PROPOSAL FOR EFFICIENCY AND PROCESS IMPROVEMENTS AT LUBAMBE COPPER CONCENTRATOR

It is proposed to conduct a two-day plant audit of the Lubambe concentrator and draw up a road map for improvement. Without seeing the plant, the following proposal is based on our experience of similar copper concentrators.

The biggest cost savings can be made in the area of milling as milling constitutes one of the most significant cost elements in a concentrator. Our approach has been:

Critically analysing slurry rheology with the aim of increasing mill throughput. This constitutes a variable cost component. Our greatest achievement has been a 15% increase in throughput, with 12% being our average. A typical guide is shown in Table 1.

Table 1: Showing operating guidelines for increasing SAG mill throughput, based on slurry rheology analysis, this circuit used HPGR also

No MILL LOAD

POWER DRAW

HPGR LOAD

ACTION

1 DOWN DOWN UP INCREASE MILL DENSITY 2 DOWN DOWN DOWN INCREASE MILL FEED 3 UP UP UP DECREASE MILL FEED 4 UP UP DOWN DECREASE MILL DENSITY 5 UP DOWN DOWN DECREASE MILL DENSITY

(OVERLOADING/BOGGING IMMINENT) 6 DOWN UP UP DECREASE MILL DENSITY

(FINES ARE BEING FLUSHED OUT) A critical imperative in the operation of a SAG mill is the density at which the

mill operates. Often, mill operators run a SAG mill at the same density like that of a ball mill. This is a fundamental error. Incorrect mill density is usually detected by the heat generated in a SAG mill. This leads to the waste of energy, and inefficient grinding. Figure 1 is an example of one of the plant surveys we conducted.

Figure 1: The effect of regulating SAG mill density on power draw

Analyse the grinding media used in the milling section. Often, mill ball breakage, Figure 2, leads to high balls consumption and reduced throughput, as broken balls cannot perform useful work but sit in the mill and takes up space while consuming energy!

Figure 2: Broken mill balls which could add to consumption, and reduce throughput

In addition, broken mill balls add to iron ions in the pulp. Iron in flotation pulp has been known to depress sulphide minerals flotation rate and copper recovery.

The application of reagents suite on the flotation circuit affects both concentrate grade and copper recovery. In addition, a stubborn and persistent froth could lead to pump sumps overflowing and a dirty plant floor. Apart from copper lock-up, a dirty floor could lead to accidents from slips and falls. Often, our recommendations result in the type of flotation froth shown in Figure 3.

Figure 3: A “sweet” flotation froth, as evidenced by the clear patches, resulting from the manipulation of existing plant’s reagents

Premature damage and failure of conveyor belts also adds to operating cost. We have implemented methodology for attenuating risks that can cause premature belt wear and failure. A simple technique is the installation of conveyor ploughs to remove deleterious rocks from getting trapped between the belt and the tail pulley, damaging the carcass of the belt in the process, Figure 4.

Figure 4: A simple plough on a conveyor belt, preceding the tail pulley

Poor stockpile management is the bane of any mill operator. Our recommendations ensure that the coarse ore stockpile is at its maximum level, Figure 5, at the commencement of day shift. This allows the mandatory day-shift maintenance to be carried out in the crushing plant without interrupting mill production.

Figure 5: Coarse ore stockpile at the commencement of day shift

Should plant availability permit, we often recommend the treatment of “low grade resource” in order to fill plant capacity? This usually involves a variable cost component. The plant manager can have a financial model on his computer which will aid in this decision making process. That model will consider:

o The selling price of copper o The grade of the “low grade material” o Known processing cost, and o Anticipated plant recovery, Figure 6

Figure 6: Grade/recovery relationship graph for the treatment of “low grade resource”

By explaining the deport of sulphide minerals in a froth column, Figure 7, we can get operators to judiciously skim the desirable minerals, thus improving concentrate grade

Figure 7: Deportment of sulphide minerals in a froth column

Froth floating on the surface of a concentrator thickener and reporting to the overflow launder contributes to copper loss. We have successfully installed mechanisms for eradicating such problems, Figure 8. This significantly improves the accuracy of metallurgical accounting.

Figure 8: A means of trapping “fugitive” froth escaping from a concentrate thickener

By implementing the above proposals, it is confident that the all-sustaining cash cost will be as shown in Figure 9.

Figure 9: Anticipated all-sustaining production cost at the end of the exercise

Ramoutar (Ken) Seecharran Process Consultant