F. Under such conditions, the screeners -especially the gyratory units -were susceptible to what Dusterbeck called “consumable parts issues”: Small parts made from products such as rubber or canvas were unable stand up to the rigorous conditions. Changing failed parts required taking equipment offline, resulting in lost production.

“The plant has always run at higher temperatures, and thermal effects have always been a burden in the service cycle of the machines,” Dusterbeck said. “Since we operate 365 days a year, 24 hours a day, we don’t like to go down and change out agitating balls or screen cloths.”

The existing screeners also experienced potash flow problems. Duster-beck said the potash would tend to “cake out” or “candy off.” The potash is hot during the process. If the potash cools against vessel walls or sits idle, it can become cohesive and stop flowing (“cake out”). Product can also build up inside chutes and fall off in chunks (“candy off ’), Dusterbeck said.

Finding a Screener That Does It All

When plant managers looked for new equipment to increase plant screening capacity, they wanted to eliminate many of the problems they were experiencing with existing screeners. They also wanted a versatile screener capable of producing many cuts of potash.

IMC Kalium’s plant design philosophy is to operate with as few pieces of equipment as possible, Dusterbeck said. The plant is designed so equipment can be bypassed when it fails, keeping the rest of the plant online. With this criteria, operating with many low-capacity screeners, connected with feeding and collecting conveyors, was unacceptable. Also, a screener that handled greater capacity but wasn’t able to produce potash ranging from -8 to +10 mesh wouldn’t meet company standards. The company wanted a screener that would produce a variety of potash sizes at higher production rates – achieving a balance between cut and capacity.

“We elevate most of our crushing and screening circuits, and we distribute and collect material by gravity,” Dusterbeck said. “If I have screening equipment that is of lower capacity per machine, I need more machines spread out over the floor -which means I need a conveyor to distribute to them and collect from them. Our plant objective in design work is to eliminate all conveyors because they increase the cost, and typically are one of the most unreliable elements of dry process handling systems. Gravity is very reliable. So we need machinery with high unit capacity per machine and a minimum number of machines to make gravity chute layouts work.”

IMC Kalium’s gyratory screeners worked well when producing fine grades of potash, but they weren’t able to handle higher capacity. The inclined vertical screeners produced coarse grades of potash at higher capacities. Regardless of capacity, both screeners had limited capabilities.

“Specifically, the inclined vibratory and gyratory screeners will not make two, three, or four different cuts,” Dusterbeck said.

IMC Kalium also wanted a screener that could accept large football-sized lumps along with very fine potash. The screener they searched for also had to use interchangeable spare parts that could stand up to the plant’s high production temperatures.

Initially, plant officials surveyed manufacturers’ brochures and other promotional materials, examining nearly every type of screener. What they found closely matched what they were already using, with only slight variations. While many of the gyratory screeners could produce the required grades of potash, Dusterbeck said IMC Kalium quickly wrote them off because of their low capacity.

One screener, however, operated noticeably different than the rest. Dusterbeck said the shaker screener had lower inclined-angle screening surfaces like a gyratory screener, yet it used a rectangular screening surface, “straight-line linear motion.” It caught the attention of plant officials.

“[This screener] really embodies the best of all the different designs,” he said. “It has a horizontal screen [surface], which enables extremely high screening efficiencies as demonstrated in the sifter machines such as the [gyratory screeners].Specifically, the whole box shakes, so the pans [that collect oversized material] don’t plug.”

To determine if the screener would meet IMC Kalium’s needs, the potash producer decided to conduct tests. The  company sent potash samples to the screener manufacturer and also to a gyratory screener manufacturer. The tests simulated production screener loads on a scaled-down version, IMC Kalium also compiled their own in-plant data for screening efficiency.

“Efficiency is a real red herring in screening,” Dusterbeck said. “You have to figure out what efficiency means. Usually it’s a ratio of something. And efficiency means different things to different people. If you’re trying to remove a little bit of fines from a bunch of product, it’s a different efficiency number than if you’re trying to get most of the material through the screen and remove a little bit of the coarse. And you can calculate product recovery ratios, but they’re not directly related to screening efficiency if the screen cloth size and other actors aren’t the same in all tests.”

To determine a more accurate efficiency rating for each screener, Dusterbeck used a technique called the method  fractional efficiencies. “Not many people in the screening business, even the screener manufacturers, understand how this works,” he said. The technique has been commonly used to test cyclone efficiency calculations.

“The way we analyze efficiencies we plot probability of passing each incremental size fraction near the cut point of the screener versus the log size of the incremental size fraction, what’s called the fractional efficiency,” he said. “We look at each individual size fraction in between sieves both for what went over the screen and what went through the screen. We plot how much passed over and what was available for each size fraction. We join the data points and end up with the slope of an efficiency line. The flatter the slope of the line, the sharper the machine cuts.”

Dusterbeck said this method measures screening efficiency as a ratio of product recovered compared to product available to be recovered. It doesn’t consider whether oversize or undersize is a desirable product or the particle size distribution for specific size specifications.

During the tests, the unique shaker screener  performed with a higher fractional efficiency than the inclined vibratory and gyratory screeners already in operation in the plant as well series and parallel arrangements that as those in the gyratory screener test permits up to 10 decks to be grouped center. The shaker screener was able in parallel pairs. The shaker screener was able to produce the correct cuts of potash and to separate cuts as diverse as 5 mesh, 80 mesh, and 48 mesh. It also IMC Kalium installed a shaker operated at up to twice the capacity of screener with the series-parallel arrangement.The potash is distributed comparable screeners.

