The following brief paper is intended, to apply to the concentration of the silver-lead ores at Broken Hill, New South Wales, generally, but more particularly to the process as applied to the ores in the Block 10 mine. The attached plans are similar in many respects to those prepared recently for the Block 10 Company.
Most of the information here given is already well known to those members who are conversant with the concentration practices of this field, but it is the writer’s object to give just a few of those figures which he found difficult to obtain when first engaging in practical work. At that time no text-books were at hand from which it was possible to get sufficient details for practical working purposes.
It is first supposed that the mine has been thoroughly explored and has been found to contain a lode sufficient in size and quality to warrant the erection of a concentrating plant. The points then to determine are:
- The quantity and cost of water available for steam-making and ore-dressing purposes.
- The most convenient site for the mill.
The capacity of the mill, supposing that the necessary capital is available, will be governed mainly by:
(a.) The amount of water obtainable at a reasonable cost.
(b.) The size of the ore body.
(c.) The rate at which the ore may be mined.
In the generality of cases the water pumped from the underground workings may be used for ore-dressing purposes, but it may be necessary to augment the quantity from other sources. Generally speaking, the daily consumption of water for steam and ore-dressing purposes may be taken at 100 to 120 gallons per ton of ore treated. Much of the water is used over again several times, but there is considerable loss owing to the amount of water that goes away in the tailings, slimes, concentrates.
Having satisfactorily arranged the magnitude of operations to be carried on, the next move is to select the mill site. The first point to be borne in mind is that the mill must not be situated on top of the lode, or in such a position that it will be affected by the extraction of ore from below. This is a point of vital interest, as more than one case may now be instanced on the Broken Hill field where the placing of plant on the surface above or near the lode has led to most disastrous consequences. Even though there should be a site on the outcrop ideal in all other respects, it should be rejected solely on account of its insecurity.
Having fully determined that the mill must be situated away from the lode and be out of all danger of depressions of the surface, surface cracks, or creeps, the most suitable hill should be selected. In some cases this may be situated some distance from the hauling shaft, but it is not advisable to go to a very great distance. A hill is chosen for the location of the contemplated mill in order that there shall be no further lifting of material after the crude ore has once been placed in the mill bins. If a good site be obtainable, matters can be so arranged that the work of transporting all material from one stage in the dressing to the next, until it finally leaves the mill in the form of concentrates, slimes, or tailings, can be done by gravitation. The only materials then to be elevated are the re-treatment returns. The return water, of course, must be pumped up again.
Having chosen the mill site, it will next be necessary to determine the position of the storage bins for crude ore. On the Broken Hill field the crude ore is generally broken underground to such a size— about 10 in. x 8 in.—as can be fed into a No. 5 Gates breaker. The breaker is sometimes placed at or near the brace, and here the first reduction takes place, but the breakers are often contained in the mill building itself. Where the mill is situated at any distance from the shaft, the first reduction should take place at the brace, and care should be taken to have matters so arranged that the breaker is placed below the first storage bin instead of above it, as is occasionally the case.
By tipping directly from the landing brace into the first bin, the Gates breaker is to a great extent independent of any temporary stoppages in the hauling arrangements, and the hauling need not necessarily stop on account of any slight derangement of the Gates breaker. At the foot of these bins, which should hold at least eight hours’ ore supply, there should be at least one spare breaker beyond actual requirements, for, if the mill is to be run continuously day and night, repairs must be made from time to time to the reducing machinery during working hours. By having one spare breaker this could be done without hindrance to the mill supplies.
After the ore has been reduced, by passing through the Gates breakers, to a size sufficiently small to go to the rolls, it should pass into small bins of a few tons capacity from which to be taken and placed in the mill storage bins at the top of the mill building. The mill bins should be capable of storing 16 hours of ore supplies at the least.
