Sulphide ore of lead and zinc containing considerable silver was submitted for testing with the purpose of determining a flowsheet for the production of separate lead and zinc concentrates for marketing at their respective smelters. It is necessary to recover as much silver as possible in the lead concentrate as a higher return for this silver is realized than for the silver in the zinc concentrate. The ore contained sphalerite, a portion of which was easily floatable but difficult to depress in the lead flotation circuit.
Also, the recovery of silver minerals occurring in a lead, zinc sulfide ore is efficiently accomplished using Flowsheet #2. The process consists of selective flotation to produce a mixed silver, lead concentrate for maximum smelter return and a separate zinc concentrate. Over-grinding of silver minerals is detrimental to efficient flotation recovery, so the Flash Flotation Unit-Cell is used in the grinding circuit to recover a large part of the silver and lead values as soon as liberated.The flowsheet is for a plant having a capacity in the range of 300 to 500-tons per day.
The crushing section of this 50-65 ton mill consists of a conventional layout of single stage crushing. The mine ore is fed from the coarse ore bin to a 9x 16 Forced Feed Jaw Crusher by means of a Apron Ore Feeder. The crushed ore is conveyed by a Belt Conveyor to the Bolted Steel Fine Ore Bin. A Adjustable Stroke Unit Flotation Cell are incorporated in the Belt Ore Feeder delivers the fine ore to the ball mill.
The Mineral Jig and the grinding circuit for immediate recovery of a substantial amount of the lead and silver at a relatively coarse grind. The 5 x 5 Steel Head Ball Mill discharges into an 8x 12 Selective Mineral Jig which in turn discharges into a small flashFlotation Cell. The tailings from the Unit Cell flow by gravity to the 30 Cross-Flow Classifier. The Mineral Jig and the flashcell treating an unclassified feed, produce high-grade concentrates of lead and silver with a minimum amount of zinc. Recovery of these important amounts of lead and silver at this point not only prevents detrimental sliming of the lead mineral and possible subsequent loss, but also increases the amount of new feed that can be fed to the ball mill. By taking advantage of recovering a clean product representing a high recovery of the lead leaves only a small amount of the lead to be recovered in the selective flotation section.
This section of the flowsheet uses two 6-cell (32 x 32) Flotation Machines. The classifier overflow is fed by gravity to the first rougher cell of the lead machine. Three rougher cells provide ample contact time for the flotation of the lead. This rougher lead flotation concentrate is then delivered by gravity to the cleaner cells. Three cleaner cells are used for triple cleaning of the lead concentrate. This triple cleaning was recommended because of the easily floatable zinc that could not be effectively depressed by conventional zinc depressant reagents. Roughing, plus triple cleaning in a 6-cell machine with no pumps or elevators is an example of flexibility a distinctive feature of Sub-A Flotation Machines.
The lead circuit tailing is then conditioned with reagents in a 6 x 6 Super-Agitator and Conditioner prior to zinc flotation. The conditioned pulp is then floated in a 6-cell No. 18 Special Sub-AFlotation Machine for the production of a cleaned zinc concentrate. This machine is arranged for four rougher cells and two cleanings of the rougher zinc concentrate.
Soda ash and zinc sulphate are fed to the ball mill by means of Cone Type Dry Reagent Feeders. Cyanide, sodium sulphite, MIBC frotherand xanthate (Z-3) are fed to the grinding circuit and lead flotation circuit using a multi-compartment Wet Reagent Feeder. Lime and copper sulphate (CuSO4) are added to the zinc conditioner and pine oil and xanthate (Z-5) are stage added to the zinc rougher circuit using Wet Reagent Feeders.
The Visual Sampler, consisting of a Suction Pressure Diaphragm Pump and a No. 13A Wilfley Concentrating Table, takes a portion of the final zinc tailing. This unit enables the operator to determine visually the results of flotation. Any necessary change of reagents is immediately indicated by observation of the concentrate streak shown on the table. Many installations of the Visual Sampler have proved this unit to be a money-saving necessity in any flotation plant.
Thickening, prior to filtration, was not recommended in this case because of the rapidity at which these concentrates filtered and the relatively small tonnage of this mill. Thickening is advisable on slower filtering ores and on larger tonnages.
The final lead-silver concentrates (including the Flash FlotationCell concentrate) are filtered on the 44-disc Filter, the filter cake discharging directly into concentrate bins. The dewatered Mineral Jig concentrate is combined with the filtered lead concentrate in the storage bin.
The above flowsheet incorporates the first rule of milling procedurerecover the mineral as soon as freedthis is accomplished by the Jig and Flash Unit Cell in the grinding circuit. Note that a high-grade lead product representing 2/3 of the total lead (very low in zinc), is recovered in the grinding circuit. This flowsheet successfully answers The Problem by recovering 84% of the total silver in the lead concentrate.
