Improving thickener performance

Carpentaria Gold Pty Ltd Ravenswood Operation, in Queensland, Australia, continually strives to increase their overall gold production by improving mining and processing efficiencies. FLSmidth was approached by Carpentaria Gold to conduct a review performance of their 22mØ tailings thickener with a focus on optimization and modernization options to improve gold, water and cyanide recovery. FLSmidth designed a new feed system incorporating E-DUC® feed dilution technology and our latest revolutionary thickener innovation — E-Volute™ feedwell.

The customer
Carpentaria Gold Pty Ltd Ravenswood Operation is located approximately 95 km southwest of Townsville in northeast Queensland. The processing plant processes 1.5 million tons per annum of high-grade underground ore using single-stage crushing, SAG (semi-autogenous grinding) and ball milling, and carbon-in-leach processing with a gravity circuit for recovery of free gold. In 2013, the Ravenswood Operation produced 141,846 ounces of gold. Ongoing improvement initiatives have been successful in increasing overall plant recovery — in 2013 gold recovery was 94.9%. This highly optimized plant continues to emphasize continuous improvement. Despite the already excellent recovery rates, the plant still sought to further increase gold recovery and target operational improvement projects aimed at reducing cyanide consumption and minimizing raw water requirements.

Improvement opportunities
Gravity gold recovery
Gravity gold is recovered with a Knelson® concentrator early in the process, which accounts for approximately 60% of the overall gold recovery at Carpentaria Gold. Slurry is pumped into the Knelson concentrator cone. The concentrator cone contains a series of concentric rings that collect the gold as it spins at a high speed to create centrifugal force. Water is pumped into the rings through capillaries (narrow hollow tubes) to fluidize the bed, which optimizes the separation of particulate gold from the slurry.

Clean process water is essential to maximizing gravity gold recovery from the Knelson concentrator. If there are solids present in the process water, the capillaries become blocked, which increases the water pressure and reduces gold recovery. The concentrator then needs to be shut down frequently and taken offline for time consuming and costly maintenance to unblock the capillaries, which impacts gold recovery and raises maintenance costs.

Raw water usage and cyanide recovery
Operation of slurry thickeners is generally a compromise between clean overflow water and underflow density. Either the 22 m tailings thickener will achieve a high underflow density, which enables maximum water and cyanide recovery, or it operates to improve overflow water clarity and enables a positive impact on gravity gold recovery.

The second option is achieved primarily through reducing mud bed levels and increasing flocculant dose. However, the consequence of this approach is inevitably lower underflow density, reduced water recovery and therefore increased raw water usage, lower cyanide recovery and excessive flocculant usage.

Alternate solutions considered
New thickener
A new thickener with higher side wall height and a longer mud residence time would enable cleaner overflow water with higher underflow density. This would enable improved gravity gold recovery and maximum water and cyanide recovery. This option would involve a significant capital expense.

Process water filtration plant
This option would enable the thickener to achieve maximum underflow density, water and cyanide recovery, and remove the solids from the dirty overflow in a filtration plant. Again, this option would involve a significant capital expense.

Upgrade the existing thickener
The lowest cost option would be to upgrade the feed system of the tailings thickener to enable high underflow density (aiding water and cyanide recovery) and the production of clean overflow water for optimum gravity gold recovery.

Final solution
The original feed system on the tailings thickener installed in 2000 was based on a higher feed rate using the technology of the day. The feed system was corroded and in need of replacement. After years of being fed leach tailings, it had reached the end of its serviceable life. This presented an excellent opportunity for Carpentaria Gold to investigate modern alternatives.

FLSmidth designed a new feed system incorporating E-DUC® feed dilution technology and our latest revolutionary thickener innovation—E-Volute™ feedwell.

E-Volute™ feedwell technology
The E-Volute™ feedwell is the result of years of laboratory, CFD (computational fluid dynamics) and pilot scale development work. It is designed to maximize the critical parameters of “best practice” feedwell performance, and incorporates all the benefits of both closed and open bottom feedwells, maintaining all the positive attributes of both designs without the drawbacks. These include:

Larger aggregate growth with less flocculant
E-DUC feed dilution and flocculation prior to the feedwell allows for excellent mixing conditions for contact of flocculant. This, followed by the low turbulence and further flocculation in the E-Volute feedwell, provides the optimal environment for flocculation and aggregate growth, minimizing flocculant and increasing settling rates.

Maximizing aggregate size with minimal flocculant allows for improved compaction in the mud bed, improved rheology and increased underflow density.

A full-width entry E-Volute shelf increases residence time and reduces short circuiting.

The unique full-shelf width entry of the E-Volute feedwell allows for maximum residence time within the feedwell, increasing mixing and aggregate growth as well as reducing unagglomerated fines and unabsorbed flocculant.

The involute shape of the E-Volute feedwell promotes even distribution of agglomerated solids into the thickener. This minimizes the risk of point loading the raking mechanism and maximizes the settling capacity of the thickener, all while reducing internal dilution within the feedwell, which negates any benefits of closed-bottom feedwells.

Low shear exit improves settling rate and clarity
The open exit of the E-Volute feedwell provides a large surface area for aggregate throughput. This large surface area reduces velocities and turbulence as the slurry exits the feedwell, reducing shear and minimizing aggregate breakage that can occur in closed bottom feedwells. The reduced shear-induced breakage increases settling rates, improves clarity and maximizes density.

Installation
FLSmidth and Carpentaria Gold collaborated at the start of the project to establish measurable performance criteria to determine qualitative and quantitative benefits of the upgrade.

The criteria included:

  • Overflow water clarity
  • Knelson concentrator feed water pressure
  • Knelson concentrator scheduled maintenance intervals
  • Flocculant consumption
  • Underflow density
  • Cyanide recovery
  • Raw water usage
  • Scale inhibitor usage

Baseline qualitative and quantitative performance was measured for a period of time prior to the feedwell upgrade. Naturally, the performance was re-measured after the installation period.

The E-Volute feedwell upgrade was designed specifically for retrofitting onto the existing structure. FLSmidth engineers designed the new feedwell for easy installation, utilizing a segmented shell and clamps for attachment and leaving minimal work to be done on site. This allowed for the installation to occur without having to lift the bridge and mechanism. The removal of the old feedwell and installation of the E-Volute™ feedwell took only two shifts to complete on site.

Results
The upgrade to the E-Volute feedwell on the 22 mØ tailings thickener at Carpentaria Gold completed in October 2013 brought about immediate improvements in thickener operation. Process data from the site PI and laboratory data from grab samples have shown:

  • The Knelson concentrator process water feed pressure stabilized, indicative of negligible capillary fouling — the result was the security of full-plant availability ensuring gravity recovery of gold as well as a reduction in maintenance costs
  • Visibly significantly improved thickener overflow clarity
  • Significantly improved solids settling rate, leading to a 40% reduction in flocculant
  • Reduced pumping costs for tailings dam water reclamation
  • Improved settling coupled with the reduction in overflow solids allowed for increased mud bed growth and underflow density
  • Reduced raw water intake
  • Increased recovery of cyanide that would have been lost to tailings
  • Lower solids in the process water allowed for a ~35% reduction in scale inhibitor chemical

David Williams, FLSmidth General Manager Customer Service – Australia & Southeast Asia, commented that this successful upgrade was only accomplished through the skill and expertise of the local FLSmidth customer service team, which included Clint Faulks, Dane Smith and Richard Jenner with Global Product Manager, Fred Schoenbrunn, to name a few.

CONTACT: David Williams