Based on the test results, IMC Kalium gather in common discharge spouts decided to install a Texas Shaker made by Triple/S Dyn​amics.

The Shakes

In 1992, IMC Kalium installed the shaker screener in a circuit producing potash for industrial uses. The screener augmented the capacity of existing screeners, allowing the plant to meet increased demand.

The shaker screener screens a variety of agricultural and industrial products. It can be mounted on the floor or suspended from steel cables. At the Belle Plaine plant, the screener was hung from cables. Plant personnel  installed walkways near the shaker screener for operators to use during maintenance.

The shaker screener comprises several screening surfaces called screen decks. Each deck consists of screen cloth stretched across a frame that can be changed according to the size of the material being screened. The shaker screener’s decks are available in three arrangements:parallel, series, or series-parallel.

In the parallel flow arrangement, incoming material is evenly divided between two decks of the screener to produce two cuts or fractions. The series deck arrangement allows up to four decks with different screen apertures, graduating from the coarsest on top to the finest on the bottom. The oversize from each screen and the fines from the bottom screen are delivered to individual discharge spouts. The series flow can produce from two to five fractions from the same pass of material. The series-parallel flow arrangement is a combination of the series and parallel arrangements that permits up to 10 decks to be grouped in parallel pairs.

IMC Kalium installed a shaker screener with the series-parallel arrangement. The potash is distributed equally to the top screen of each pair of screen decks, and the fractions gather in common discharge spouts corresponding to each screened fraction. This allows one screener to complete multiple separation applications.

At IMC Kalium, the shaker screener was configured to achieve four streams of material coming out of two groups of parallel-fed decks. “And it’s all inside the same machine,” Dusterbeck said, “SO it’s like two screeners in one.”

In operation, potash from the dryers falls through chutes into the screener’s infeed. A proportioning feed splitter directs the potash to a corresponding screen deck. The screener’s inertia drive generates a long-stroke, straight-line oscillating motion that conveys oversize potash off the screen decks. Potash is either directed through the screen or to the appropriate overs chute.

“The straight-line motion lets this screener convey and handle very high bed-depth loads,” Dusterbeck said. At times, operators have tested the limits of the machine by overloading it. Unlike other screeners the company has used, the shaker screener didn’t show significant changes in the sieve results with increasing loads. Operators used to seeing changes during overloads weren’t aware that the shaker screeners were overloaded.

“Screening efficiency isn’t a function of load until you start grossly overloading the machine and backing it up,” Dusterbeck said. “These screeners won’t tolerate backing up chutes on the discharge side. So you have to make sure that if the downstream equipment trips that there’s another overflow place for the material to go or you cut feed to the screener. You have a large vibrating box and if you fill it full of product and back it up, it starts beating itself to death.”

Once the screener was installed,operators made several adjustments to meet the plant’s needs. Some changes included increasing the spaces between screen decks, increasing the oscillating speed, and changing the bearing lubricants from oil to grease in order to eliminate frequent maintenance.

“Basically it was just minor tuning,” Dusterbeck said of the changes. “And the screener’s been performing reliably ever since. Some of the changes have become design standards for [the manufacturer].”

Efficient Screening

With the new shaker screener, the company increased plant capacity and efficiency without sacrificing plant space. Since the screener was installed in 1992, the plant has added nine more units at the Belle Plaine plant site and 17 more at other IMC Kalium plant sites. The increased screening capacity has allowed the plant to meet market demand for more potash, both for agricultural and industrial uses.

The shaker screener has also improved plant screening performance in general. Dusterbeck said the screener has curbed the loss of precious 10-mesh particles to under size. The shaker screener also operates at up to twice the capacity of the plant’s inclined vibratory screeners and up to three times the capacity of the gyratory screeners, while occupying the equivalent floor space of one of the other screeners.

The multi-deck screener has allowed the plant to operate more efficiently as well. By using a single machine to make many potash fractions, the plant hasn’t had to invest in extra screening equipment, giving the plant more bang for its capital investment buck.

“The same basic screen design does a superior job of handling fines separation,” Dusterbeck said. “We use the same machine for removing football sized lumps out of a product stream and making a quarter-inch or one-inch separation.

” The screener has also been able to stand up to high temperatures much better than other screeners in the plant. As a result, Dusterbeck said, the plant has been able to “significantly reduce our maintenance costs.” He said some of the screeners have operated an entire year before they required screen cloth changes.

“Screen life is almost indefinite,” he said. “[On the shaker screener] screen cloths aren’t tensioned like a violin string, as they are in most screeners. They’re just basically riveted down to the deck, and that’s one of the reasons the screen cloth lives an extremely long time. There are no rubber balls or plastic, canvas, or silicone parts in this screen to wear out. All of [the shaker screeners] use exactly the same spare parts and the same maintenance procedures and exhibit the same sort of high efficiency numbers.”

Dusterbeck said the greatest savings have been achieved with the screener’s versatility. The plant has been able to create new products and increase production simply by adding shaker screeners. “The functionality of one [shaker screener]is equal to two of our other screeners,” he said. “Fewer [shaker screeners] produce more tons of potash.”

And by adding one machine where two might have been needed in the past, Dusterbeck said, the plant has increased capacity without adding conveyors, structural steel, instrumentation, or maintenance procedures for extra equipment normally required for plant expansions.

“The design flexibility that these machines afford allowed us to significantly reduce our capital investments in expanding our capacity,” Dusterbeck said. “We’ve been able to add significant screening capacity with existing circuits rather than designing and building an entirely new plant.”

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