24 September 2003 18:08
Ground to Nano-dust
St. Petersburg engineers have developed a device capable of not only mining diamonds from rubble but also making nano-materials for electronics and biotechnologies
Irik Imamutdinov
Crushing and grinding materials are the most widely used production processes in industry and agriculture. These processes consume over 20% of all electricity currently generated throughout the world. The orbital mills developed by St Pete’s scientists, if used in the mining industry, will reduce the electricity required for the ore grinding process by half. The useful metals recovered from concentrated ores, which currently seldom exceeds 70%, may come near 100%. However, orbital mills promise to make a revolution not only in the metal mining industry but also in metallurgy and the chemical industry, not to mention in the manufacture of cement, ceramics, and paints and varnishes. Developing new materials and technologies thanks to the submicron-sized powders, which are mechanically activated in orbital mills, is an area of no less importance.
From mortars to drums
Early humans used a couple of stones to grind roots and grains, as well as minerals for paints. Then, someone invented the mortar and pestle and later on, millstones. Grinding raw materials between millstones had remained the principal grinding technique for many centuries before the rattler was invented. It operates simply: lumps of rock, loaded in a crushing cylinder that revolves on its axis, are reduced to fine particles as they rub and strike against each other.
Drum or ball mills use metal balls inside this cylinder. Thanks to the additional abrasion, the material can be pulverized more completely and grade size can be adjusted more easily, which results in a more uniformly ground powder. About 110 thousand mineral deposits are mined across the globe, and each has from 10 to 50 thousand ball mills operating on site. The most efficient mills, which can grind up to 100 tons of rock per hour, have a cylinder as big as 6 meters in diameter and up to 15 meters long. A powerful electric motor is required in order to spin these hundred-ton beasts. It is not unusual in the mining industry for a single mill to require four megawatts, and a 10-megawatt engine drives the one of the world’s biggest mills. Thus, even in developed countries, mechanical grinding accounts for as much as two thirds of mineral concentration capital and operating costs.
This technology has hit a dead-end and not only due to its monstrous power consumption. Ball mills simply cannot achieve the finer powders in demand over the last decade. As a result, this affects the efficiency of all subsequent links in the input processing chain, designed for a certain range of particle sizes. Ball mill technology fails to extract as much as 20% of useful elements from produced ores. “A classical ball mill is too stupid for intelligent grinding, but an orbital mill isn’t,” argues Vladimir Kochnev, General Director of the St. Petersburg company, Disintegration Techniques and Technologies (Rus. TTD). This is where the first continuous action orbital mills in the world were developed. “Out orbital mills will immediately improve the efficiency of ore concentration exponentially. We cannot promise that they will get the remaining 20% left after the ball mills, but we can certainly guarantee 10%.”
On two axes
Unlike ball mills, orbital mills have, as a rule, three or four drums. These drums revolve on the orbital mechanism’s central axis (similar to planets traveling round the sun). In addition each drum revolves on its long axis in the opposite direction. Due to the drum’s movement, the balls inside have a centripetal acceleration, which significantly exceeds acceleration due to gravity. Significantly, these mills can make the superfine powders, which in principle are impossible to grind in ball mills.
The idea’s simplicity makes it sound simple to build. In the early twentieth century, hundreds of engineers from the UK, Germany, France and the US attempted to build an orbital mill. But they immediately were confronted with a seeming insoluble problem: how can the mill feature continuous loading of materials into drums that quickly revolve on two axes? In the early 1950s, the first patent was issued for a continuous orbital mill in France. However, no one was able to create a working model until the seventies.
A simple solution
In the early seventies, the South African Mining Chamber provided a quite generous budget for a 15-year program to develop highly productive industrial continuous action orbital mills for the diamond industry. A short report on the work going on in Johannesburg was leaked in an industry journal, and Anatoli Leites at the Yakutia Research Institute for the Diamond Industry (Rus. YakutNIIpromalmaz) took note. The Institute’s leaders thought Leites’s proposal to create a continuous orbital grinding process for improved recovery of diamantine looked promising. Leites gathered a group of young engineers, one of which was Vladimir Kochnev.
The orbital mill feeder (a device that continuously feeds materials into a working mill) was created in 1972. “It was a simple mechanical solution,” Kochnev says, modestly describing his invention and wondering at the fact that in South Africa, an international team of engineers with German expertise and multimillion dollar budgets, failed after ten years of work.
In 1978, a prototype of the continuous orbital mill was assembled at YakutNIIpromalmaz. The mill had an output of 10 tons per hour and passed 100-hour industrial tests. Just then, another five-year plan ended and so did funding. The industrial orbital mill was put on hold.
For the sake of African diamonds
During perestroika, the demand for small laboratory orbital mills skyrocketed. They were purchased by research institutes and cooperatives that separated from scientific organizations that needed fine powders for their research. The group, led by Kochnev, didn’t have time to fill all the orders.
In 1991, Kochnev set up TTD, where the core of the Institute’s young team went. They hoped that the new economy would need more efficient and productive equipment, and the company little by little began to siphon revenue into R & D of new orbital mills for industry.
South African diamond producers rescued the company and got interested in TTD’s work. Several of Kochnev’s colleagues took two laboratory orbital mills--one continuous feeding and the other periodic feeding--to South Africa and started making money. They ground rock samples and charged a fee for demonstrating the machinery until De Beers representatives finally got interested in the continuous mill. “The core of our business relations with De Beers was verifying whether it was possible to ensure 100% diamond recovery from kimberlite using orbital mills. This was why they were originally designed,” Kochnev explains. Available grinding technologies just couldn’t do it and quite a lot of precious stones went to waste. The idea of complete recovery seems simple: a diamond is 50-300 times harder than the rock around it, so under certain conditions, kimberlite can be ground to dust while keeping diamonds intact.
Orbital mills are promising not only for the diamond industry. Other valuable materials, such as gold, can be recovered thanks to their particular properties, such as their plasticity.
On Hint’s trail
According to Kochnev, introducing orbital mills is like moving from piston-engine to jet planes. The technology is revolutionary not only because mining industry efficiency can be increased exponentially. The greatest demand for the mills is coming from the hottest segments of the new economy. New scientific and technological products need finely-dispersed and nano powders, areas that have emerged from traditional mechanical chemistry.
During highly energy intensive pulverization, an agent is mechanically activated in the orbital mills, after which its physical properties undergo significant changes. For example, mechanically activated ceramics or composites can be baked at 1,300° C rather than 1,700° C. When activated, the agent’s mechanical properties change, too. At the St. Petersburg Institute of Hydraulic Engineering, they ran the 300 grade cement through an orbital mill, and products’ strength increased to 800 grade.
Interestingly, commercial production of mechanically activated agents began in the Soviet era. Tallin scientist Johannes Hint was the first in the world to apply technologies based on mechanical chemical effects to industry, creating processes for preparing drilling mud, water-fuel suspensions, dye solutions for textiles, mineral fertilizers, and polymers as well as catalysts reduction and activation.
In 2000, Galina Chernik together with her colleagues at the Chemistry Research Institute, Elena Fokina and Nadezhda Budim suggested to Vladimir Kochnev at TTD that they develop machines for the commercial production of various activated micron, sub-micron and nano powders.
The market for such powders is small at present but promises to become huge. Orbital mills can be used in powder metallurgy and to produce light composite materials for the auto and aerospace industries, fillers for polymers, abrasives and catalysts, as well as biomarkers. Nano powders are especially in great demand in electronics and biotechnologies. Demand is growing rapidly. TTD and the mechanical chemistry group at the St. Petersburg University plan to get a piece of this high-tech market.
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