Thursday 18th Aug, 2022

When screen classifying cutters outperform hammer mills and knife cutters

Steve Knauth from Munson Machinery explains how screen classifying cutters work and where they work best in a bulk handling operation.

Steve Knauth from Munson Machinery explains how screen classifying cutters work and where they work best in a bulk handling operation.

Screen classifying cutters combine the shear of a knife cutter with the impact of a hammer mill at high speeds, reducing friable, semi-friable, fibrous, semi-hard and hard materials into controlled particle sizes at high rates with minimal fines.

Like knife cutters, screen classifying cutters shear material against bed knives but can handle a greater diversity of materials and retain sharpness far longer. At medium to high speeds, they impact material similar to a hammer mill, but have greater flexibility from variable rotations per minute (RPM) for greater control of particles sizes. 

As a result, screen classifying cutters are often more effective at reducing friable, hard and fibrous materials into uniform particle sizes from 15 centimetres down to 20 mesh (850 micron) or less, with fewer fines and/or less heat generation.

A horizontal rotor – comprised of tightly packed, staggered parallelograms or ‘stars’ – generates greater force per inch with each cut than a conventional knife-type cutter of equivalent horsepower. This is due to cutter tips on each star that shear materials against twin stationary bed knives at lower speeds, with higher impact at higher RPM.

As material enters the infeed chute, it is subjected to successive mechanical shearing against the bed knives until small enough to pass through the screen apertures, the size of which determines residence time. The result is uniform, coarse grinding down to 20 mesh (850 micron) with minimal fines or heat generation, and reduced energy use.

Typical applications include reducing bulk foods, tobacco, plastics, batteries, chemicals, minerals, fiberglass insulation, ceramic honeycomb filter media, leather, glass bottles, trim stock, wood products and a broad range of scrap.

The rotor can range in length from 25 to 183 centimetres, and its geometry can be adapted to optimise the application. 

It is available in food-grade, industrial and abrasion-resistant finishes, and in a range of sizes from mini laboratory units to ultra-large units 1.8 metres in length. Screen classifying cutters can mince, crush or crumble materials that are soft, moist, sticky, medium-hard, fibrous, or friable offering great flexibility in size reduction.

There are a number of different examples and uses for screen classifying cutters across a diverse range of materials.

Increased yield and uniform grinding for date processor

Jewel Date Company of California, US, switched from using a hammer mill to using a screen classifying cutter to reduce dates into granules and powders.

The company grows, processes and packages approximately 3.6 million kilograms of dates and date products a year. In addition to whole dates, a portion of the crop is processed into date granules and date powder.

A hammer mill previously processed 907 kilograms of dates in an eight-hour shift. The screen classifying cutter processes the same amount of product in just one hour with more uniform results.

Dates have a high moisture content and range in size from 3.2 centimetres to nearly 7.6 centimetres long. To process them, the dates are air-dried for two days to reduce the moisture content from 50 to 7 per cent.  According to the company, anything greater than 7 per cent moisture would cause the dates to clump during processing.

Unlike a hammer mill which uses a series of hammers strike and break the material apart, the screen classifying cutter repeatedly shears it into uniform-sized particles.

With the variable speed motor, the plant can run the cutter at 1500 RPM to produce particles down to 1.6 millimetres, or at 1200 RPM for particles down to 4.8 millimetres to achieve the output of powders or granules required by its customers.

“The screen classifying cutter has not required parts or maintenance in its first four years of operation, and it has fewer moving parts than our Hammer Mill,” Jewel Date Company Sales Manager John Ortiz says.

“Stainless construction with a food-grade finish allows us to clean it quickly.”

Producer of calcium chloride satisfies high demand

Cal-Chlor in Louisiana, US, operates five 76-centimetre screen classifying cutters to meet demand for calcium chloride (CaCl2) powder used by the petroleum industry.

The plant reduces CaCl2 pellets into a powder of uniformly sized particles that is used in drilling shale formations, flushing mud from oilfield holes, and filling casings when drilling ends.

“Each screen classifying cutter processes up to 11,793 kilograms of product per hour,” says Brett Davis, Cal-Chlor Operations Director.

The cutters are so important to meeting oilfield demand that Cal-Chlor runs four of them and keeps the fifth for emergency use if one goes offline. The plant’s daily CaCl2 powder production ranges from 181 to 363 tonnes.

According to Davis, the cutters are “near bulletproof” when it comes to processing CaCl2, which is abrasive, generates heat when collected in large volumes, attracts moisture, and is extremely difficult to handle. He notes that as little as 84 to 112 grams of CaCl2 in a cup with water will become too hot to hold in minutes. 

