I THINK MINING

Home at last after nearly three weeks away in places where mining is flourishing.  Too much going out delayed blogging.  Here is one piece of advice I got while working and partying.  It pertains to control of dust on mines.

There are many patented products out there to spread on loose soil in an attempt to control dust.  This piece of insight came from a farmer, not a miner, and not a product salesman.  It is implemented with success on a tailings facility in Nambia where the wind blows over 100 kilometers an hour.

The idea is to stop the first sand grain being picked up.  For if you can do that, you stop the first grain from dislodging two more, that in turn dislodge four more, and so on in exponential growth to a full-blown dust storm.  To stop the first grain of sand, put waste rock on the leading edge of the pile, the sandy area, or the windward edge of the dust-prone deposit.  And put some waste rock on the downwind edge of the same dust-creating area.  Seems this is very effective.

If this does not entirely cure the issue, doze down a series of windrows.  Take any old handy mine machine and let the operator gouge depressions about 300-mm deep into the flat area; thereby create small piles of soil downwind of the depression; and do this every three or four meters.  Apparently the rule of thumb is that depressions at ten to twelve times the depth of the depression works best.

If you have tried and succeeded with this approach let us know.  If you have other equally cost-effective approaches, let us know.  Thanks.

PS.

Here is more on htis topic:

• I am a tailings engineer who works as a consultant to the mining industry.
• The first thing to recognise is that that dusting is really a function of particle size.  Fine, colloidal dust of a respirable size will travel large distances once mobilised.  Larger grains move by bouncing or saltating over the surface.
• In 1990 I was working with a mining company in the desert of Namibia who have a tailings storage facility of about 700 ha and who were experiencing dust issues during the windy months.  They had been battling this issue for many years putting up fences, windbreaks, scrap fishing nets, old haul truck tyres even rock at the down-wind edges of the tailings facility to try and stop the dust leaving the facility.
• I read a paper by R A Bagnold published in 1954* and learned about stopping the first grain of dust as this leads to an exponential growth in the number of saltating grains when each grain itself displaces two or more grains each time it bounces back on the surface.
• I recommended to the mine that they should place rockfill at the leading (up-wind) edge of the facility.  This was done and the principle of stopping the first grain showed merit straight away.
• Moreover, since the top surface of the facility stepped I recommended and had implemented the application of rockfill on all windward and most leeward crests.  The zones where rock was placed worked to great effect and are still in place today and are effective.
• At about that time one of the foremen on the mine responsible for tailings management read in weekly farmers magazine that windrows were very effective in controlling dust.  The suggestion apparently was to plough the lands as the ridges serve to reduce dust.  The engineering principle for this is that there is a phenomenon known as the boundary layer which is a zone just above the surface where the wind velocity is lower due to frictional drag with the surface (the very thing that lifts the particles in the first place).  If, however, this boundary layer can be thickened there will be less velocity available to lift the saltating grains.
• The foreman concerned set up a trial where he used an ordinary road grader to grade up 300mm windrows perpendicular to the wind direction and spaced these a grader-blade width apart.  This worked a treat.
• The performance was further enhanced by using an organic grain fixing agent which is a by-product from paper manufacturing.  At first it was thought that the fixing agent was the primary cause of the very significant reduction in dusting but over the years re-application of this agent has been stopped as it has been found that the windrows without fixing agent are just as effective.  These windrows have been in place for more than 20 years and are still in very good condition.
• In 2000 the environmental personnel on the mine decided they needed more science.  So Professor G E Blight from the University of the Witwatersrand in Johannesburg was retained to provide advice and do some field measurements.  Now Prof Blight is probably the real person you should have on your radio.  Anyway he had a PhD student who did wind velocity measurements over the facility and found that wind velocities at the sharp crests of the paddocks increased.  In the case of windward crests the issue is the higher velocity at the leading edge caused by the fact that the wind has to move over the crest..  In the case of leeward crests there is a drop in velocity as the wind leaves the crest and this causes suction and at high enough wind velocities this suction is enough to mobilise the grains.  Prof Blight and the student published a paper on this and I attach it for your interest.
• This tied in very well with our observations of the performance of crests covered with rock – there were a few leeward crests that were not covered in 1990 and these eroded very badly over the years.
• The PhD student also took samples of the tails back to the lab and conducted wind tunnel tests on a range of windrow geometries and spacing.  He found that the optimum was when the distance between the windrows is about 10 to 12 times the height of the windrows.
• As it turned out back in 1990 when grading up the windrows the spacing between the windrows was about 10 ie 300mm every 3m.  Quite fortuitous.