The installation of three tube mills has eased the work of the stamps and enabled them to be kept in better order than formerly without effecting repairs on Sunday, the output being about the same as before but more finely ground.
Grading Tests Before and After Installing Tube Mills. [table omitted - E]
When first erected the tube mills were run by friction rollers carrying the body of the tubes on tyres. The wear was excessive and the present drive (Fig. 8), as arranged by Mr. J. A. Turnbull, mechanical engineer to the company, was substituted in September, 1910, since when there has not been the slightest trouble. The teeth of this drive on both pinions are of ample size (10 inch face) for the power to be transmitted. The failure of bevel driving-gear is generally due to the teeth being to short. The diameters at the bases of the cones are 1 foot 8 inches and 7 foot 8 inches, a ratio of 5 to 23, and the mill starts easily when the belt is only one quarter of its width on the driving pulley. The mills are run at 26 to 27 r. p. m. The shells are ½ inch boiler plate and are 12 feet long by 4 foot internal diameter. Two are lined with H. P. Barry’s patent sectional honeycomb liner and one with F. C. Brown’s iron plate and rib liner, the object being to compare the two when running side by side upon the same product, no strictly comparative tests having yet been published.
So far as the mill-man is concerned, the cast-iron liners are the best, for should a rib or a plate break or loosen, a small leak starts promptly at the bolt holes, thus giving timely warning that something is wrong. The ribs are the first to give way and when this occurs the shell of the mill is still protected by the lining plates. With the Barry liner the contents of a quarter section may be dislodged without the defect being discovered until the mill is opened up for examination.
Further, in replacing or renewing the lining, time must be allowed for cement to set thoroughly. E. g. 3 or 4 days when ordinary cement is used. With ribbed liners a new plate or rib can be put in in half an hour or less.
The arrangements in the tube-mill house are as follow- the pulp from the battery passes to a launder behind the stamps and so to the tube-mill house. A ¼ inch screen, intercepts stray coarse material which is removed occasionally by the attendant, the coarse sands from the elevator classifiers join the pulp and after passing another ¼ inch mesh screen, the launder is divided into three equal sections feeding three square pyramidal diaphragm classifiers of the form and size shown in Fig. 9. The diaphragm is inserted near the bottom of each pyramid leaving a ½ inch space all round, as advocated by Caldecott, the object being to ensure an even thick pulp. A water supply is arranged at the throat for use when the pulp packs. Four tests were done to determine the thickness of the feed, the Brown and one Barry mill being tested simultaneously. The percentages of dry ore present were 55 and 49.5% for the first and 50 and 45% for the second mills, so that the pulp is practically 1 to 1, and fairly even in character.
The tailings from the tube mills are joined by the overflow from the three classifiers and then elevated by two bucket elevators. The buckets are 0.2 cubic feet capacity and run at 160 p. m. A clearing pit 6 foot x 6 foot x 5 foot is provided to each elevator and if the boot chokes up, the door communicating with the pit is opened so that its contents can be run out, and the elevator restarted. The pit can be cleared afterwards. Each elevator delivers its pulp to a classifier of the same design as that shown in Fig. 9. The overflow from these passes by launder to the plates in the battery house and the coarse sand returns to the tube-mill feed-launder.
Grading Tests of Tube Mill Products.
There was no arrangement to ensure that the pulp passing to each mill should be of the same thickness, and the quantity fed could only be adjusted by altering the launder divisions. However, it has been shown that the thickness of pulp does not vary greatly and the feed was divided so that the two mills appeared to get equal quantities.
Sampling was done by passing a square tin across the stream of pulp from one side to the other. The sand adhering to the outside of the tin was rinsed off and the contents then transferred to a sample bin of about 2 cubic foot capacity, only the water of the sample itself being used for rinsing out the inside of the tin. The thickness of feed was determined by measuring and syphoning off the clear water from the tailing sample and weighing the remainder before and after drying. It took just three hours to take 19 cuts from each stream, viz., feed and tailings from one Barry and the Brown mill. The weights of dry ore obtained represent from 1/280th to 1/475th of the pulp flowing through one mill during the time of sampling. The samples were allowed to settle for 20 hours and the clear water syphoned off, the remaining material being transferred to large pans and dried in an oven. The dried samples were rubbed through a 10 mesh sieve and reduced to 1/8th of the original bulk by riffles. After transport to Auckland, they were re-dried, mixed thoroughly, riffled down to about 300 grm., and then graded by sieving with standard I. M. M. screens. This sieving was performed dry until apparently finished, and the process was then completed with the aid of water. The results are given in Table E in percentages.
The amount of grinding effected by the tube mills is seen by comparing the grade of the battery pulp (screen samples) with that of the fine pulp returned from the tube-mill house (plate heads) see Table E, remembering that about 160 long tons are crushed per 24 hours. These screen and plate samples were taken over a space of 2½ hours and consist of 11 cuts across the stream at regular intervals. The dry weights of the samples only represent 1/2740th and 1/4180th respectively of the total flow of pulp in this time, but the cuts could not be made more rapidly without risk of bad work.
Grading Tests. [table omitted - E]