Many different classes of Gates breakers are now in the market, some of which are of good and others of inferior design. It is unnecessary to go into any details of these machines as full information can be obtained from trade catalogues. The main features to be observed when ordering a Gates breaker are:
- That the bearings are well protected from dust.
- That the bottom plate may be lowered and the eccentric removed without having to pull the cone and gyratory shaft out through the top of the breaker.
- That where hard material such as rhodonite has to be crushed, it is best to use a solid brass eccentric. The eccentric usually supplied is made of steel or cast iron with babbetted wearing faces and is unsuitable for crushing the hardest materials.
With regard to the crushing of ore, the mill engineer should lay down for himself the following rule:
“ Never feed into a crushing machine, material which is already small enough to pass through it without further reduction.”
If this matter is not attended to, extra work is given to the rolls and crushers, thereby causing unnecessary wear, tear, delays, and excessive sliming. In order to separate the coarse from the fines, the ore is first of all tipped over a “grizzly” or grid of inclined steel bars placed over the first bin. The ore delivered from the brace having been tipped on to the “ grizzly,” the material already fine enough to go to the rolls passes between the bars, and the large material passes to the bins in readiness for the Gates breakers. The fine material which has passed through the “ grizzly ” goes either into a separate bin ready for removal to the mill, or else runs into the same bin as the crushed product issuing from the Gates breakers.
The ore, having been placed in the mill bins, is now ready for further reduction and subsequent concentration. An up-to-date mill will be so arranged in sections that each section is totally independent of the others. By this means it is possible for the whole process of concentration to go on in one part of the mill while the rest of the mill is idle. In the older form of mill, in which this used not to be the case, much loss was sustained, as an interruption to one part of the mill often meant the stopping of all machinery for a considerable period.
Where the mill is arranged in sections, each section will probably consist of rolls, trommels, coarse jigs, fine jigs, hall mill, slime tables, vanners, spitzkasten, and the necessary elevators, pumps, &c. On leaving the bins the ore for each section passes through a conical trommel about 6 feet long and 3 feet in diameter at the large end and 2 feet in diameter at the small, having a cover made of 14-gauge iron punched with round holes 1/8 in. diameter. The trommel, supported on external rollers, makes 12 revolutions per ; minute, and about ½ h.p. is required to drive it. The oversize from the trommel is fed to the rolls, the trommelled material going towards the jigs.
Many kinds of rolls are in use at the different mills, but those constructed for the present plant are of the Cornish type, driven by gear wheels, the gear being 7 to 1. The shells for these rolls are 2 ft. 6 in. in external diameter and have coned centres, bolted together in such a way that the shells when worn may be expeditiously removed. Each pair consists of a plain roll and a flanged roll into which the plain one fits. Either manganese or toughened steel is used for making these shells, which work at the rate of 15 revolutions per minute, are capable of crushing 1,000 tons per week from 1½ in. to 1/8 in. mesh for average material, and require a driving force of 25 horse power. Such material as has passed through each set of rolls goes to two parallel trommels 6 ft. in length and 22 in. diameter. These trommels make 20 revolutions per minute, are set in an inclination of 1 in. to 1 ft., and have screens composed of 14-gauge iron punched with round holes 2½ mm. in diameter. Such material as is too large to pass through the mesh of the screen is returned again to the same set of rolls, whilst the trommelled material, meeting the trommellings from the conical machine, passes to the hydraulic classifier at the ; head of the coarse jig.
The hydraulic classifier is a cone-shaped hollow casting of iron 5/8 in. thick, is 2 ft. in diameter at the base, and 2 ft. 6 in. deep.
The apex of the cone is placed downwards, and is perforated by two holes 5/8 in. diameter for water inlet, and by one discharge hole, the stream entering by the inlets being so regulated that there is always a slight overflow at the upper edge of the cone. The feed then coming in at the top of the classifier meets the rising flow of water from the inlets; the slimes are then carried away with the overflow water, the heavier particles falling into the, bottom and finding their way through the discharge opening on to the head of the coarse jig.
