By Steve Fiscor, Editor-in-Chief

One of the more exciting pieces of equipment on display at the 2011 Coal Prep trade show was the StackCell from the Eriez Flotation Group. Readers might recall the technology being discussed in the article covering Mechel-Bluestone’s new K2 prep plant (Coal Age, April 2011, p. 32). Eriez displayed the unit for the first time at Coal Prep, giving delegates a chance to kick the tires or in this case kick the tailings outlet.

In addition to having the StackCell at the exposition, the Coal Prep 2011 Technical Program included a presentation on the system, “Industrial Evaluation of the StackCell Flotation Technology.” The authors offered data collected from recent field trials. They also recommended some guidelines for installation, operation and maintenance.

The technology evolved from column flotation. Based on
its experience designing and engineering coal flotation circuits, Eriez has developed the new flotation device, which offers high capacity in a small footprint. Unlike column flotation, the
tank is shorter and smaller, not only reducing the footprint, but the load as well. The system is designed to operate at low pressure so the total energy consumption is lower per ton of fines processed. A low-pressure blower delivers the flotation
air, resulting in lower operating and maintenance costs, according to Eriez, when compared with systems driven by compressed air.

This new technology, according to Eriez, is ideal for use in a wide variety of applications, including those involving fast floating coals or overloaded conventional cell circuits.

StackCell Technology
Designed with a small footprint and for gravity-driven fed applications, the cells can be easily “stacked” in series or placed ahead of existing conventional or column flotation cells. During operation, feed slurry enters the separator through either a side- or bottom-fed nozzle at which point low pressure air is added. The slurry flows into an internal pre-aeration sparging device that mixes the slurry with the bubbles inducing high intensity bubble-to-particle interaction before arriving at the the primary tank. A liquid slurry level is maintained inside the tank to provide a deep froth that can be washed to produce a high-grade float product. Froth is then carried into the froth launder via mass action.

Technically, the unit uses feed pre-aeration and focused turbulence to increase the flotation rate of hydrophobic particles within the recovery system. The bubble-to-particle interaction takes place in an enclosed aeration chamber. The sparging energy, according Eriez, is concentrated and used solely for bubble generation.

The tank is constructed in a similar fashion to column flotation cells. It is used primarily for the phase separation between the pulp and froth. According to Eriez, a deep froth can be maintained and washed, which generates a high-grade float product. In the case of coal preparation plants, the high-grade float would be fine coal.

According Eriez, the new device will provide metallurgical results superior to conventional mechanical cells, while offering benefits similar to that of column flotation cells. “Eriez developed this new flotation technology based on flotation fundamentals with an emphasis on carrying capacity restrictions, circuit design for cell mixing, lower retention times and more efficient sparger design,” said Jaisen Kohmuench, manager-R&D Process Group, Eriez.  “The efficiency of this flotation device surpasses that which can be achieved by other individual techniques or separators.”

The system offers distinct advantages over column flotation and conventional cells, including:

  • Improved sparging incorporating “preaeration” techniques;
  • Lower required residence time;
  • Significant drop in cell size (and both static and live loads);
  • Significant drop in horsepower;
  • Uses “wash water” and deep froths to provide column-like metallurgy; and
  • Can be easily retrofitted into existing plants for de-bottlenecking.


To lend perspective, Kohmuench cited an existing installation that employs a three-stage unit to treat 45 tph. Using column flotation technology, this would have required two 14-ft diameter by 28-ft tall cells with two compressors with a power requirement of at least 410 hp. The new three-stage device treats the same material in less than half the volume, one-third the footprint and one-third the installed power.

Challenges Associated with Column Flotation
One of the challenges associated with column flotation results from the aspect ratio of the column itself. A column cell must be tall to achieve the desired residence time and minimize internal mixing conditions that are detrimental to cell performance. The design minimizes plant floor space