Historically fine grinding applications have used inefficient, conventional ball or Tower Milling using steel media. The effect of improved liberation was often negated by the detrimental effects of steel media on flotation. While the benefits of inert grinding have been well known there was not a practical fine grinding machine to deliver these benefits… until the IsaMill™.
The benefits of inert grinding
If a mineral or metal is immersed in water it assumes an electrical potential with respect to the water (Kocabag 1985) where the principal reactions are oxidative (of the mineral and xanthates) and reductive (of the oxygen). When sulphide minerals are brought into contact with steel media in water an electrochemical or galvanic cell is formed where the element with the highest rest potential (ie: steel media) is the anode, and the lowest is the cathode.
Oxidative Reaction: Fe -> Fe2+ + 2e- oxidation of steel media: anode
Reductive Reaction: 2e- + H2O + ½ O2 -> 2OH- reduction of oxygen: cathode
Grinding in a steel media environment has several detrimental effects:
- The Eh effect: the reducing environment lowers dissolved oxygen and Eh of slurry. Collector adsorption is Eh dependent and may require oxidation of xanthates to dixanthogen. The Eh must be increased to ensure adequate flotation (aeration may also be required after inert grinding if the newly created surfaces have a high oxygen demand).
- Oxidation of steel grinding media causes iron hydroxide coatings on mineral surfaces. This is even evident at the coarse sizes of autogenous grinding. The surface coatings reduce flotation selectivity for both coarse and fine particles. The impact is worse at finer grinds as media consumption is higher and more surfaces are created.
- Oxygen reduction on mineral surfaces promotes precipitation of hydrophyllic, insoluble metal hydroxides on the surface of sulphide minerals. This effect is more pronounced on fine particles.
Some of the flotation impacts of steel media can be overcome by increasing pH and higher reagent addition – but at a cost of overall flotation selectivity. High intensity conditions can remove some of the surface coatings, but at high capital, operating and maintenance costs.
A much better solution is to address the root cause of the problem – keep all mineral surfaces clean by using inert media. While considerable work has been done demonstrating the advantages of high chrome media in flotation and the beneficial effects of autogenous over conventional milling little commercial based work has been done on fully inert media – until recently it has not been a practical option. Grinding in the IsaMill™ with fully inert media is the next step to improved flotation recovery – improved liberation without detrimental effects on flotation chemistry resulting in good flotation performance across all size ranges.
Improving flotation performance with inert grinding
Minerals need to be adequately liberated before flotation. Grinding in a steel mill can achieve liberation, but often at the cost of poor flotation chemistry. This is because of the effect of iron hydroxides on particle surfaces. A case of two steps forward from liberation, but one step back from chemistry.
The negative effects of steel grinding are worse for fines – sometimes one step forward from liberation, but two steps back from chemistry. As a result, most plant operators believe that "fines don't float". Plant size – recovery graphs often have the classic "hill" shape – high recovery in the mid sizes and low recovery at the fine and coarse ends.
In fact, fine particles can float very well in conventional cells, achieving over 95% recovery down to 1 micron at high grades. The problem with fines in most plants is a combination of the steel grinding effects and circuit design. Most plants treat all sized particles together, as if they had the same properties. But they don't – fines tend to have more surface coatings, need more collector, and need more time to float. But when mixed with coarse particles with clean surfaces, the operator can't achieve the right conditions for both coarse and fines. This is compounded if there are mid-sized composites that have to be rejected in cleaning to achieve target concentrate grade – the composite may have more exposed valuable surface area than the liberated fine, so rejecting it also rejects the fines. A final complication occurs if the plant then sends this back to mix with fresh new feed at the head of the roughers, building big circulating loads of particles with very different needs.
In contrast, fine particles can be the best performing part of your circuit if they are treated properly. They are fully liberated, so if they are given the right surface chemistry and you protect against entrainment, you should expect very high recoveries and grades, using much less reagents.
The key design principles for good fines flotation are :
- Don't grind anything more than you need to. Grinding is expensive, so only liberate what you need to in each stage, and recover mineral as soon as it is liberated. Often you can grind coarser before roughing and then regrind the lower tonnage of rougher concentrate. Staged grinding significantly reduces total power consumption, and is easy to do with the IsaMill™ - the IsaMills small footprint means it can be installed easily in several locations throughout the flotation circuit. Further, the inert grinding improves circuit chemistry, not harms it.
- Use grinding to improve surfaces, not harm them. Staged regrinding is much less attractive with steel grinding because it harms flotation chemistry. The inert grinding in the IsaMill™ changes this – you can grind exactly where you need to quickly producing fresh surfaces. Two steps forward for liberation, and two steps forward for chemistry.
- Float minerals in narrow size distributions. This doesn't mean "sand-slime" circuits, it means designing for the needs of different minerals in different parts of the circuit. The staged grinding approach achieves this – recover minerals as they are liberated, then regrind the remaining minerals and recover them. The very sharp size distribution produced by the IsaMill™ further assists flotation.
- Minimise circulating loads, and open-circuit as much as possible. If a mineral isn't recovered early, it either needs more liberation or better surface chemistry. Both can be achieved in the IsaMill™. Don't send fine particles back to the head of your circuit to mix with fresh coarse particles. They float very well if you tailor the conditions just for them in their own flotation area.
- Take care of entrainment. With clean surfaces and the right collector, valuable fines float very well in any flotation cell, however fine gangue particles will also be recovered by entrainment with water. To minimize diluents, circuit design should minimize flotation time (ie speed up fines kinetics with small bubbles), minimize flotation density in conventional cells, or use froth washing. A combination of a high intensity froth washing device like the Jameson cell with conventional cells can be a powerful way to achieve the combination of concentrate grade and recovery for fines in a small footprint (www.jamesoncell.com).
The principles described above have been known for decades. What was lacking was the tools to apply them. Steel grinding mills haven't changed in the last 80 years, they just got bigger. The IsaMill™ has changed this. Now you can install up to 3MW of grinding power in a small footprint in the middle of your flotation circuit producing a sharp size distribution without cyclones. Inert grinding improves chemistry – faster flotation rates with less reagents. Staged grinding allows coarser grinding up front, lower circulating loads, total grinding power and required flotation capacity.
A new tool to apply old principles. Enabling smaller plants, using much less energy, and getting better performance – a transformation in circuit design.