PART I PROJECT DEVELOPMENT
6. LEACHING AND ADSORPTION
problem for most agitator suppliers, who specify an average shear rate of about 2.3 s–1. However, high-clay slurries can have apparent viscosity measurements of up to 3000 cP. This may require a specialist agitator de- sign. The simple solution to this is to dilute the slurry. Most slurries exhibit a kink in the viscosity/density curve between 30 and 45% solids. The design therefore needs to take this into consideration. However, at the same time, dilution of the slurry leads to an increase in tank volume to maintain residence time. A shift from 40% solids to 35% solids increases the slurry volume by about 20%.
The ability of the slurry to pass through the intertank screens. The key to achieving counter-current flow of slurry and carbon in an adsorption cir- cuit is the facility for the slurry to pass through the screen to the next stage while leaving the carbon behind. The flowrate of slurry should include both the normal slurry flow and the additional flow of slurry from carbon transfer operations. At feasibility study level, a typical intertank screen will be based on a superficial flowrate of about 60 m3/h/m2. This provides an indicative screen size for the purposes of cost estimation and drafting.
However, for high-viscosity slurry, this may reduce to 30 m3/h/m2. For a large-capacity tank train, this may necessitate the use of multiple screens per stage.
Thehead lossper stage. High-viscosity slurries develop a higher head loss per stage and therefore require a higher static head to maintain slurry flow. Maintaining a higher freeboard in the system for a flat-deck design can accommodate this; however, a maximum of seven tanks is normally used before introducing a step. Alternatively, the tanks can be stepped on ring beams. Steps usually employed are 75 mm for a leach tank and 100 mm for an adsorption tank. In either case, the height of the tank train increases.
6.3. Requirement for leach feed thickener
For efficient cyclone operation, dilute feed and high cyclone operating pressure usually produce a sharper cyclone split. This results in more efficient classification and avoids coarse grit in the cyclone overflow and excess fines in the underflow. As a result, the recirculating load is decreased and milling power is used more efficiently. This is particularly important for grind sizes of less than 106mm. The leach circuit is generally operated at the maximum slurry density that does not impact on leach and adsorption efficiency and does not cause carbon to float. These two operations require different op- erating conditions and the leach-feed thickener provides a means of sepa- rating the two processes.
Larger gold plants have moved towards inclusion of leach-feed thickeners.
Smaller plants are still divided on the issue. It generally depends on the
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magnitude of the difference in operating conditions between milling and leaching. The main disadvantage of the leach-feed thickener is cost, partic- ularly for poorer-settling ore types. The advantages of the thickener are:
It separates the milling circuit from the leach circuit in terms of operating conditions.
It provides some limited surge capacity for minor milling-circuit break- downs.
It may allow the height of the cyclone tower to be reduced in some cases.
It provides a means of separating the milling-circuit water from the leach- circuit water, thus reducing the volume of water requiring cyanide de- struction.
6.4. CIP or CIL
Selecting between a CIL circuit and a leach/CIP circuit depends on a number of factors, including the following:
Preg-robbingores are generally treated in CIL circuits to avoid re-adsorp- tion or desorption of gold onto carbonaceous material in the ore.
CIL circuits generally have a lower capital cost for the same leach residence time, as there are fewer tanks. However, they have a higher carbon and gold inventory because the tank volume holding carbon is larger.
Leach/CIP circuits usually achieve higher carbon loadings as the pregnant liquor tenor is higher. This allows smaller carbon inventory to be achieved.
The CIL circuit normally consists of six or seven agitated tanks of identical size arranged in a staggered formation. This allows the minimum footprint to be achieved while still providing the facility for bypassing each stage. Each tank is equipped with an agitator and intertank screen(s). The tanks are joined by launders or large-diameter piping, which allows the slurry flow to be diverted as required.
The traditional leach/CIP circuit consists of a series of leach tanks (three or more) providing the full circuit leach residence time. This is followed by a series of adsorption tanks (six or more) with a short residence time for adsorption of precious metals onto carbon. This results in two separate tank sizes with layout consequences in linking the access ways.
The traditional leach/CIP circuit has evolved into a hybrid circuit that might consist of eight identical tanks. The first three tanks provide the initial leaching and the subsequent five tanks provide further leaching and adsorp- tion. This hybrid configuration has the advantages of higher pregnant liquor tenor, lower carbon inventory and identical tank-size.
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6.5. Number of stages
The number of leach and adsorption stages is dictated by a number of issues, including:
The choice between CIPand CIL, as described above.
The kinetics of leaching and adsorption. Preliminary modelling using the Fleming model or equivalent is normally done at the feasibility study stage.
Given the leach feed-grade, target loaded-carbon and tails-solution grades, this will dictate the number of stages required while still managing the gold and carbon inventory.
Consideration of potential bypass conditions.
6.6. Aeration requirements
The oxygen demand of different ore types varies widely. Oxygen is gen- erally introduced in two ways:
Low-pressure blowersprovide aeration, generally down the agitator shaft or via sparges under the lower agitator impeller. This is suitable for low to medium oxygen-demand ores and is generally limited to one tank volume per hour.
Cryogenic or pressure-swing adsorption facilitiesprovide oxygen that is di- rectly sparged into the slurry, through the side of the tank or down the agitator shaft. The sparge is generally located just below the agitator. This is used for high oxygen-demand ores often containing active sulfides.
6.7. Bypassing requirements
The tank train is generally arranged to allow each tank to be bypassed.
This can be done using launders running over the top of the tanks (for the Kambalda type screens) or via a dart valve located in the discharge box.
Screen discharge can be hard piped from tank to tank; however, this may lead to flooding if the density of the slurry in the two tanks varies. Selection of seven or more tanks allows one tank to be bypassed at a time while still maintaining the minimum of six tanks to prevent short-circuiting of pulp.
6.8. Carbon movement
The loaded-carbon grade and the precious-metal extraction efficiencies dictate the rate of carbon movement required. Carbon is generally advanced by either airlift (for smaller plants) or by recessed impeller pumps. The plant design must take into consideration the impact of the return slurry flow from carbon advance on the intertank screen capacity. For average-grade gold plants this is not an issue. However, for ore with a high silver grade, the carbon movement may be significant. This can lead to large slurry flows
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coming from carbon movement. In this situation, separate carbon-transfer screens may be included to allow the slurry to return to the tank from where it came while still allowing the carbon to advance.
6.9. Bunding requirements
Concrete bunding around the leach/adsorption tank train is designed for access and spillage management rather than total containment. If total con- tainment is required it is generally achieved using a separate containment pond.
6.10. Barren carbon return
Following elution, barren carbon is either returned directly to the ad- sorption circuit or is regenerated. The trend today is towards a horizontal regeneration-kiln. For smaller plant this is located on top of the CIL tanks.
This allows the barren carbon to gravitate directly into the requisite adsorp- tion tank after screening and avoids the need for a separate support structure.
For larger facilities, the regeneration kiln and associated feed hopper can be substantial and may provide significant structural loads. In this circumstance, the regeneration kiln is located at a lower level and the regenerated carbon is hydraulically transported to a screen on top of the adsorption tanks.
6.11. Leach tails thickener
A leach tails thickener is generally included if water recovery or cyanide recovery is critical to project economics. This may be the case where water is in short supply or a start-up water supply is not available.