How to Select the Right Ferrosilicon Grade for Dense Medium Separation

Selecting the right ferrosilicon grade for dense medium separation requires more than choosing a product from a particle-size table. The medium must create a stable, controllable suspension at the required operating density while remaining compatible with the separator, feed material and recovery circuit.

A grade that is too coarse may settle rapidly and produce density instability. An unnecessarily fine grade may increase medium viscosity, reduce separation efficiency and create recovery or handling difficulties. The most economical option is therefore not automatically the cheapest powder or the finest available grade. It is the grade that delivers reliable separation at an acceptable total medium consumption.

Dense medium separation plants also differ considerably. A static bath treating a relatively coarse feed has different medium requirements from a high-pressure dense medium cyclone processing smaller particles. Ore mineralogy, clay content, feed sizing, target cut density and the condition of the magnetic recovery circuit all influence grade selection.

This guide explains how particle size, powder form, medium stability, viscosity and plant operating conditions should be assessed before specifying a ferrosilicon grade.

What does a ferrosilicon grade describe?

In dense medium separation, the term “grade” normally refers primarily to the particle-size distribution of the ferrosilicon powder. Suppliers may identify products by nominal mesh size, micron range, fineness category or a proprietary grade name. These labels are useful, but they should never replace the full technical specification.

Two powders carrying similar grade descriptions may have different proportions of coarse particles, ultrafines and material within the main operating range. Those differences can affect settling behaviour, slurry viscosity, magnetic recovery and medium losses.

The specification should therefore be reviewed for:

• The complete particle-size distribution rather than a single nominal size.
• The percentage retained above the upper size limit.
• The percentage passing the lower size limit or reporting as ultrafines.
• Whether the material is atomised, milled or a defined blend.
• Apparent and particle density data, where supplied.
• Chemical composition and any relevant handling information.
• Batch consistency and the supplier’s test methods.

Grade should not be confused with the silicon content of the alloy. Dense-medium ferrosilicon is selected as a complete medium product, with alloy composition, density, particle shape and size distribution working together. Comparing only the alloy chemistry does not establish how the powder will behave in a plant suspension.

The main criteria for selecting a ferrosilicon grade

Separator type

The first decision is whether the medium will be used in a static or relatively low-force separator, or in a centrifugal device such as a dense medium cyclone.

Baths, drums and similar separators commonly process coarser feed and can often operate with a comparatively coarser medium. The lower viscosity associated with a suitable coarser powder can assist drainage and medium recovery. The grade must still remain sufficiently stable throughout the vessel to prevent an uncontrolled density gradient.

Dense medium cyclones generally require a finer and more stable medium. The suspension must withstand rapid acceleration and complex flow conditions while maintaining the density needed for separation. A grade with excessive coarse material can segregate within the cyclone and contribute to inconsistent performance. Selecting an extremely fine grade without testing is not the answer, however, because excessive fines can raise viscosity and interfere with particle movement.

Feed top size and bottom size

Feed sizing affects both separation dynamics and medium contamination. Coarse, narrowly sized material is usually less sensitive to moderate slurry viscosity than fine feed. As the lower feed size decreases, the risk that high viscosity will hinder particle movement becomes more significant.

Desliming performance is especially relevant. Clay, slimes and fine mineral particles entering the dense-medium circuit can accumulate in the circulating medium. This changes rheology even if the fresh ferrosilicon meets its specification. A plant treating clay-bearing ore may therefore need to place greater emphasis on feed preparation, bleed control and circulating-medium testing rather than trying to solve every problem through a change in ferrosilicon grade.

Required separation density

The target operating density determines how much ferrosilicon must be present in the water-based suspension. Higher medium densities generally require a greater solids concentration. As solids concentration rises, viscosity becomes more sensitive to particle-size distribution, particle shape, contamination and water chemistry.

A grade that performs well at one operating density may become too viscous when used at a substantially higher density. Selection trials should consequently be conducted close to the actual density range expected in the plant, not only at a convenient laboratory concentration.

Medium stability

Stability describes the medium’s resistance to rapid settling and segregation. A stable suspension helps maintain predictable density through the separator and associated pipework. Finer ferrosilicon particles generally remain suspended for longer than coarse particles, but stability should be balanced against viscosity.

Too little stability can cause density variations, solids accumulation in low-flow areas and unreliable cut points. Too much stability is not automatically beneficial if it is achieved through an excessive ultrafine fraction that makes the suspension difficult to drain, rinse or separate magnetically.

Medium viscosity

Viscosity affects how freely the feed particles can move relative to the medium. If viscosity is excessive, particles may report to the wrong product because their movement is hindered rather than because their density is above or below the intended cut point. This effect becomes more pronounced for fine feed.

Ferrosilicon particle size is only one influence on rheology. Solids concentration, particle shape, ore fines, clay, temperature and water chemistry can all change the measured result. Grade comparisons should therefore use representative process water and realistic levels of contamination wherever possible.