The jigs, which are of the class known as May Bros.’ patent, are divided into two classes—coarse and fine. Each coarse jig , consist of 8 working and 2 tailings compartments or divisions—5 on each side—and each working compartment consists of a hopper with a hutch and separate plunger at the top. Each hutch is 3 ft. 6 in. by 2 ft. 6 in. and has a bottom composed of 6-mesh. English wove brass wire screening, which rests on crossbars of iron, and is kept in position by iron grids placed on top and bolted through to the crossbars. Each plunger is 3 ft. 6 in. long and 14 in. wide, and has a clack opening in it 2 ft. 6 in. x 6 in.
The plunger and hutch being in the same hopper, which is kept filled with water, and being separated at the top only by a wooden partition, any motion given to the water by the plunger is communicated to the material resting on the sieve. The object in having a clack and clackway in the plunger, instead of having the plunger solid, is that there shall be as little downward suction in the hutches as possible, and that the water shall have, as far as possible, a quick upward motion and slow return. In this way the ore particles are allowed to settle, more or less, according to their specific gravities, the denser particles passing through the ragging and sieve into the hopper below, the less dense being carried forward on to the next hutch. In this way the gangue eventually finds its way over the end of the fourth hutch and is removed to the tailings dump. The plunger clack is a pine board loosely bolted to the bottom of the plunger, from which it has a clearance of 5/8 in. on the upward, stroke. The use of a clack on the plunger does away with much of the classification which would otherwise be necessary before jigging. Each plunger of the coarse jigs makes 180 pulsations per minute, and each jig, requiring 2 h.p. driving force, is capable of treating 6 or 7 tons of ore per hour. The discharge of products from the hoppers goes on continuously, material from Nos. 1 and 2 being carried away to bins for final shipping to the smelters, that from Nos. 3 and 4 to undergo further crushing and re-treatment in the fine jigs. No. 5 is taken to the tailings dump.
The re-crushing of material from the last two hutches of the coarse jigs to 20 to 25-mesh is done in Krupp hall mills which make 30 revolutions per minute and require 8 to 10 h.p. for motive force. The material leaving the ball mill passes through a system of classification at the head of the fine jigs similar to that which took place at the head of the coarse jigs.
The fine jigs are the same in principle as the coarse, but are run at a speed of 200 vibrations or strokes per minute. The hutches, and plungers are smaller—3 ft. 4 in. x 24 in. and 3 ft. 4 in. x 12 in. respectively—but their number is the same, viz., 5 of each on each side of the jig. About 1½- h.p. is required to drive each jig having a capacity of about 4.5 tons per hour. Each fine jig has to deal with about 60 per cent, of the crude material which had, in the first place, been sent to the coarse jigs. The material discharged from the first two hoppers of the fine jigs, i.e., from hutches 1 and 2 on each side, is sent to the shipping product bins, whilst that from hoppers 3 and 4 is returned to the jig after having again passed through the ball mills. The material from the fifth compartment is sent to the zinc middlings dump.
The fines and slimes from the classifiers at the heads of the coarse and fine jigs are settled in spitzkasten, the coarser material being treated on Wilfley tables, the finer flowing away, and, after further classification, being treated on belt vanners.
In the mill under consideration no middle product is returned to any table over which it has already passed, and therefore the return launder and elevator on the Wilfley table have been dispensed with. The Wilfley is too well known to need any description, beyond saying that it is run at a speed of 220 to 240 vibrations per minute and has a capacity of about 1 ton per hour, for material up to 30-mesh. If, however, the material is from 30-mesh down to slimes, the capacity is lessened to about half a ton per hour. About ¾ h.p. is required for driving a fully loaded Wilfley table. Krupp tables, very similar in action to the Wilfleys, are also to be used, but all the finer slime work is to be done on Warren belt vanners.
