PART I PROJECT DEVELOPMENT
8. ELUTION AND GOLD ROOM
At this point, it is assumed that the size and type of elution circuit has been selected for the process plant. The selection of type of circuit is complex and may be based on a number of issues, including capital cost, cycle time, water quality or other considerations. While the type and size of the elution circuit is an important part of the feasibility assessment for the project, the capital cost of this area of the plant is a relatively small part of the overall project cost. It generally ranges from 5 to 10% of the treatment plant cost. Design considerations for this area include the following:
Type of elution circuit One or two columns Column location Degree of automation Location of gold room
Complexity of gold-room operations Security.
8.1. Type of elution circuit
The more common types of elution circuit include:
Atmospheric or pressure Zadra circuit. The key issue here is the continuous nature of the strip/electrowinning process and the need to ensure that the discharge from the column can feed the electrowinning cell and thereafter the eluate tank. The Zadra circuit can generally accept poorer-quality water although this may extend elution cycle time.
AARL circuit. The key issue with this type of circuit relates to water quality and automation of the process. AARL circuits generally operate best where the water contains less than 500 mg/L total dissolved solids and the dis- solved salts are unlikely to cause scaling. Where higher likelihood of scaling is anticipated, a regular cleaning schedule with sulfamic acid should be anticipated. The level of automation in the AARL circuit is generally higher to ensure the correct sequence of heat-up, soak, elution and cool-down
Feasibility study plant design 143
stages. The separation of electrowinning and elution makes this circuit more suited to fast cycle times.
Split Anglo circuit. The issues here are similar to the straight Anglo circuit although with the added complication of having two eluate streams. This circuit is favoured where there is a limitation in water quantity.
Integrated pressure strip (IPS). The IPS system is a packaged plant where both elution and electrowinning operate under pressure.
8.1.1. One or two columns
The key issues here are elution cycle time and elution temperature. A single acid wash/elution column can be used if there is sufficient capacity in the system to accommodate the extra time required for acid wash, rinse and carbon transfer as well as the normal elution cycle. This can be appropriate for lower-temperature Zadra circuits that do not need to operate every day. If the elution temperature is below 1101C, a single rubber-lined column can be used for both acid washing and elution. Above this temperature, two sep- arate vessels are required. Typical Zadra cycle time is about 16 h.
If an elution circuit with a relatively short cycle time, such as an AARL circuit is used, then two separate columns are required. One of the advan- tages of the AARL circuit is the ability to complete the elution cycle in 6–8 h, allowing two cycles to be completed in a day. If a single column is used, then this cycle time can be extended by 2–3 h, making multiple strips more difficult to achieve in a single day.
8.1.2. Column location
The location of the acid-wash column is normally driven by the location of the loaded-carbon screen and the ability to gravitate loaded carbon into the column. This may be dictated by whether the circuit is CIL, CIP or a hybrid.
For a Zadra circuit, it may also be influenced by the location of the gold room in the milling circuit, which can push the stripping circuit further down the tank train. Favoured locations are:
Between milling and the first tank (for CIL) located under the cyclone tower for smaller throughput facilities and where a gravity circuit is included.
Halfway down the train nestled between Tanks 3 and 4 (for hybrid circuits).
In a line parallel to the tank train incorporating gold room, eluate tanks and columns. This can impact on constructability.
In a separate independent compound if gravity concentrate processing is not an issue.
A. Ryan et al.
144
8.1.3. Degree of automation
Elution circuits can be fully automated or fully manual. As a general rule, Anglo circuits are usually automated and controlled by a stand-alone PLC.
Zadra circuits can be automated but are frequently fully manual as there are fewer valve changes to make and elution occurs over a longer time.
8.2. Gold room
8.2.1. Location of gold room
The location of the gold room is dictated by a number of competing factors:
If there is a gravity circuit, the gold room is normally close by to allow the concentrate to gravitate. This means it is either adjacent or under the milling area.
If there is no gravity circuit its location is dictated by proximity to the elution column as described above. In the case of a Zadra circuit where the elution and electrowinning operations are inextricably linked, this is a re- quirement. This is less so for the AARL circuit.
In some more remote overseas operations, security is a major concern and the gold room is placed within sight of the offices/security complex in a separate compound.
8.2.2. Complexity of gold room operations
Gold room operations can be as simple as a single electrowinning cell and eluate tank, drying oven and smelting furnace. Or they can incorporate gravity-concentrate tabling, separate concentrate leaching, multiple electro- winning cells, cell-mud handling and filtration and large-scale smelting op- erations. The complexity of the operation generally dictates the layout. The following issues are typically taken into account:
Occupational health is paramount and adequate fume extraction and shower facilities must be provided.
Gravity flow is used wherever possible with electrowinning cells located on a mezzanine floor.
All hoppers and wash vessels must encourage flow, because cell mud is a high SG material.
Operator access to key control points such as electrowinning rectifiers must be maintained external to the gold room for security.
Adequate wash-down and silt trap facilities must be provided.
The building must be secure.
Feasibility study plant design 145
8.2.3. Security
The level of security attached to a facility is site specific and is driven by a number of considerations:
The economic development of the country.
The level of institutionalized gold theft.
Client preference.
At the lowest level, the gold-room security will include motion sensors, closed-circuit television (CCTV) cameras and a locked door.
At the highest level it will include all the above together with a separate perimeter fence with CCTV,in situguards and body searches.