Atomised and milled ferrosilicon powders

The production route influences particle shape as well as particle size. Atomised ferrosilicon is formed by breaking molten alloy into droplets that solidify into particles with a generally rounded form. Milled ferrosilicon is produced by crushing and grinding solid alloy, resulting in more angular and irregular particles.

Rounded atomised particles can offer favourable flow and rheological behaviour at a given solids concentration. The smoother particle form can help a suspension achieve the required density without the same degree of interparticle resistance associated with highly angular material. Atomised products are widely considered for cyclone applications and other circuits where tight control of medium behaviour is required.

Milled ferrosilicon can be suitable for bath and other dense-medium applications, depending on its sizing and the operating conditions. Its angular particles may produce different settling, viscosity and drainage characteristics from an atomised powder with a nominally similar size.

Neither production route should be selected from shape alone. The correct comparison includes the full particle-size distribution, operating density, rheology, stability, magnetic recovery and delivered performance. Some circuits may also use a controlled blend to obtain a particular balance of characteristics.

The DMS Powders ferrosilicon product range provides a starting point for comparing powders intended for dense medium separation. Plant operators should match the relevant product specification to their own circuit and test conditions.

How particle size changes plant performance

Particle-size distribution influences several parts of the process at the same time. Changing grade to correct one issue can create another if the entire circuit is not considered.

If the grade is too coarse

An excessively coarse medium may settle quickly in tanks, launders or low-velocity pipe sections. Within a separator, it may develop an undesirable density gradient or respond poorly to short-term flow changes. Coarse particles can also behave differently under cyclone forces, contributing to medium segregation between product streams.

Possible operating symptoms include unstable density readings, fluctuating product quality, solids deposits and a greater difference between sampled medium densities at different circuit locations. These symptoms are not proof of an incorrect grade, as pump performance, instrumentation and flow distribution can produce similar effects.

If the grade is too fine

A powder containing more fines than the circuit needs may create a medium that is stable but overly viscous. Drainage from the product can become more difficult, and additional medium may remain attached to wet ore after the drain stage. Very fine particles can also place different demands on magnetic separation and rinse-screen performance.

High medium losses should not automatically be blamed on a fine grade. Damaged screen panels, unsuitable aperture selection, inadequate spray water, poor magnet performance, leakage and incorrect flow balance must also be investigated.

The importance of the distribution between the limits

Plant behaviour is not controlled only by the largest particle or an average size. The shape of the distribution matters. A broad distribution can pack and flow differently from a narrow one, while a small ultrafine fraction may have a disproportionate effect on viscosity.

For meaningful supplier comparisons, request data produced by stated test methods and compare like with like. Sieve analysis and instrumental particle-size methods do not necessarily report identical results, particularly for irregular particles. Sampling and dispersion procedures can also affect the reported distribution.

A practical selection process for a new or existing DMS plant

1. Define the separation duty. Record the separator type, feed rate, feed top and bottom size, target operating density, expected density range and required product split.
2. Characterise the feed. Review ore density distribution, liberation, clay content, degradation during handling and the quantity of fines likely to enter the medium circuit.
3. Assess the water and circulating medium. Use representative process water for testing. If the plant is operating, analyse the circulating medium rather than relying solely on the fresh make-up powder specification.
4. Shortlist suitable powder forms and sizes. Use the separator type and feed sizing to identify realistic atomised, milled or blended options. Remove grades that cannot provide adequate stability or that are likely to create excessive rheology.
5. Compare full technical data. Review particle-size distribution, powder form, chemistry, density information and test methods. Do not treat nominal grade names from different suppliers as directly interchangeable.
6. Conduct laboratory tests. Measure suspension density, settling or stability and rheological behaviour at representative concentrations. Where possible, assess contamination by ore fines and process water.
7. Run a controlled plant trial. Introduce the selected grade under documented conditions. Allow enough time for the new make-up medium to influence the circulating inventory before drawing firm conclusions.
8. Evaluate total performance. Compare separation efficiency, product quality, medium consumption, recovery performance, density control and operating stability rather than focusing on purchase price per tonne alone.

Account for the circulating medium, not only fresh powder

Fresh ferrosilicon begins to change once it enters the plant. Preferential losses may remove one part of the size distribution faster than another. Attrition can generate finer particles, while corrosion and oxidation can change particle surfaces. Ore fines and other contaminants may accumulate if they are not adequately controlled.

As a result, the circulating medium can differ materially from the make-up grade. Routine sampling should cover relevant locations such as correct-medium circuits, dilute-medium circuits and separator feed, subject to the plant’s flowsheet. Samples need to be taken consistently because settling within a line or vessel can make an unrepresentative grab sample misleading.

