Description

911MPE offers all the major components of this complete process plant designed using these key design parameters:

Process development testwork and detailed engineering are essential services 911Metallurgy Corp. offers separately. The equipment package described herein does not include any permitting, infrastructure, foundation, electrical, assembly, reagents/supplies or commissioning. These are all additional paid-for services we do offer if you need them.

Procedure to Determine the Amount of Gravity Recoverable Gold

1. The test should be statistically reliable. This calls for a minimum mass to be treated which varies from ore to ore (depending on gold content and particle size), but has been found to be around 40 to 70 kg for most.
2. The test should rely on technologically up-to-date separation equipment. Centrifuge units have been shown to outperform devices that rely on the earth’s natural gravity field. Since these units (especially the iCON for gold) are now commonly used at plant scale, they should also be used at lab scale to characterise recoverability.
3. The test should indicate not only how much gravity recoverable gold (GRG)’ the ore contains, but its size distribution and the grind at which it is liberated. This should preferably be available with a single test, to minimize the mass of sample required (as sample mass is often in short supply, especially for greenfield applications).
4. The test should be free of the usual pitfalls of gravity testing, such as gold traps, using samples from circulating loads non representative of steady-state operation, or producing a concentrate that cannot be upgraded to smelting grade (i.e. recovering gold that is not GRG).
5. The test has to be inexpensive, as gravity will not be the main recovery method, and its use justified only on the basis of economy of effort, inclusive of the planning stage.

The test is based on the treatment of a sample mass of typically 50 kg with a laboratory gravity concentrator. Three stages are used, the first on the sample crushed and rod milled to 100% -850 µm, and the next two on part of the tails of the previous stage, ground to achieve further gold liberation. Stage two is performed on typically 24 kg ground at 45-55% -75 µm, and stage three on 18 to 21 kg ground at 75-80% -75 µm.

The GRG tests are performed at increasingly lower feed rates and fluidization water pressures to match the finer feed, typically from 1000 g/min and 25 kPa for stage 1 to 400 g/min and 12 kPa for stage 3. These correspond to optimal settings as determined by extensive test work with both gold ores and synthetic feeds, but must be adjusted for gangue density. Because the test is optimized, it yields the maximum amount of GRG; actual plant recoveries will be lower, because of limitations in equipment efficiency and of the usual approach of processing only a fraction of the circulating load. Linking projected plant recovery to the results of the GRG test will be briefly discussed later.

Gold

For each stage, all of the concentrate and 600 g of tails are screened from 25 to 600 µm (the tail sample is wet screened first). The tail fractions above 105 µm are further pulverized prior to assaying. All of the concentrate screen fractions and up to one assay ton of tails are fire-assayed.

The test was used on more than 30 ore types, ranging from completely oxidized to complex sulphides. Some ores were tested twice with the regular procedure to assess natural variability. Additional work included performing either a single or all three gravity stages at final grind, to check the validity of the basic approach and explore possible simplifications. These additional tests are discussed at length in; they show that the progressive grinding (as opposed to testing only at final grind) is necessary to obtain the correct size distribution of GRG, as well as a measure of progressive liberation. Testing only at final grind normally underestimates the GRG content, because of over-grinding. Testing feed masses below 10 kg can result in a slight overestimate of GRG content for ores with a low sulphide content.

The size distribution of the GRG is very useful in determining how gold should be recovered. This includes both the choice of recovery unit and feed preparation (usually screening). For example, it is pointless to present to the recovery unit a coarser fraction that is barren and lowers both its capacity and efficiency. Prior screening would then be appropriate, at a size that should hinge on the coarsest GRG. If GRG is fine enough and the main process route is flotation, it may be more effective to use flash flotation, which can significantly decrease the circulating of GRG below 75 µm, and can even be followed by gravity recovery.

Even when the response of the test is intermediate to highly amenable, results should be used cautiously. It should be understood that since the laboratory iCON recovers gold very efficiently, actual plant performance will always be inferior to the measured GRG. By how much depends on the efficiency of the gravity circuit. Circuits that are extremely efficient can probably achieve a recovery equal to two thirds of the measured GRG, but this has never been observed in plant practice. The economic incentives of gravity recovery do not normally warrant achieving the full gravity potential. Two factors can limit recovery. First, gravity is used only in the first of two loops in the grinding circuit, which implies that unliberated GRG in the primary cyclone overflow will never be recovered by gravity. Second, the recovery effort in the primary loop is limited to treating 25% of the circulating load, which is certainly reasonable.