Saturday, May 10, 2014

Waterjetting 21a - Of peat, coal and New Zealand

During the historical development of waterjets there was not a lot of change from the time of the Ancient Greeks, who found water flow being used to uncover and move minerals such as gold, and the Northern British miners of the 1800’s who were still using this tool in a form known as “hushing” as a way of helping to get lead ore out of the ground. The use of a directed higher-pressure stream (even though only powered by gravity) through a nozzle, as a way of mining the gold ores of California was the first step in a major change.

In part this was because the higher pressures sped the process up considerably, and in part it was because there was more method in the way that the jets were applied, so that the operators were kept safe and away from the area where the jets were mining, and where the surrounding rock was unstable. This had advantages in other applications.

Consider that peat is a fuel that is still mined and used in various parts of the world. (As a young man I helped my father dig peat in the spring using a shovel, and then carry it back to my Grandma’s house where it would dry over the summer and be burned the following winter.) But many peat bogs are relatively unstable places, so that bringing in heavy equipment is difficult given the soft ground. Modern practice is to cut large drainage channels around the mining area, allow the water to drain out and then mine the dried peat, which is more often these days used in gardening.

However, before such large operational processes were around peat miners had used water jets (along the lines of those used in California) to break out the fibrous material, and then to move the liquefied slurry in pipelines which are easier and lighter on the land than road beds. Back in 1979, for example:
Western Peat Moss Ltd in Vancouver is currently using hydraulic mining and pumps a peat moss slurry of a concentration varying beteen 0.75 and 1.5% by mass, a distance of 3.65 km in a pipeline 0.3 m in diameter, at a rate of 340 cubic m/hour (1,500 gpm). A similar operation is being used in Alberta by Hood Manufacturing Ltd, for the mining and transport of Peat moss.

Figure 1. A peat mining operation (the peat is transported away in the pipeline. (SRS Crisafulli)

Given that peat covers about 3% of the world land surface it is sometimes surprising that more has not been made of the resource, although it is physically strenuous to mine manually, and the high water content means that it must be dried before it can be used. This is why the largest market in America has become the garden market where the costs can be more easily absorbed by the market, and modern vacuum mining of the dried material has become the more common modern method of extraction.

Figure 2. Peat distribution around the world (SRS Crisafulli).

The first mention of hydraulic mining of peat came from Prussia, and by 1914 it was used as the primary source of fuel for the Electroperedacha Electric Power Station in Russia. After the war that usage had grown to mining roughly a third of all the peat mined in the Soviet Union. (Yufin A.P Hydromechanization, State Scientific Technical Press of Literature in Mining, Moscow 1965). There was, however, another place where remote mining can have a considerable advantage.

There are many places in the world where coal beds, originally laid down horizontally, have over the course of time been tilted to steep angles. These mines are very difficult to work in, since the steep slopes can be dangerous to workers, and productivity is generally quite slow. It appeared to dawn on mining engineers in both Russia and New Zealand at about the same time, that using water to mine and move the coal might get away from many of the problems of moving men, material and supports to a working face that isn’t that stable or needed for very long. A simple schematic shows the process:

Figure 3. Early New Zealand hydraulic mining operation (New Zealand Ministry of Culture and Heritage.

In the initial operations the coal was first weakened by blasting it with explosive, and as pressures and flow rates were increased this was found to be unnecessary, and the coal could be mined from the sold. Further, although the illustration shows that a man is directing the stream at the coal face, earlier methods had the stream being directed along a face, carrying the coal from one entry to another, where it could be flumed and carried away, by the same water flow.

By a suitable choice of geometry this did not need an operator to guide the jet stream, and so the practice arose back then of firing the face, and turning on the water flow and going home for the evening. When the miners arrived back the following day the coal had been moved down to storage (or in some cases out through a lower adit and out of the mine) and the face was cleaned off. All that was needed was to insert new roof supports in the exposed part of the seam, move the hydraulic channel over to the new face, drill it, and then, at the end of the shift, blast down the coal and go home. I will confess this is a much less strenuous way of mining than the 15 yards length of blasted coal some 6 ft deep that I had to physically move onto a conveyor each day with a shovel and pick (to break up the big lumps and to drive wedges above the props to hold the roof up) back when I was an Indentured Apprentice.

There are a number of different ways in which the coal was mined, with the pattern of extraction changing with the slope of the coal seam, the thickness of the seam and the strength of the overlying rock (since as the jet mining distances grew greater the area mined was no longer supported) and I will discuss some of these in more detail over the next couple of posts or so.


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