SPECIALIZED MIXERS

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Large volumes of gas are injected into the vessel-reactor through some type of dispersion unit, be it a sparge ring or pipe. Impellers for these applications must be carefully selected to prevent flooding of the impeller by the gas and to ensure effective dispersion, blending and solids suspension. The mechanical design of the systems is complex, with the agitator seeing large fluid forces and consequential shaft deflection.

AFX has many years of experience in the process and mechanical design of these agitators. The impeller of choice is the AFX high solidity P4 impeller, providing efficient operation at high gas induction rates. Gas dispersion applications include bio-oxidation processes (bacterial leaching) and dual impeller technology. 

The lower impeller, known as the P4, is a downward-pumping, four blade, high-solidity hydrofoil impeller that has become the standard in bioleach reactors. It is designed to disperse high volumes of air, while also maintaining the solids in suspension and promoting heat transfer. Such impellers which typically have a solidity ratio of more than 90%, are characterised by their ability to operate at high gas volumes without flooding.

The upper impeller is the innovative aspect and is an upward-pumping triple- blade-medium solidity hydrofoil impeller. This impeller known as the P3 impeller was originally designed for high-viscosity applications but the diversity of the P3 impeller to operate through three-phase or gas- liquid- solids applications have also been realized.

The oxygen mass- transfer requirements of the BIOX® or bioleaching process with be satisfied at a lower input and aeration rate when compared to the previously established single- impeller system. The improved mass-transfer performance is achieved through surface air induction created by the top impeller which is enhanced by the gas hold-up form the specific mixing pattern created by the dual impeller configuration.

The air induced from the surface by the dual impeller installation results in the reduction of blower capacities and therefore results in blower power decreases. In addition to the improved process performance, the dual impeller system also demonstrates a reduction in a bending moment, torque and thrust which will extend the mechanical life of the agitator.

 There are several processes available to destruct cyanide, with the most common process being the INCO method. This process route included the destruction of the cyanide in the presence of oxygen and sulphur dioxide. The oxygen is provided through large volumes of air or pure oxygen with sulphur dioxide gas or a solid reagent sodium called metabisulphite, to achieve maximum oxygen utilisation during the treatment process, specialised impellers are developed to yield oxygen utilisation from the compressed air ranging from 20% to 40 %. This depends on the percentage of solids in the slurry being treated. When pure oxygen gas is introduced, however this number could be as high as 80%.

When your pregnant liquor solution is contracted with an organic solvent, enough energy is required to prevent the phases from separating. This energy is entered into the system by a pump mixer, when too little energy is entered, the phases will separate.This is clearly evident in SX as the phases separate within the settler naturally and hence mixer-settlers (SX).

The mixers need to create dispersion and not an emulsion, as this would be the result of too much energy. The ideal dispersion is the minimum energy required to maintain a complete dispersion with in pockets of phase separation. Since the surface is the furthest away from the pump, this is the region where phase separation is most probable. This is convenient because the surface of a mixed box can be seen if the lids are removed.

At the minimum power per unit volume for complete dispersion, the droplets are the biggest stable droplets possible. This is ideal if top speed is kept down, to minimize fines and facilitate the separation in the settlers. The minimum power for phase separation has been determined experimentally. The minimum power per unit volume is a function of residence time and the height of the fluid. Tall skinning tanks need more power to resist phase separation than short, squat tanks. This is important to be considered during the design phase of the mix box. The best way to look at the design of the mix box is to determine the maximum possible hydraulic efficiency that can be achieved for the given head and flow and then determined the pump box volume. 

Apart form the correct pump mixer design consideration being considered, we consider all available options when designing the mix box. We can assist with the best tank geometries as well as improve your mixer’s efficiency.