The spitzkasten used for classifying the slimes are generally made of timber planks about 2 in. thick, and have dimensions after the following proportions:
Length .. Top 15 ft., bottom 10 ft.
Depth .. 2 ft. 1 in. at head, sloping to 4 ft. 8 in.
Breadth .. 2 ft. 8 in. at top, widening out to 7 ft. 1 in. at the end.
Two different kinds of elevators are used for elevating returned material, viz., raff wheels and elevators. The raff wheels are 14 ft. in diameter and make 15 revolutions per minute. A good form of elevator is one having buckets 7 in. x 5 in. x 5 in. bolted to a belt 8 in. wide at a distance of 15 in. from each other. The driving is done from above by cog wheels geared at 3 to 1, the bucket-belt passing round drums 2 ft. in diameter, top and bottom, and having a speed of 250 feet per minute. A good slope for elevators is about 80 degrees.
As regards the quantity of water in circulation it is estimated that for every ton of ore undergoing treatment 1,500 gallons of water are in use in the mill at the same time. The loss of water, as has already been stated, is estimated at 100 to 120 gallons per ton of ore treated. The mill circulating tanks, which are placed
at sufficient height for all mill purposes, are generally made cylindrical in form, and composed of iron, the thickness of which, of course, is in proportion to the depth of the tank. Other separate sets of tanks are used for settling the slime from the water before the latter is pumped back to the mill circulating tanks. The slime settlers are generally rectangular in plan and have a sloping bottom; they may be made of iron, timber, or masonry. The tanks are fitted on the lower side with suitable discharging doors and launders for periodically carrying away the slimes.
For a mill capable of treating 4,000 tons per week, and laid out in four independent sections, the following horse power would be required:
The above list corresponds closely in most respects with that of the machinery to be installed in the new Block 10 ore-dressing mill, the motive power of which will be electricity throughout, thus making a new departure as far as the Broken Hill field is concerned.
A mill arranged as above will probably give from each ton of crude ore 20 per cent. of concentrates, 20 per cent, of jig tailings, 7 per cent. of fine slimes, 7 per cent. of vanner tailings, and 45 per cent. zinc middlings.
A most important and yet very difficult matter in connection with milling operations is the sampling of the crude ore and mill products and by-products. The practice of “working back” for assay values is one which should be discountenanced in all milling operations. By ” working back ” is meant the calculation of the assay value of the crude ore from the values of the mill products which have been obtained by sampling and subsequent assay. For instance, the weight of the crude ore is known, as are also the weight and assay of the concentrates, middlings, and tailings; the assay value of the slimes is known and the weight calculated. From these figures an assay value is calculated for the crude ore, and from this assumed assay value the recovery of metal contents is calculated, much, as a rule, to the apparent advantage of the concentrator. The only true and correct way to calculate the recovery is to take a proper sample of the crude ore after the first crushing and before the ore has in any way come in contact with the mill water, then, having its correct weight and the correct weight and assay of all other material, a very close approximation of the true state of affairs can be arrived at. It is true that the recovery as calculated from week to week varies from the theoretical amount, being sometimes above and sometimes below, but this can only be expected, where large and varying quantities of material are continuously in transit. When averaged over a considerable period, say three months, the quantities come out very closely if due care has been observed in the sampling.
Various methods of sampling the material leaving the mill are in vogue at the present time, but it is often found that samples taken from the trucks with a “spear” are very unreliable. A good system is to sample each truck as it is being tipped and is only part full, and further to sample each dump every 24 hours. The former assays should be used only for comparison of the work done
on the various shifts, but for all official purposes the latter method of sampling should be adopted, as there is much less likelihood of error.
In conclusion, whilst admitting that there are many points in connection with the subject of concentration which are open for discussion, the writer would state that the practices as set forth in the above brief paper are the outcome of the efforts and experience of the many engineers who have given much of their time and attention to the treatment of the silver-lead ores of the Broken Hill field.