Key considerations in thickener modernizations

• Increased throughput either in tons per hour (tph) or volume (m³/h)
• Changing ore conditions such as lower grades or smaller particle size distribution (PSD)
• Changes in environment and social license to operate  

Cumulative effect of upstream capacity improvements

The number one driver for thickener modernization is increased capacity.  Often incremental increases occur over the life of a plant and it is not immediately recognized the impact this may have on thickener performance.  When a plant embarks on an efficiency drive which results in a small 1-3% increase and the cumulative effect of this on thickener performance is not easily seen, as each change only marginally impacts thickener performance.  Focus in these improvement projects is often upstream at the mills, leach or flotation circuits.  However, when viewed as whole over a 10-20 year period these 'efficiency' gains can amount in throughput increases of 20-50%.  The resulting decrease in underflow (UF) density, increasing flocculant consumption and torque trips are not recognized as deviating from the design by rotating and changing plant staff who may not be aware of its original design parameters.

Changed ore conditions

The second major driver for modernization of thickeners is changed ore conditions. It is well established that mine operators are processing ever-lower grades of mineralization (refer figure 1). This occurs when easy to mine, high value targets are exhausted and the ore body properties change. While also contributing to the push for higher throughput to maintain plant output and adequate Return-On-Investment (ROI), these lower grades are associated with smaller PSD. Smaller PSD has a direct impact on the settling characteristics of the solids materials and require more tightly tuned feed conditions and efficient flocculation to obtain the desired outcomes.

These changed conditions can result in a thickener performing at levels below design or sub-optimal to the process requirements. Reduction in returned water to the plant, increase in flocculant consumption, loss of recovery and increased tailings costs can all be expected if a thickener is not performing to its 'benchmark' or performance target.

Environment and social license

Over the last 10-20 years there is increasing awareness and focus on the long term impact human activity can have on the environment. The issuance and ongoing retention of an operating license is now more closely controlled than ever before. Local communities are increasing their demands on owners to operate their mines with sustainable and minimum footprint on their environment. This “social license” is increasingly important in decision making for plant operators on ongoing operations optimization.

To continue to operate through these changes, one option open to operators is to modernize or upgrade parts of the thickeners with newer technology or correctly sized components. Modernization represents an effective use of capital when compared to the alternative to install a new thickener, which in addition to increased replacement costs may have additional costs to run services and piping to a new location.

Modernization project phases

The process of modernizing a thickener requires certain steps to be taken. The timing of each step is dependent on availability of information and may not run sequentially as shown here.

Firstly, after identifying a challenge, the possibility for upgrading must consider the suitability of the tank, the one fixed component, being suitable for the current or planned duty. This can be determined by either drawing on Metso’s experience and database of over 2,000 installed thickeners to benchmark and provide an indication of the possibility of success or conducting test work to determine the thickening characteristics of the material.   If analysis of experience, or from test work results, indicates the tank is suitably sized then the modernization possibility can be further developed.

For process-related modernizations, which comprise over 90% of modernizations, the focus will be around the feed system, including feed well, feed pipe and feed delivery prior to the thickener. The criticality of viewing the complete feed system as a unit is clear when we recognize the purpose is to ensure the efficient utilization of flocculant for improved solids separation. If conditions in the feedwell are not created for the best flocculation to occur, then the process will not operate as intended, with poorly flocculated material unlikely to compact to achieve density before rising to report to the OF. In the feedwell, too much or too little energy, high solids density or insufficient time for floc creation, will lead to this undesirable outcome.

The design of the feed system will consider such factors as:

1. Solids rate
2. % solids in feed
3. Optimal feed dilution
4. Material physical properties such as SG and PSD
5. Routing, elevation changes and number of streams

Once the feed system design and scope is decided, consideration of the mechanical requirements can begin with an engineering study. This would include reviewing all structural and mechanical parts to determine:

1. Are in suitable condition, free from corrosion and damage, and
2. Can they serve any changed process conditions?

This is accomplished with access to as-built drawings or in their absence a full site survey with sufficient data to enable an FEA on the structural parts to determine their suitability to accept new static and dynamic load conditions. The engineering study will consider:

1. Drive unit
2. Mechanism and rakes
3. Underflow
4. Overflow
5. Bridge

Scope and implementation of modernization

Now that all aspects are known and considering the customers’ requirements for the thickener, the scope and implementation plans are finalized. The scope may include:

1. Reactorwell™
2. Vane Feedwell™
3. Feed or Deaeration tank
4. Feedwell dilution
5. Modifications to OF and UF handling
6. Drive and power units
7. Bridge and rake mechanisms

A key success factor in modernization is the timely completion of implementation. The plan is always to facilitate the work to coincide with plant-planned shutdowns such as Mill relines or maintenance periods to minimize disruption to plant operations and production.

Benefits of thickener modernizations

It is clear that thickener modernization can be an effective and efficient way to maximize equipment use for changing conditions and to meet external requirements and expectations.

The benefits include:

• Improved process efficiency
• Improved capital efficiency
• Decreased operating costs
• Improved environmental performance
• Safer plant operation

The outcome of this effort can provide sustained and quantifiable benefits. Some examples include: