Rotary Copper-Concentrate Drying Plant Energy Efficiency

Rotary Copper-Concentrate Drying Plant

Processes to dry concentrate have developed in recent decades. Starting from direct-heated dryers such as rotary-drum dryers and flash dryers, drying technology has evolved into indirect-heated steam dryers. Direct-heated dryers employ hot gas from combustion or other processes to heat up and dry the material by direct contact. The combustion gas is usually produced in a separate chamber by burning fossil fuels. A dryer of this type has certain space requirements, and it produces emissions that may harm the environment, emission gases such as CO2 and SO2. Flash dryers consist of a rotary drum, a cage mill, and a flash drying tube. The cage mill associated with this type of dryer may be problematic: it is subjected to wear and needs to be repaired fairly frequently.

Indirect steam dryers came into operation. In this application, steam instead of combustion gas is used as the heating medium. Steam is generated in the waste-heat boilers of the plant; additional fossil fuel or a separate combustion chamber is no longer needed. The dryer is consequently smaller. The application also provides for energy to be used throughout the plant. The first application of a steam dryer in copper smelters was of a stationary type. In 1999 a rotary steam dryer was successfully commissioned. Energy consumption and off-gas flow vary with dryer type. Both variables affect the energy efficiency of the drying process. 

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Features of the Hengyang Rotary Steam Dryer

The Hengyang steam dryer is a rotary-type dryer.3 It was developed for copperconcentrate drying. One of its key features and advantages is the rotation of the steam-tube package together with the rotary drum. The structural properties of the dryer improve water evaporation and eliminate tube wear.

Steam as an indirect heating medium offers at least two benefits: (1) it lowers temperatures in the dryer, and (2) with an outlet temperature of 100–120°C, the risk of fire ignition is very low.

Mechanical Features

The Hengyang steam dryer consists of a rotary drum with a steam-tube package of individual tube elements. Two riding rings support the drum; each ring rolls on two support rollers. The drive mechanism consists of a girth gear and pinion connected to a speed reducer and electric motor with a frequency converter. The rotation of the drum improves water evaporation, as the material is properly mixed and the bed is relatively loose. Other advantages include the prevention of build-up and clogging of the concentrate. Following a cycle in the drum, the wet material is heated up, water is evaporated and the vapour released to the dryer chamber. As the steam-tube package rotates with the rotary drum, material in the drum turns with the drum shell at the same rate as the steam-tube package. This eliminates wear on the tubes by an abrasive wet feed, be it copper concentrate, sand, reverts, or slag. The wear rate of tube elements is a function of the distance of relative movement of the tube elements with respect to the bed and applied pressure on the tube surface. The 270 turning bed hardly moves with respect to the shell and the tube elements.

Therefore, the distance of relative movement is close to zero and tube wear is eliminated.