Straight Story on Spirals, The

Straight Story on Spirals, The

Benson, Steve

Spiral technology has steadily developed over the past 60 years and is now routinely used in industrial minerals, base metal and coal operations worldwide.

Humphreys introduced spiral concentrators in the 1950s. The early models, made of cast iron and occasionally cement, had one trough profile and were used in the treatment of iron and chromite ores and other developing applications. In the 1970s, PVC, fiberglass and urethane became the materials of construction and different trough designs were introduced to accommodate a variety of applications. Spirals have steadily improved both in performance and versatility over the years and today are found to be the separator of choice in many mineral concentration plants.

Benefits of spiral separation include:

* High upgrading capability

* Flexibility to readily accommodate different ores and capacities

* Relatively simple operation

* Low cost

* Small footprint (more mobile, important in mineral sands industries)

* Environmentally friendly, reagent free

A spiral concentrator is a flowing film separation device. General operation is a continuous gravitational laminar flow down an inclined surface. Although different trough designs have led to many models, spirals can be categorized into two broad varieties: washwater and washwaterless, with the latter being the more dominant variety by a substantial margin.

For both washwater and washwaterless spirals, the mechanism of separation is the same and involves primary and secondary flow patterns. The primary flow is essentially the slurry flowing down the spiral trough under the force of gravity. The secondary flow pattern is radial across the trough. Here, the upper, more fluid layers, comprising lower density particles, move away from the center while the lower, more concentrated layers of higher density particles move towards the center. The conceptual cross section of the slurry shows the trough divided into three zones (See figure 1).

The innermost zone generally comprises higher density particles transported downward, (i.e. primary flow). The rising component of the flow has a certain capacity to lift lower density particles and transport them outward to the intermediate zone (i.e. secondary flow). The intermediate or transition zone is a region of free motion above the bed. This zone is relatively less concentrated and more fluid than the inner zone. Particles in this region move with the secondary flow and are transported according to their relative position within the bed. In the outer zone, particles may settle into the lower layers and be transported toward the center of the spiral. Particles of higher density will have a tendency to migrate into the lower, inward-moving stream.

Particle size is also a significant factor in the mechanics of separation. The finest, highest-SG particles will be distributed in the main (inner-most) concentrate band whereas the coarser particles of the same SG minerals are often the most difficult to recover to the concentrate.

In applications where the particles are relatively fine and/or higher density particles are the predominant component of the slurry, the addition of washwater provides greater separation efficiency. In these situations, the innermost zone may lose its fluid nature since water can be removed along with the higher density material or crowded-out by the particles themselves. Replenishing the slurry with washwater enhances the upward and outward movement of the lower density particles.

Washwaterless spirals are used in most applications, particularly for concentrating low-grade ores. The only water required is that which is added to the solids prior to introducing feed onto the spiral. A concentrate is collected at the bottom of the spiral or from several intermediate take-off points on the transition down the spiral. Washwater spirals require the addition of water at various points down the spiral and therefore provide a “washing” of the concentrate, i.e., transporting away light gangue from the concentrate band.

Testwork is Critical

Testing in the early stages is critical to determining the most efficient spiral model and circuit. During testing, feed distribution, pulp density and feed rate are adjusted to establish the optimum separation parameters for a specific mineral suite. Outokumpu’s spiral test rig at its Perth laboratory, for example, can test and simulate all three stages at once. This means that even the often difficult-to-model scenarios involving recirculating streams can be adequately catered for.

It is important to note the following general rules:

* By maintaining a consistent distribution to each spiral, consistent products are achieved.

* Generally, low pulp density will produce high heavy mineral concentrate grades while high feed pulp densities will result in lower concentrate grades with the higher recovery of heavy minerals.

* A spiral will normally achieve a minimum 3:1 upgrading ratio (ratio between head feed grade of heavy minerals and concentrate grade). Therefore, as with most gravity concentrators, a multi-pass flowsheet is generally required to achieve a desired grade and recovery of minerals.

Another element key to achieving the desired processing goal is accurately determining a spiral’s performance. Sometimes, the high recovery of heavy minerals (HM), which includes alumino-silicates, can cause processing problems in subsequent separation circuits. In these instances, it is necessary to differentiate HM recovery from valuable heavy mineral (VHM) recovery and recognize that, even though the overall HM recovery may be lower the VHM (e.g. TiO2 and ZrO2) recovery might be higher and thus the flowsheet is more efficient.

Ideally, a testwork facility that offers expertise in both mineralogy and processing should be chosen. No two deposits are alike and a mineral suite, even in similar geographic regions, can vary dramatically. In a mineralogy suite, confounding mineralogy issues can arise, particularly agglomerated particles. Testwork programs need to include some examination by microscopy to ensure that test results are not distorted through mis-analysis.

Steve Benson is manager-physical separation technology for Outokumpu Technology in Perth, Australia.

Copyright Mining Media Apr 2006

Provided by ProQuest Information and Learning Company. All rights Reserved