Useful operating measurements include:

• Correct-medium density and its short-term variation.
• Density differences across relevant streams or sampling points.
• Medium viscosity or an agreed site rheology indicator.
• Particle-size distribution of the circulating medium.
• Magnetic and non-magnetic solids in the medium.
• Ferrosilicon make-up consumption per tonne of feed.
• Medium recovery through drain, rinse and magnetic separation stages.
• Product yield, ash or grade results, depending on the mineral processed.
• Separation efficiency indicators such as probable error and misplaced material, where partition testing is available.

Trend data is usually more informative than one isolated result. It helps the plant distinguish a persistent grade mismatch from short-lived disturbances caused by feed changes, water balance or equipment condition.

Check compatibility with the medium recovery circuit

Ferrosilicon selection affects the whole medium circuit. Drain-and-rinse screens must recover adhering medium while preventing excessive product contamination. Magnetic separators must recover the magnetic medium from dilute streams, and pumps and pipework must maintain suitable transport velocities.

Before changing grade, inspect screen condition, spray arrangement, magnetic separator settings, dilution-water control, sumps and pump performance. A finer grade may expose limitations in the recovery circuit that were less visible with a coarser product. Conversely, poor recovery can make an otherwise suitable grade appear uneconomical.

Magnetic recovery performance should be assessed using the plant’s actual dilute-medium stream. Ore-derived magnetic minerals can report with ferrosilicon, while non-magnetic slimes can remain entrained. Measuring only total solids does not show how much recoverable ferrosilicon is present.

Common ferrosilicon selection mistakes

• Choosing by nominal grade name alone. Product naming systems are not necessarily equivalent between suppliers.
• Assuming finer is always better. Additional stability can be outweighed by higher viscosity and difficult drainage.
• Ignoring feed desliming. Clay and mineral fines can alter the circulating medium more than a modest change in fresh powder size.
• Testing with clean laboratory water only. Results may not represent the plant’s process water or contaminant load.
• Changing several variables during a trial. Simultaneous changes to grade, density, pressure and feed sizing make the result difficult to interpret.
• Judging cost only by powder price. Consumption, product recovery, downtime, quality variation and medium losses form part of the operating cost.
• Overlooking inventory turnover. A new make-up grade does not instantly replace all material circulating through the plant.
• Failing to verify instruments. Density gauges and sampling systems should be checked before a powder grade is blamed for instability.

Questions to give a ferrosilicon supplier

A useful technical enquiry contains enough process information for the supplier to narrow the options. Provide the separator type, mineral or ore being treated, feed-size range, target medium density, throughput, current grade and the problem the plant is trying to solve.

It is also helpful to share available circulating-medium analyses, viscosity or stability results, medium consumption trends and details of the recovery circuit. State whether the project is a new plant, an optimisation exercise or a replacement for an existing supply. For a global operation, delivery location, expected volume, packaging requirements and consumption schedule should be addressed separately from technical grade selection.

DMS Powders supplies ferrosilicon powders for dense media separation to the global mining sector. Readers can review information about DMS Powders before starting a product discussion.

Frequently asked questions about ferrosilicon grades

Which ferrosilicon grade is best for a dense medium cyclone?

Dense medium cyclones generally require a relatively fine, stable medium, but there is no single grade that is best for every cyclone. Feed size, cyclone geometry, operating pressure, target density, ore fines and medium recovery performance must be considered. The choice should be confirmed through representative laboratory work and a controlled plant trial.

Can the same ferrosilicon grade be used in a bath and a cyclone?

It may be technically possible in some flowsheets, but the optimum grade is often different. Baths can commonly use a comparatively coarser medium, while cyclones tend to need greater suspension stability. Using one grade for both should be based on measured performance rather than procurement convenience.

Does finer ferrosilicon improve DMS separation?

Finer powder generally improves suspension stability, but it can also increase viscosity. Excessive viscosity can restrict the movement of fine feed particles and raise medium carryover. Improvement depends on finding the balance appropriate to the separator and feed.

Why does circulating medium become more viscous over time?

Ore fines, clay and other contaminants can accumulate in the circuit. Ferrosilicon may also undergo attrition, and parts of the size distribution can be lost preferentially. Water chemistry and solids concentration can contribute as well. Testing the circulating medium helps identify the actual cause.

How should two ferrosilicon products be compared?

Compare full particle-size distributions, powder form, chemistry, density information and test methods. Then test both products at the plant’s operating density using representative water and contamination conditions. The final comparison should include separation results, medium recovery and consumption, not only the delivered powder price.

Select a grade against measurable plant objectives

The right ferrosilicon grade for dense medium separation is the one that maintains sufficient stability without creating excessive viscosity, supports the intended cut density and can be recovered efficiently by the plant. Separator type and feed sizing establish the starting range, while representative testing confirms the choice.

If you are specifying medium for a new project or reviewing an existing DMS circuit, send DMS Powders the separator, feed, density and recovery details behind your requirement. Contact DMS Powders about ferrosilicon grade selection to discuss an appropriate product from its global DMS supply range and make an enquiry.