High-Intensity stirred milling for Energy-Efficient Fine & Ultra-Fine Grinding

There have been groundbreaking findings from case studies of mining projects where stirred milling techniques have been used as an alternative to tumbling ball mills in fine and ultra-fine grinding applications. Thus far, the results gathered substantiate that there are huge opportunities for energy savings as well as improved liberation and recovery in concentrators of metalliferous ores. 

Energy constitutes the biggest component of a mine’s operating costs, which significantly impacts the bottom line. That’s why mining companies are in continuous pursuit of energy-efficient techniques to be used in concentrator circuits.

This is no exception with fine and ultrafine grinding requirements for effective liberation and recovery of metalliferous ore in concentrators circuits. It is emerging that what is considered the traditional technique of using tumbling ball mills falls short, necessitating the exploration of viable alternatives.

Interestingly, there has been a growing preference for high-intensity stirred milling technologies, as an energy-efficient and effective grinding option. Stirred milling uses very inert high-intensity attrition and small media size. Typically, cheap natural products (local sand, slag and ore) can be used as grinding media.

It has to be pointed out that reference to energy efficiency in high-intensity stirred milling in this context is the optimal use of energy in various concentrators.  Doubtless, one would appreciate the relevance and business case of stirred mill technology better by first analysing the shortcomings of tumbling ball mills.

Shortcomings of traditional techniques 

There has been extensive literature highlighting the inherent limitations of tumbling ball mills in fine grinding. Two papers expound on this subject vividly from energy consumption and efficiency perspectives. The two papers are  Technology Selection of Stirred Mills for Energy Efficiency in Primary and Regrinding Applications for the Platinum Industry and Fine Grinding as Enabling Technology – The IsaMill.

•   Energy consumption

1. Ntsele, Metso Minerals South Africa (Pty) Ltd, and J. Allen, Metso Minerals Industries USA are authors of Technology Selection of Stirred Mills for Energy Efficiency in Primary and Regrinding Applications for the Platinum Industry. They mention the alarming increase in electricity costs contributing to high operating costs. Worse still, the declining grade quality of ore bodies means that primary grinding circuits are overexerted to use more power to process more feed with smaller output.

There is a more compelling business case for energy-efficient processing, especially platinum and gold mining, where, traditionally, tumbling ball mills are used in grinding. This is when one considers statistics on energy consumption.

The two authors estimate that, in gold and platinum mining, comminution (crushing and grinding of ore) uses about 30 per cent of the energy and 19 per cent in processing activities within a concentrator.

•   Limitations in grinding  

J.D. Pease, M.F Young, D.C. Curry, three specialists from Xstrata Technology, are authors of Fine Grinding as Enabling Technology – The IsaMill.  Their accounts are gathered from extensive experience in mineral processing plants in which high-intensity stirred grinding technology replaced traditional techniques.

They raise two points – the limitations of tumbling ball mills to grinding within bigger microns and how they impact the leaching process.

The traditional method of fine grinding of coarse particles using tumbling ball mills has proven to be inefficient below 75 μm. The authors observe: “Below 30 microns the advantage of stirred milling becomes dramatic.“ In fact, there is evidence that balls cannot produce 10-micron particles at whatever level of power consumption. 

However, every grinding task must be approached according to its own merits, as ore bodies have unique characteristics. Typically, fineness is dictated by the liberation characteristics of the ore body treated. 

Furthermore, using tumbling ball mills in the ultra-fine of grinding pyritic concentrates of precious metals can be onerous, time-consuming, and ultimately, costly in leaching. The three authors demonstrate this drawback: “The use of steel media, however, can still be detrimental to a leaching process. When fine-grinding pyritic concentrates of precious metals, it is common to follow the fine-grinding stage with a pre-aeration stage to remove active pyrite and pyrrhotite before cyanidation. Worn steel media in the ground pyrite can significantly increase the pre-aeration time needed.”

Clearly, the process of pre-aeration would consume more energy, and effective ultra-fine grinding can forestall this.

Sufficient grounds for high-intensity stirred milling technology 

Cases bearing testament to the shortcomings of tumbling ball mills are numerous. Nevertheless, the abovementioned examples present sufficient grounds for the adoption of grinding technologies that produce particles from 20 μm to less than 10 μm to enable increased liberation and recovery.

However, the question is: What options are at the disposal of mining companies?

There could be different technologies that are employed to enable energy-efficient and effective mineral recovery and liberation in ultra-fine grinding in concentrators. However, based on documented successes globally, no doubt, stirred milling technology has grown in prominence as an effective alternative.

High-intensity stirred milling technologies have been installed platinum and gold mines, where it has excelled in ultrafine grinding of coarse particles. Small wonder, bouyed by this success, it is being adopted in metalliferous mining projects worldwide.

Achieving Energy-efficiency  

A South African mining journal quotes Nsele highlighting what makes stirred mill technologies used in ultra-grinding applications energy efficient: “An ultrafine grinding mill is particularly more efficient because of the energy intensity available with each unit volume. Its normal energy input ranges from 5 kWh/t to 100 kWh/t, which meets the majority of metalliferous fine and ultrafine grinding requirements.”

Specifically, among others, based on results, energy efficiency is achieved in the following ways:

• Reduced circulation loads (less pump and flotation energy)

• Increased concentrate grade (necessitates less fuel consumption in smelters).

• Improved classification eliminates the necessity of grinding, which also saves energy.

Besides energy efficiency, there are other benefits of high-intensity stirred milling technology. The following are relevant in this context:

• Liberation impact (eliminating the need for dissolution of host mineral) and significant improvement in recovery due to improved classification.

• Lower consumption of floatation reagents after inert milling, saving costs.

Viable option 

On the whole, from the aforestated, SMD stakes its claim as a viable option in meeting energy efficiency, improved recovery and efficient liberation in gold and platinum processing in concentrators. Ultimately, a mine can generate additional revenue from existing deposits.