Getting the Most from Spiral Separators

Sampling and maintenance improve product quality and extend equipment life

By Steve Fiscor, Editor-in-Chief

Spiral separators are used to recover the fine coal fraction (2-mm to minus 325 mesh). They are expected to recover fine coal over a wide particle size range without the use of chemicals or reagents.

Supported by Downer in Australia, Mineral Technologies designs and engineers cost-effective separation technologies for fine mineral applications. The company specializes in gravity, electromagnetic and electrostatic separation, and it’s quite well-known for its spiral separators, including the LD7, the LD7RC and the LC3.

Citing its compact size and weight, Mineral Technologies said its spiral banks have a low footprint, which ultimately impacts installation costs. The units require minimal maintenance, and they have a relatively long life if they are used properly.

“We are known for our spirals,” said Justin Reed, technical sales manager for North America for Mineral Technologies. “We also perform process engineering and develop flowsheets. We have electrostatic and magnetic separators, but coal operators are mostly interested in our spirals.”

The headquarters for the U.S. subsidiary of Mineral Technologies is in St. Augustine, Fla., and they have a test facility located at an industrial minerals mine, which operates nearby in Starke, Fla. The spirals are made in Australia. The demand for spirals has been so high lately, especially with industrial minerals and iron ore, that the company is currently adding more capacity at its South African facility.

Recovering Fine Coal With Spirals

Prep plant operators use spirals in heavy media circuits to treat the size fraction finer than 1 mm, but they can also be used to supplement coal washing jigs treating the fraction finer than 2 mm. They can be used to replace coarse flotation treating the fraction coarser than 0.1 mm. They have also been used to re-treat 2- x 0.1-mm material from slurry ponds, gob piles and refuse streams.

Describing what sets Mineral Technologies apart from the others, Reed said one aspect is the polyurethane (PU) material they use to manufacture the spirals. “It’s very high wearing and it works well, especially for more abrasive applications,” Reed said. “It’s the durability of the spiral troughs themselves. We spent a ton of money and a lot of time researching and we developed a PU with a proprietary blend. We reverse cast the spiral troughs in one piece to reduce any surface imperfections that would cause irregular currents and wear.”

The most popular coal spiral from Mineral Technologies would be the LD7RC. It combines both a rougher and cleaner (RC) processing stage into a single spiral. This model is particularly suited to duties more demanding than those of conventional coal spirals, especially where levels of high-gravity and near-gravity material in the feed require better separation. By recirculating the middlings, the separation performance of the 2-stage system can be further improved.

The LD7RC has three turns with an auxiliary reject splitter. A re-pulper redistributes the slurry back onto the spiral trough, Reed explained, and that serves as the cleaner function for the middlings and clean coal.

A typical setup would be triple start spirals operating at roughly 35 gallons per minute (gpm) or a nominal 2.5-3 tons per hour (tph) with a cut point specific gravity (s.g.) range of 1.8-1.9. Feed consistency is key, Reed explained, which includes the feed across the spiral bank and the feed over time. “In general, once those splitters are set, they remain where they are,” Reed said. “It’s not something that you would adjust on a regular basis. That’s why feed consistency is key. As the volume increases and decreases on the trough, it functionally adjusts the splitter setting automatically.”

Mineral Technologies has a new spiral, the LC3, the low cut point spiral. With its unique trough design, the LC3 targets a cut point in the 1.45- to 1.60-s.g. range. It has demonstrated effectiveness at reducing losses of clean coal to reject, Mineral Technologies said, while considerably enhancing processing capability.

That’s highly dependent upon tonnage, throughput, and volumetric flow, Reed explained. “Looking at the system volumetrically, it will be hydraulically overloaded at about 28 to 30 gpm, where it would physically start to throw slurry off of the spiral trough, whereas an LD7 or an LD7RC can handle up to 40 gpm,” Reed said. “From a tonnage standpoint, if the goal is to get to that low cut point of 1.5- to 1.55-s.g., the LC3 must run somewhere down around 1.5 tph — that’s short tons per hour — to truly get that. If the plant pushes capacity up, the percent solids will increase and the LC3’s cut point will inevitably end up closer to where a traditional coal spiral would run, the 1.8-s.g. range.”

At 35 gpm, the percent solids on an LD7RC would be 35% and 30%-35% solids with 25 gpm on an LC3.

