Dredging

Korero (AEWS), Volume 2, Issue 7, 10 April 1944, Page 21

Dredging

They tell a story on the Coast of an inquisitive tourist who asked where the Pancake Rocks at Punakaiki came from. A Coaster replied that the glacier brought them. Rising neatly to the bait, the tourist exclaimed, “ Where’s the glacier now ? ” “ Gone back to get some more ! ” shot back the Coaster.

But the inhabitants are easier on the visitors who inquire the reason for the huge heaps of stones that are often seen when travelling through the country districts of the Coast. The non-com-mittal and unqualified reply “ Tailings ” is not very enlightening, so the Coaster will go on to explain that these “ tailings ” are the spoil left after an alluvial dredge has worked over gold-bearing ground. These great piles, up to 60 ft. high, cover acres of land and are furrowed as though they had been worked with a huge plough. Desolate grey mounds, occasionally coloured with a red fungus, they are unsightly blots on the face of a green and bush-clad countryside—monu-

ments to man’s search for gold with modern machinery. Alluvial gold is carried in a gravel “ wash,” and to' free it the “ wash ” must be passed through some process which will separate the gold from the rubble. The main-spring of this process is water. Wash the gravel with water and the heavy gold will sink. The remaining sand and stone is cast aside as “ tailings.”

The old diggers in sluicing, left tailings beyond their race. Modern sluices with greater power leave behind a proportionately greater amount of spoil. But the huge gold-dredges treating thousands of yards of ground a day, leave behind them a huge hill of grey stones. Because much of the early sluicing was done in hills and creeks and terraces, these scars are not easily visible. The dredge works on flat or semi-flat land, and its despoliation is open for every one to see. If you’ve ever tried to put back in a slit trench or fox-hole the amount of spoil you took out of it (which isn’t likely), you will understand the difficulty experienced in leaving the land level. Approximately one-third of the spoil remains over the surface.

Many of the locations now worked by the dredges were previously prospected by the early diggers (some were passed over), but where the early miners were looking for rich strikes and left alone land which would require too much capital to work, or would yield only small returns per yard treated, the dredge, because of the amount it handles can be run economically if it returns 6d. per yard.

Wooden dredges were first used in New Zealand by a Chinese in Central Otago many years ago and were most successful. They have since been developed, until to-day the modern steel dredges, electrically powered, weigh 3,000 tons and handle about 400 cubic feet of ground per minute. They look very like ordinary harbour or river dredges with huge buckets that bite deep into the earth, depositing the gravel inside the dredge for treatment and ejecting the spoil on a conveyer belt at the other end. They also float on the pond they dig. Where they are working above water -

level, water must be carried to the site by means of a flume or race.

First the ground is prospected by boring and the results assayed. If the ground is found to be payable, the 12 ft. steel pontoon is erected and floated and the superstructure built. Just how huge the dredges are will be realized by the fact that on the biggest of them the stern gantry, carrying the elevator, is 104 ft. above the pond-level. A massive steel ladder carrying the bucket-chain can be raised or lowered from a huge bow gantry by large blocks, each with twenty - eight sheaves of if in. steel cable. A 250 h.p. winch controls the ladder.

The buckets, of which there are 94 on the largest dredges, each carry 18 cubic feet of spoil and tip at the rate of 2if per minute. Tearing into the shingly earth they rip the ground from top to bottom to a depth of over 100 ft. With groans and wails (they cannot be lubricated) the buckets steadily climb the ladder, and with an especial shriek crash their boulders and rubble into a huge steel hopper. A strong jet of water flushes each bucket as it deposits its load.

Inside the dredge is like the engineroom of a gigantic ship. Steel riveted plates form the deck, and strong steel girders the framework covered by corrugated iron. Long ladders lean up to the heights where the buckets can be watched as they deposit their spoil.

From the hopper the wash passes into an immense revolving cylindrical screen perforated with Jin. holes. Strong jets of water play on the protesting

stones as they slide down the angle of the screen. The finer wash drops through into a distributor, while the larger stones pass over a nugget screen of larger perforations and out on to a long elevator belt which carries them out to the tailings pile at the back of the pond. The fine wash is sent out by the distributor to the ripple tables below the screen — twelve to each side. Each 12 ft. table carries 200 ripples of angle iron on a wooden base, and over these the gravel is washed by a continuous stream of water boiling over the slight elevations of the ripples and depositing its fine gold. The distributor ensures an equal proportion of wash to each ripple table, where most of the gold is caught in the first few feet of ripples. These are locked off with a wire grid.

In the older wooden dredges the ripples are 1 in. wide, and i in. apart, and 1J in. deep. Without the distributor the first three tables collect the majority of the valuable concentrates. On the modern dredges a pulsating screen is used in conjunction with the ripples, the wash flowing first over the ripples, then over these “jig boxes,” and on to further ripples. On the older dredges mercury is scattered about the ripples to amalgamate with the gold. The modern type collect the wash,first, by lifting the ripples out and washing them, collecting the sand by shovel and passing the resultant wash through the “ streaming-down ” box over fine ripples and coconut matting. To these concentrates is then added mercury to collect the gold.

The screen of one of the smaller dredges is 35 ft. long by 5 ft. in diameter. That of a modern dredge is almost twice this size.

The method of manoeuvring the dredge differs in the newer types, which have two great steel spikes at the stern known as “ spuds.” These are over 100 ft. in length and are driven into the pond-bed alternately to provide a pivot on which the dredge swings in a 300 ft. arc.

Bowlines attached to anchors ashore are used to pull the dredge around on this pivot. The old-type dredges used head and stern lines as well as bowlines to manoeuvre because they take a straight cut. The tailings can be directed down to the spud, instead of up to the elevator, to provide firm ground to hold the spud. Huge double springs press against the spud to take the kick while the dredge is working. Up in the control room, where the winch-man controls the dredge, the

scene is similar to that of a signal-box with huge levers controlling the winches set in the floor. A clock shows the depth at which the dredge is working, while an automatic recorder shows both depth and time on a rotary graph. The noise of a large dredge is terrific. The ring of boulders on steel, the wail and groan of the buckets, the creaking of the structure as the dredge works, can be heard a mile away. Yet from these masses of boulders torn up from the depths of the earth comes sufficient gold to make dredging a payable concern. Gold that the old diggers would have scorned as requiring labour out of all proportion to the return is dug out regularly to the tune of 800 oz. a month. Admittedly, many yards of waste spoil are handled, but dredging pays well at a return of 6d. a cubic yard. The old-timer would have laughed at such a revenue. Companies with capital of several hundred thousand pounds pay dividends on it.