SOME DEFINITIONS AND PERSPECTIVES
We often note that communication between different operating plants is confusing because we all use different definitions of “coarse”, “intermediate” and “fine” particles. An operator of a “coarse” grained orebody may call minus 37 µm “slimes”. To an operator at Mount Isa or McArthur River this is gravel. To them, anything above 20 µm is coarse, between 10 and 20 µm is intermediate, and less than 10 µm is fine or ultrafine.
We avoid using generic terms like “fines” in this paper. But if you hear someone saying that “fine” particles don’t float well, ask them two questions:
- What do they mean by “fine”?
- Are they aware that in the last decade, Mount Isa Mines and MRM have produced over 10 million tonnes of concentrates at an average sizing less than 10µm, at over 80% recovery, in conventional flotation cells, with a simple xanthate reagent system?
Particle surface area per tonne increases rapidly as size gets finer. One tonne of 7 µm particles has 5 times the surface area of a tonne of 37 µm particles; for 2.5 µm particles the surface area triples again. This explains why grinding energy increases exponentially as grind size decreases (Figure 3), and why finer media with higher surface area is needed for grinding below about 30 µm. It also explains the higher collector need of fines.
In this paper reference to a 7 µm particle means a single particle of that diameter. By convention a grind size of 7 µm refers to the 80% passing size (P80). For example, the regrind size at MRM is 7 µm, so only 20% of final product is 7 microns or coarser. Fifty percent weight of MRM concentrate is below 2.5 µm. Since flotation works on individual particles interacting with bubbles, consider this from a different perspective – 50% by weight less than 2.5 µm means that 96 % of individual particles recovered at MRM are less than 2.5 µm ! In spite of popular perception, fine particles float very well indeed.
ULTRAFINE GRINDING CIRCUIT DESIGN
If you have to grind below 25 µm, then you need to choose the right equipment. Three issues are particularly important:
- power efficiency
- classification within the grinding circuit
- the impact of grinding on flotation performance
- Power efficiency is demonstrated by Figure 3, comparing the power required to grind a gold ore in a ball mill with 9 mm balls with an IsaMill with 2 mm media. The IsaMill is much more efficient below about 30 µm – to grind this ore to 15 µm would take 28 kWh/t in the IsaMill, but 90 kWh/t in a ball mill. Traditionally this has been attributed to the difference between attrition grinding and impact grinding. However by far the most important factor is media size, as shown by Figure 4, which shows the breakage rate in Tower Mills drops dramatically – the breakage rate for 20 µm particles is ten times lower than the rate for 40 µm particles. Even though the Tower Mill is full attrition grinding, practically it is constrained to using relatively coarse media, 9mm balls in this case. In contrast, the IsaMill (Netzsch mill in Figure 4) can operate with much finer media and much higher intensity of power input (Table 1), meaning the peak breakage rate occurs at 20 µm, and doesn’t drop as quickly below that.
Good classification is vital for power efficiency in ultrafine grinding, just as it is in conventional grinding. However it is not generally practical to use cyclones to closecircuit a grinding mill with a target below about 15 µm. To get good cyclone efficiency at these sizes requires small cyclones, eg two inch (50 mm) diameter or smaller. This is virtually inoperable on a large scale, so the circuit is either compromised (and less power efficient) by using bigger cyclones, or an alternative solution is needed. The IsaMill achieves this by the internal classifier mechanism, using the high centripetal forces generated inside the mill to classify the discharge, ensuring a very sharp product size without external cyclones. The very short residence time in the IsaMill also minimises “overgrinding”, further contributing to the sharp product size distribution. As an added advantage this mechanism also retains fine media very effectively, meaning that low cost media can be used, eg local sand, or granulated smelter slag.
A cautionary word to those designing circuits – the benefits of good classification on power efficiency and media retention does not show up in laboratory tests. These tests are done in batch mills, and many technologies will show the same power efficiency in a closed device. The crucial questions are, what is the power efficiency, media retention, and product size distribution in a full-scale continuous installation.
Managing the impact of grinding on flotation performance is the third crucial factor in plant design. Even if you can accept the low power efficiency of a mill with steel balls, you may not be able to deal with its impact of surface chemistry.