The auxiliary splitter on a coal spiral is generally used to make a coarse refuse cut. On an LD7RC, the
auxiliary splitter makes the primary reject cut and it’s followed by the re-pulper box, Reed explained. “It’s similar to the start of another spiral on the same stick,” Reed said. “Essentially two spirals are stacked on top of each other on the same stick. It has a smaller footprint, but they are taller, and that can present a problem for some existing plants. For any greenfield application, we would highly recommend using them for a traditional 2-stage circuit.”

Another recommendation is for plant operators to wash the spirals down once a shift, or at least once a day. “The pyrite will bind with the clay and create a little dam on the trough if they are not washed regularly,” Reed said. “That dam will negate the effect of that auxiliary splitter. Instead of rejecting a stream as it normally would, the buildup prevents the pyrite and ash from finding the auxiliary splitter. Skipping the washdown hurts the quality downstream. It really negates the whole point of having a rougher cleaner two-stage spiral.”

When the plant has scheduled downtime, technicians should inspect the feed distributors that sit above the spiral bank. “They generally don’t wear very quickly,” Reed said. “Plant operators can expect 10,000 hours before they really need to replace any parts. Operators should check the orifices, the wear plates, and the diffusers in the feed pipe for the older Mark VII distributors.”

Again, the feed distribution across the bank is key. Reed said he once saw a spiral distributor with a dead pigeon in it. “Operators should check for blocked starts,” Reed said. “If the plant is running at a lower capacity, starts can be blocked intentionally to keep the tonnage on each trough the same.”

Sampling will indicate if the spirals are not operating properly. Just like anywhere else in a heavy-media circuit, sampling around the spiral circuit is important, Reed explained. Plant technicians should check the capacity (tph per start), the feed volume, the feed’s percent solids and perform a sink float test for the clean coal products. 

Oversized materials can generate problems for spirals. “Plant managers really want to protect the spirals from oversize because, again, that leads to a buildup in the trough,” Reed said. “Normally, the underflow from the classifying cyclones reports to a protect sieve that screens out anything that might have made it through the raw coal classifying screens.”

Everything in mining is site-specific, however, under normal operating conditions the average lifespan for a set of spirals should be eight to 12 years, Reed explained. “We have seen them last for 15 years in a well-maintained coal application,” Reed said. “The lifespan is really a function of the plant’s capacity or how many tons per hour pass through the spiral and the maintenance program.”

Looking toward the future, Reed said the R&D department at Mineral Technologies, working with Australian Coal Association Research Program (ACARP), is looking at a much smaller, self-contained spiral. The XX International Coal Preparation Conference (ICPC) will be held near Mineral Technologies’ head office, manufacturing and testwork facilities at the Gold Coast Exhibition & Convention Center, in Queensland, Australia from October 16-19, 2023.

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12-turn Spiral Eliminates Need for Multiple Units

Process equipment specialist Multotec has introduced its first 12-turn spiral concentrator. According to Christina Ramotsabi, general manager-process engineering at Multotec, the HM12 spiral was designed and tested in-house, evolving from the company’s popular NHM heavy minerals spiral. The first order has been manufactured and shipped to an international customer.

“The customer required higher recoveries from their mineral sands plant, while also achieving optimal grades,” said Ramotsabi. “Our HM12 spiral concentrator will deliver these increased recoveries while still maintaining a limited footprint in the plant.”

Conventional spirals generally feature seven or eight turns, she noted. Two stages of these spirals would be required to achieve similar recoveries with one 12-turn spiral. The use of the 12-turn setup reduces footprint requirements as well as the use of additional equipment such as pumps, distributors, and piping.

The new model, said Multotec, was developed through collaboration between its R&D, process, and engineering teams. Kgabo Mashita, applications engineer at Multotec, said the initial phase of testing used a conventional spiral to generate benchmark results. In the next phases, the feed box was re-designed and evaluated, and re-pulpers were introduced.

“The length of the spiral made it necessary for us to introduce more energy to the slurry mix at certain stages, to renew the separation process,” said Mashita. “We placed re-pulpers strategically on each spiral, to receive and re-energize the slurry, essentially creating a multi-stage separation process.”

He emphasized the critical role of the feed box in optimizing the functioning of the spiral. The improved presentation of the slurry feed allows even and consistent distribution as it enters the spiral. This ensures that more residence time is spent on separation.

The first units were ordered through Multotec International’s Eurasian division and have been shipped to the customer. A technical team from South Africa will assist in the commissioning and optimization of the equipment.