SUCCESSFUL DREDGE

Wanganui Chronicle, Volume LXXXIII, Issue 19512, 15 February 1926, Page 2

SUCCESSFUL DREDGE

MOST POWERFUL IN NEW ZEALAND. | DIFFICULTIES OVERCOME. | Some notes on the American type of I dredge operating at Rimu Flat on the | West Coast were presented to the Minting Conference at Dunedin in a paper !by Mr J. F. Downey (Inspector of I Mines). ! Mr Downey stated that the Californiian dredge of the Rimu Gold Dredging Company at Rimu Flat, near Hokitika, was the most powerful yet put into commission in New Zealand. It also presented in construction and method of working a number of features novel to dredging in this country, and represented the last word in modern dredge design. It was built during the period followwing the war, while high price.s ruled for steel, and could be installed now for considerably less than the sum mentioned. Measured by American standards it was not an unusually large dredge there being a number at work in America greater in size and capacity. With the exception of a ! similarity in the size of the pontoon •this dredge differed greatly from those with which New Zealanders had been 'familiar. The most novel features ; wore the application of electricity for all power purposes, the perfect and : easy control of nearly all operations 1 from one central position, the absence of a link between the buckets, the use □f a conveyor belt for stacking, and he employment of spuds instead of head-lines to keep the dredge up to the working face. Every effort was made to provide that all parts subject to i severe wear or strain should be of a i size, strength, or quality of material i permitting them to meet the utmost [demands that could be made on them. [The main drive as well as all auxiliary I machinery were all operated by electric ! motors, as were also a sawmill and a number of machines in the blacksmith j and fitting shops. The current was [ generated at Kanicri Forks, at the 'power station of Kanieri Electric Limited ((a subsidiary company), but pun fortunately, the total generating capacity of the power station was limited. and. although the whole of the

motors seldom worked at one time, the ■apply of current had not been entirely adequate for the needs of the drudge, and consequently its efficiency had been somewhat impaired. The digging ladder, of plate-girder construction, was 135 ft long, and was designed to dig to about 46ft below water level. In actual work a face of G9ft was dredged, but below water level the depth for which it was designed could not be exceeded. There I were 73 buckets of 10 cubic feet capacity on the ladder, the rate of delivery was 19 buckets per minute, and no link was used between the buckets. The [stacker was 135 ft long, and was of lat-tice-girder construction, with the lower end supported at the stern of the boat so as to allow it to move up and down and also from side to side. The screen was of the revolving kind, 4Gft in length, with an internal diameter of 7ft. What was known as the “spuds” might be described as large steel piles, working in guides, and capable of being raised or lowered with ease. There were two of them, situated at the stern, one on each side of the outlet end of'the i screen.. Each spud was 56ft in length, 'and weighed 18 tons. In the ordinary course of working only one was used at a time, and it was made to serve several purposes. The chief of these was to take the place of the head-line customarily used to keep a dredge up to the working face, and it was claimed that they did this more satisfactorily than the older method. The pumps were of centrifugal type, directly coupled to motors of all-enclosed design, : the lii <*;h pressure having a working

head of about 60ft and the low pressure of from 20ft to 30ft. The firstmentioned supplied the water for the spray pipe and pets in the screen, and the latter for the tables. The main winch had eight drums of cast steel, four of which were for operating the corner lines, two for operating the spuds, and two for head line and spare.

For saving the gqjd the dredge was fitted with 4800 square feet of tables—24 on each side of the dredge, or 48 in all. It was doubtful if such a very large spread of tables was necessary. The Rimu gold was not of excessively fine character, and nearly the whole of it was caught on the upper tables quite close to the distributor box.

In opening out the Rimu area a cut was first made for about three-quarters of a mile to a width of 200 ft. This was what was known as a single-width cut. On the return trip a double-width cut of 400 ft was taken, the object being partly to provide a bigger pond to manoeuvre the dredge in, and partly to avoid, as far as possible, any rehandling of tailings. From the time the dredge started up till the end of September last the total working hours numbered 30,048, of which 22,507, or 75 per cent., were actually spent in digging. The proportion of digging time had varied considerably. For as much as a year it had reached as high as 86 per cent., but over the whole period delays of one kind or another had reduced it to 75 per cent., but to have secured even this proportion for four full years was in itself an achievement.

The dredge during the whole period of operation had turned over an average of 1,365,000 cubic yards annually, and as it was only designed to treat at most 1,500,000 yards per annum, it had fulfilled in very satisfactory degree the hopes of its designers. The value of the gold won had been £183,028, equal to 8.05 d per cubic yard treated. In recovering gold to the value of 8.05 d per yard the dredge had accomplished all that was expected of it, and as the total working costs, inclusive of administration, had been only 5.5 d per yard, no further proof seemed to be needed that for the work it was called upon to perform the dredge had shown itself eminently suitable. At the present rate of progress it would take about 40 years to work out the property. A good part of the area was much deeper than any yet worked by the dredge, some of if reaching 75 feet, and carrying values well above the average. Whether or not another and more powerful dredge would be put. on to work these portions was yet undecided, and it seemed that the question could not well bo decided pending the discovery, within a reasonable radius of Rimu Flat of a further payable area. The putting on of a second dredge entailed the provision also of a new and costly hydro-el'ctric power plant and, while the ground now being worked would afford profits to justify the ’present dredge plac?d on it, it was open to question if it would yield profit to an extent sufficient to warrant the installation of a second dredge, plus the power station. Failing the putting of a second dredge on the Rimu area, the present one could be made to work the deeper ground all that was necessary being to lengthen the ladder, making it about 30 feet longer than it was now. This lengthening would not affect the stability of the dredge, for the bow of the pontoon now carried 30 tons of ballast to keep the boat down forward and their weight would just about equal the extra weight that would be given to the ladder by the alteration. Mr W. F. Sligo said the paper was of very considerable importance to the people of Otago. At the time of the dredging boom a good deal of gold was won but there were many failures. He had a look at the Rimu Flat and came to the conclusion that there was no chance of any New Zealand dredge handling the material there. Many of the failures in dredging were attributable to the fact that the machines were incapable of doing the work required. Several of the returns showed that the values were there, aud it was only a question of power. Electric power was now available from I Roxburgh to Cromwell, and there | seemed to be considerable inducement to give the Molyreaux another trial with a dredge like that working ou the Rimu Flat. |