The cutters are made of stainless steel, which resists abrasion, corrosion and other problems that CaCl2 presents. Cal-Chlor operates the machines at between 1200 to 1800 RPM to achieve the desired particle size.

Mini paper mill increases efficiency

The University of Maine Process Development Centre (PDC) provides the paper industry with collaborative pulp and paper research. The process lab — originally part of the university’s forest and paper industry program — houses a virtual ‘mini’ paper mill with a broad range of processing equipment.

The PDC is equipped to work with a range of raw materials including wood, bark, herbaceous crops, and agricultural residuals. Available processes include extraction, pulping, bleaching, papermaking, coating, and finishing. To facilitate its process work, the lab requires efficient size reduction capabilities to downsize materials into uniform particles.

At the outset, the lab used a hammer mill, which proved to be inefficient and failed to produce consistently sized particles due to its crushing and pulverising action. The PDC replaced it with a screen classifying cutter.

“We specified that unit because it efficiently reduces materials into uniform particle size with a lower energy requirement than the hammer mill,” says Mark Paradis, the PDC’s group leader of engineering. He added that it requires minimal maintenance.

The Screen classifying cutter played a key role in one initiative to reduce perennial grasses and hay into 3.2 mm particles which were then compressed into biofuel pellets.

In another project, a University of Maine chemical engineering professor developed a biodegradable golf ball partially composed of lobster shells. Here, the screen classifying cutter reduced the shells to powder form for downstream processing.

High throughput with difficult-to-cut cinnamon

ForesTrade produces and distributes tropical, organically grown spices, coffee and essential oils. In 2004, the company built a state-of-the-art cinnamon processing plant in Indonesia. 

Mary Porter, Manager of Indonesian operations, says the company equipped the plant with a 38-centimetre screen classifying cutter, based on the recommendation of another local
spice producer.

Harvested cinnamon arrives at the plant in bags as rolled sticks, known as quills, that range in length from 7.5 centimetres to one metre. The bags are then manually emptied onto a belt conveyor, where workers remove stones and other extraneous material, and arrange the quills inline for the
cutting operation.

The quills are fed pneumatically into a high-speed model 38-centimetre screen classifying cutter. The blades rotate at 2200 RPM and continuously shear the quills against the bed knives, cutting them into five-centimetre lengths. Despite the high rotation speed, there is little to no heat generation and minimum fines.

The cutter contributes to quality by producing uniform pieces and a cleaner cut. Porter says that having a clean, uniform cut is not only important for meeting size specifications but makes for consistency in drying.

Ordnance fabricator saves money, recycles tungsten heavy alloys

Aerojet Ordnance of Tennessee, United States, fabricates parts from tungsten heavy alloys and other refractory metals for defence-related products like ammunition, warheads and electronic shielding.

Tungsten heavy alloys are one of
the densest metals produced. Reclaiming fabrication scrap was difficult and costly, prompting Aerojet to sell the scrap at significantly lower prices than it was worth as a recycled and reusable material.

Looking for a better solution, Aerojet experimented with techniques to reclaim tungsten heavy alloys scrap, including using a hammer mill and manually downsizing pieces. None of the methods was efficient or produced consistently-sized particles.

Tim Brent, Aerojet Ordnance Project Engineer, says a hammer mill’s crushing, pulverising action was not effective with material of this density. With manual reclaim, “labour costs were prohibitive and the results uneven.”

“If we can reuse the material, it is worth two- to three-times more to us than selling it as scrap,” Brent says. “But we couldn’t reuse the material without an effective means of size reduction.”

After testing several size-reduction machines, comparing particle sizes, consistency and economy, Aerojet decided on a screen classifying cutter. Aerojet’s engineers were aware that the machine had a successful record in similar applications.

“The design is simple and stands up to the tungsten heavy alloys,” Brent says.

Aerojet specified an abrasion-resistant interior and a special stand to accommodate containers used in moving scrap to the process machines.

“Downsizing different grades of tungsten heavy alloys raises the risk of batch contamination if the interior isn’t properly cleaned,” he says. “The cutter is easy to clean, compared to other machines.”

When to use a screen classifying cutter

Hammer mills and knife cutters are often default choices for size reduction due to their long-term use, familiar names and broad capabilities. However, the range of applications they can satisfy efficiently is narrow.

Many hammer mill users sacrifice particle size control and excessive fines, while many knife cutter users are resigned to frequent shutdowns for re-sharpening of blades that quickly lose their edge.

In these and other situations, plant engineers would do well to test their material on a screen classifying cutter side-by-side with a hammer mill or knife cutter in manufacturers’ test labs. Only in this way can they quantify the difference in efficiency, output and product quality afforded by each machine, and reap performance benefits over the long service life of the ultimate purchase.