The recovery of silver minerals occurring in a lead-zinc sulfide ore is efficiently accomplished using the above flowsheet. The process consists of selective flotation to produce a mixed silver-lead concentrate for maximum smelter return and a separate zinc concentrate. Over-grinding of silver minerals is detrimental to efficient flotation recovery, so the Flash Flotation Unit-Cell is used in the grinding circuit to recover a large part of the silver and lead values as soon as liberated.The flowsheet is for a plant having a capacity in the range of 300 to 500-tons per day.
The crushing section consists of primary and secondary crushing with intermediate screening. Both crushers are located in the same building and conveniently attended by one operator. A minimum of conveying equipment is required by this arrangement. Dust collecting facilities are, likewise, limited to only one building.
The crushed ore after automatic sampling is subjected to two-stage grinding using a Rod Mill in open circuit and a Ball Mill in closed circuit with a Classifier. TheUnit Flotation Cell receives the discharge from the ball mill for recovery of a substantial amount of the granular silver minerals together with galena as soon as freed. Reagents are added to the ball mill. Tramp iron and occasional oversize gangue are removed from the circuit by the Spiral Screen attached to the ball mill and this prevents excessive wear or plugging of the unit cell. The classifier is of the latest design.
The Mineral Jig is not included in the flowsheet, but on many ores of this type it is applicable either alone or with the unit cell. The grade of jig concentrate is usually very high grade and ideal for blending with the flotation concentrate. If native silver or gold values are present, the jig is a very essential addition to the flowsheet and would be used on the rod mill discharge in this case.
The classifier overflow is treated in a conventional manner using Sub-A Flotation Machines of cell-to-cell design which enables double cleaning of the silver-lead and zinc concentrates without the need of pumps. For large tonnage operations the Sub A Free Flow Machine is optional for roughing and scavenging, but the cell to cell type is always used in the cleaner circuits where high selectivity is essential. The two flotation banks are arranged so that the banks face one another and can be conveniently controlled by one operator from a single aisle. Operation of the Conditioner can also be observed from this aisle. A Sampler is used on the zinc tailing to provide an instant means for the operator to evaluate plant results. Some plants find it beneficial to use a visual sampler on the lead tailing ahead of the zinc circuit. The Sampler is also useful for evaluating the lead or zinc concentrate.General view of the flotation section at a modern silver-lead-zincmill. The lead circuit is on the left and the zinc circuit ison the right.
The silver-lead concentrate (including the unit cell concentrate) and the zinc concentrate are separately treated through wet cyclones to remove the coarse sulfides as thick underflow products suitable for direct filtration. The cyclone overflow products are ideally suited for thickening and subsequent filtration with their respective cyclone underflows. This procedure avoids any overload of heavy sulfides in the thickeners and, therefore, simplifies the operation of the thickeners. SRL Pumps are engineered for use with wet cyclones and give trouble-free service.
In addition to the feed sample, which is cut by means of a Type C Automatic Sampler, the final silver-lead and zinc flotation concentrates are sampled using Type B cutters. The final plant tailing is also sampled in the same manner.
This flow-sheet incorporates all features of a modern day mill for optimum efficiency and general simplicity for ease of operations. Instrumentation devices can be included to facilitate automatic control of the plant circuits if desired.
Many factors affect the metallurgical results of every plant. However, in a study of this type it is interesting to note the recoveries and grades that are actually being made at successful mills. The figures of these two plants are included for their value in making economic studies of new deposits.
The flowsheet above was designed to treat economically approximately 200 tons in 24 hours of a low-grade dump orecontaining lead and zinc values as well as some silver in both sulphide and oxide form. Due to varying specific gravities of the values, gravity concentration in addition to flotation was indicated. As is common in many such cases, the dump contained material over 2 in size as well as some debris. This necessitated an arrangement for scrubbing and separating this coarse material at the head of the circuit.
A Drag or Conveyor is used to bring the dump material to a Revolving Trommel Screen which discards all plus 2 material and debris. A thorough washing and some disintegration is given the ore by the trommel screen. The minus 2 material passes to a cone or Forced Feed Jaw Crusher and is reduced to approximately minus . A Cross-Flow Classifier isused to dewater this minus ore and also removes fine colloids. Classifier overflow is discarded.
The dewatered minus ore from the classifier feeds to a 6x4 Peripheral Discharge Ball Mill. This type of mill was selected to produce a minus 10 mesh product for jigging with a minimum of overgrinding. A 4x8 Vibrating Screen separates the ball mill discharge at 10 mesh, the oversize returning to the classifier for additional grinding. The minus 10 mesh screen product passes over a 16x 24 Duplex Mineral Jig. This unit recovers not only a lead-silver concentrate in sulphide form but also recovers a high percentage of the lead carbonates and lead oxides.