Consuming so much power in a steel environment means high retention time and lots of steel contamination. The resultant low pulp potential changes flotation behaviour, requiring additional reagents and reducing selectivity. One early response to this problem was to use High Intensity Conditioning (HIC), eg at Hellyer, to reverse the negative impact of Tower Milling on surface chemistry. Processes like IsaMilling are far more efficient by providing this high intensity as part of the grinding action, and grinding in an inert environment. Later we will show how IsaMills significantly improved the flotation behaviour of ultrafine particles at Mount Isa.
FLOTATION CIRCUIT DESIGN AND OPERATING STRATEGY
The view that “fine” particles don’t float is caused by circuit design and the constraints of operating strategy. Simply, flotation works best when applied to narrow size distributions. A 5 µm particle has 10 times the surface area of a 50 µm particle, and fundamentally different hydrodynamics. Yet often our circuit designs assume they will behave the same, and treat them together in flotation. Texts as old as Taggart described the benefits of “sand/slimes” splits into separate circuits. This simple concept has been largely ignored in the push for circuit simplification and larger flotation cells.
We are not advocating complicated flotation circuits. However if you have fine-grained minerals then you must design your circuit to suit the needs of fine particles, not coarse particles. The Mount Isa circuit developed into an excellent balance of the needs of different minerals, relying on several stages of grinding and flotation. The design principals are:
- Don’t grind anything more than you need to. Fine grinding is expensive – technically the best solution for Mount Isa would be to grind everything to 12 µm, but this would not be economic. Therefore stage grind and float to suit the mineral behaviour – at Mount Isa this means a 37 µm grind before roughing. Some mineral is liberated at this size and can go to concentrate. Other minerals in rougher concentrate need to be ground to 12 µm. Some of these are rejected in cleaning and need to be reground to 7 µm.
- Float minerals in narrow size distributions – this happens automatically with the staged grinding approach described above, and is assisted by the inherent sharp size distribution produced by the IsaMills.
- Minimise circulating loads, and open-circuit as much as possible – this is another automatic outcome of staged grinding. It is pointless to recirculate a composite particle unless you are going to grind it to liberation. If you do regrind it, you should now float it separately with similar sized particles.
These principles can be seen in the simplified Mount Isa flotation circuit in Figure 5. Though the circuit may appear complicated, it is better than the alternatives of either:
- Grinding everything to 12 µm and floating together (too expensive)
- Recirculating regrind products and trying to float them with coarser minerals (causing poor performance of the reground minerals, high circulating loads and low recoveries).
Contrary to appearance, these developments at Mount Isa greatly simplified circuit operations. Lead recovery increased by 5% and lead concentrate grade by 5%, zinc recovery by 10% and concentrate grade by 2%. More surprisingly, reagent needs dropped, circulating loads dropped, and the circuit became far more stable. Flotation suddenly became as easy and predictable as the textbooks say it is!
COMPETING FLOTATION RATES OF DIFFERENT PARTICLE SIZES
The profound impact of a narrow size distribution to flotation feed is explained by mineralogy and operating constraints. In a system with just pure liberated sphalerite and quartz, flotation could achieve good recovery in all size ranges, even though the “fines” have slower flotation rates. But in real circuits there are two crucial constraints:
- Other contaminant minerals such as pyrite and pyrrhotite also exhibit some floatability. If so, a “coarse” pyrite particle may have the same flotation rate as a “fine” sphalerite particle.
- Composite particles. To explain the problem with composites, imagine a simple 37 µm sphalerite-quartz binary. This particle has to be rejected since typically zinc concentrates must be less than 3% silica. The low collector and high depressant needed to reject the 37 µm composite will also depress the slower floating 10 µm liberated sphalerite particle.
Some further problems arise when floating coarse and fine particles together:
- It is no point depressing the 37 µm composite particle unless you can liberate it. While plants often “send it to regrind”, this is often a conventional ball mill or Tower Mill that has very low breakage rates on sub 30 µm particles. This causes high circulating loads of composite particles.
- The high circulating loads then take up volume and reduce residence time in roughing and cleaning. Since fine particles are slower floating, this drop in residence time further hurts their recovery.
- If high pH is used for depression, and lime is used to get high pH, then a surface chemistry problem is introduced. Circuit water can become super-saturated in Calcium ions. This leads to reaction with sulphate ions, which causes gypsum to precipitate on the nearest surface – usually a mineral particle. SEM work at Mount Isa before IsaMills showed that up to 80% of sphalerite surface was masked by gypsum. This has a more serious effect on sub 20 µm particles.
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