Toroidal-shaped Flash Dryer Deagglomerates and Dries Filter Cake

By: Stephen H. Donaldson, P.Eng., Engineering Manager, Steetley Talc Inc. and Alan E. Hodel, Associate Editor, Chemical Processing

Toroidal-shaped flash drying system with jet-milling system in the background

Problem

A need to increase production capacity prompted Steetley Talc Inc., Timmins, Ontario, to look for ways to improve their drying operation. Prior to 1984, the company was shipping wet talc cake from their mine and concentrator plant over to a jet-milling plant about 50 miles away.

There the fine (80-90%, -325 mesh) material was taken from about 20% moisture to bone dryness before being passed through the jet-milling circuit.casel_01

The dryer was a toroidal-shaped flash dryer that had been installed at the plant in 1975. The unit was in continuous service, producing 5 1/2 t/hr of talc using an inlet air temperature of 1100°F There had been no problems with the dryer and it had operated virtually maintenance free. However, transportation of the wet talc cake between plant sites was time consuming and labor intensive.

Steetley Talc Inc. mines a talc-magnesite ore body and refines it to pure talc powder. The process involves taking the ore from an open-pit mine and upgrading it in a flotation-type concentrator. The purified product is recovered by filtration, dried to a bone-dry powder, and ground in a jet mill powered by super- heated steam. The average particle size of the ultrafine talc produced is 1.5 microns.casel_02

New toroidal-shaped flash dryer increased capacity by 20% due to improved thermal efficiency

Dry talc is a slippery, soft material (Mohs scale hardness of 1) which has two surfaces in its structure. One is hydrphobic and organophilic, the other is hydrophilic and organophobic. Although talc is not heat sensitive, wet talc cake retains moisture well and the clumped particles stick together if steps are not taken to prevent agglomeration during drying.

A new dryer would be needed at the mine. The only fuel available there is propane gas. In view of rising energy costs, Steetley Talc initiated a study to locate the most economical dryer for the job.

Solution

After many months of testing and evalua- tion of various dryers, it was determined that the most energy efficient dryer was a closed-loop, toroidal-shaped flash dryer. The closed-loop dryer was twice as efficient as a straight-through dryer and the unit both dried and completely deagglomerated the product. A certain amount of size reduction was also achieved due to the high circulating velocity in the small, loop body. The dryer’s small size would require less plant area and height when compared to other units.

Based on the above facts, Steetley Talc Inc. installed a new toroidal-shaped flash Levitra dryer at the mine and concentrator plant in 1984. The system is an upgraded version from the same manufacturer as the one that had been in use at the jet-milling plant. It is composed of a turbo air blower, a propane fired air heater, a toroidal-shaped drying chamber, a vol- umetric feeder, an exhaust fan, a bag collector and a heat recovery system.

Unfiltered air for the push-pull dryer system is drawn in from outside the building. A 75 hp blower boosts the air pressure to 2.5 psig. Downstream of the baghouse, a 75 hp exhaust blower draws the dried powder into the dust collector. The system is maintained under a nega- tive pressure, and moist air is exhausted into the condenser.

The drying air is heated to about 1100°F, passes into the dryer manifold and through high velocity nozzles. Wet talc is fed from the screw feeder into the high-velocity gas (300-400 ft/sec) that circulates in the torus. The talc is instantly dried and deagglomerated. The mixture of dry, fine talc and gases pro- ceed to the classifier area of the torus and are drawn off through a reverse outlet on the inside curve of the classifier. The high centrifugal force in the classifier causes any coarse or undried particles to return to the drying area for another pass.

Airflow is set manually by adjusting a blast gate at the discharge of the inlet blower. Fine control over drying is achieved by setting the drying chamber discharge air temperature (at 215°F) to automatically control the firing rate of the air heater. The feed rate of the wet talc cake is manually controlled by adjusting the variable speed drive on a screw feeder.

Results

Installation of the new toroidal-shaped flash dryer system at the mine and concentrator plant increased production capacity by about 20% due to better thermal efficiency. When the old dryer system from the jet-milling plant was moved to the mine and concentrator plant and its controls upgraded from early 1970′s standards to current standards, capacity increased to that of the new dryer. The overall effect of the changes has more than doubled talc production capacity.

Dryer system performance has been excellent. The toroidal-shaped flash dryer effectively deagglomerates the wet cake and the talc leaves the dryer as a bone-dry powder. The donut-shaped design with an inside curve withdrawal point effectively prevents wet material from being discharged.

One operator runs the plant. The up- graded system uses less operator attention than previously and the operation is more stable with smaller temperature fluctuations. Safety features include a high discharge temperature shutdown.

The system’s thermal efficiency is enhanced by using a specially designed heat recovery system.

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