The jig discharges into a 6 cell No. 24 Sub-A Flotation Machine which produces a high-grade lead concentrate which is dewatered in a Thickener. A Adjustable Stroke Diaphragm Pump discharges the thickener underflow into a 6 Disc Filter for further dewatering of the final lead concentrate. Flotation tails are fed to two Wilfley Concentrating Tables for additional recovery of the oxide values. These tables produce a final tailing (which is pumped to waste by a SRL Sand Pump), a lead concentrate and also a middling product containing both lead and zinc.
The table middlings are pumped by a SRL Sand Pump to a regrind circuit consisting of a 3x6 Ball Mill, a 24 CrossFlow Classifier and a 6 cell No. 15 Sub-A Flotation Machine. The classifier overflows a product at approximately 80 mesh directly to the flotation machine where a cleaned and recleaned lead concentrate is combined with the concentrate of the 4 cell lead flotation machine.
Tailings from the 6 cell regrind flotation machine contain the zinc values which are conditioned in a 5x5 Super-Agitator and Conditioner. The zinc is cleaned and recleaned in a 6 cell No. 15 Sub-A Flotation Machine. The zinc tailings pass over a Wilfley Table which gives the operator a constant, visual picture of the efficiency of the zinc flotation machine, and also recovers additional zinc values.
Zinc concentrates are fed to a Wilfley Table to remove any lead oxide values which may have floated with the zinc, and then are thickened in a Thickener. A Adjustable Stroke Diaphragm Pump takes the thickener underflow to a 6 Disc Filter.
This flowsheet takes advantage of recovering lead minerals at a coarse size not only by means of a Mineral Jig butalso by utilizing the ability of the Sub-A to handle a coarse feed. The two pilot tables, taking the coarse flotation tailings, cut down the tonnage fed to the regrind circuit.
One of the greatest difficulties in any flowsheet is the treatment of the middling product. Early removal of the slimes and the use of a regrind circuit greatly simplifies the problem of middlings in this mill.
By far the most important lead mineral is the sulphide, galena. In many of the ores in which it occurs it is associated with sphalerite or marmatite, and two-stage selective flotation is necessary to separate the two classes of minerals. When, however, the lead mineral is the only one of importanceand such ores are not uncommonit is usually extracted by gravity concentration in jigs and on tables, followed, after thickening and grinding if necessary, by the flotation of any middlings and tailings that are too high in leadvalues to be sent to waste; direct flotation is comparatively rare except for the treatment of tailing dumps.
The flotation of galena Pb is comparatively simple. It is usually carried out in subaeration machines, presumably because the cells provide a ready means of high-speed conditioning ; the proper pre-treatment with the necessary reagents of the various products as they come from the different parts of the gravity concentration plant might otherwise be none too easy. Single or double cleaning of the rougher concentrate of the flotation section is generally necessary, this operation often being done in pneumatic cells. Because of the variations in the flow sheets of gravity concentration plants, it is impossible to indicate the points at which the different reagents should enter the circuit, but their normal consumption is usually within the following approximate limits:
Pine oil is not often employed as it tends to produce too stiff a froth ; cresylic acid is preferred, reinforced, should the froth be too delicate, with a coal-tar creosote, or, more rarely, with pine-tar oil. The usual promoters are ethyl and amyl xanthates, either alone or in combination, or else aerofloat, either alone or with a small quantity of one of the dithiophosphates. Soda ash is commonly employed for maintaining the alkalinity of the circuit, but lime is replacing it to some extent, especially in those plants where amyl xanthate or aerofloat reagents are used. Sodium silicate is sometimes added as a deflocculator when the gangue has too much tendency to enter the concentrate. Should the ore contain an appreciable quantity of sphalerite, though not enough to warrant the production of a separate zinc concentrate, it can usually be depressed by the addition of sodium cyanide with or without zinc sulphate either to the grinding circuit or to a conditioning tank. It is not often that more than 0.1 lb. per ton of either reagent is required for this purpose.
The oxidized lead minerals, of which cerussite and anglesite are the most important, can generally be concentrated by flotation after their surfaces have been coated with a film of lead sulphide by a preliminary sulphidizing treatment. Since the consumption of the sulphidizing reagent is proportional to the surface area to be attacked,it is usual, particularly with a rich ore, to table out as much coarse mineral as possible before flotation. The table tailings, reground if required to a suitable size, are treated with soda ash to precipitate any dissolved salts which might react with the sulphidizing reagent, and with sodium silicate in addition, should it be necessary to deflocculate the gangue. The pulp is then conditioned with sodium sulphide, or, more rarely, with one of the other alkaline sulphides, the normal range of consumption being 2 to 6 lb. per ton of ore. The sulphidizing contact period may be as long as 20 minutes, but in many cases it has been found possible to reduce the time to a few minutes, especially when flotation is carried out in subaeration machines, the cells of which have the advantage that additional reagent can be introduced at any point if extra sulphidizing is needed. The minerals are floated with frothing and promoting reagents similar to those required for galena, amyl xanthate being very effective with this class of mineral.