Table of Contents
Three mills are now operated by the Consolidated Mining & Smelting Co. of Canada, Ltd.: (1) The Sullivan Concentrator, Chapman Camp, B. C. (near Kimberley, B. C.), (2) the St. Eugene Concentrator, Moyie, B. C., and (3) the Tadanac (Customs) Concentrator, Trail, B. C. Differential flotation is the only concentration method employed in all three plants. Tables are used for pilot work only.
Values in wall ores treated are as minerals of some, or all of the metals —lead, zinc, gold or silver. Occasional customs ores contain small amounts of copper. Sulfide iron content of ores treated varies from a low figure in some ores to 38 per cent, iron in Sullivan ore. Gangue minerals vary considerably. A few customs ores are somewhat oxidized and as a result present difficulties.
Flotation equipment and control are similar in the three mills, and the flow sheets at the St. Eugene and Tadanac mills, while much simpler, are modeled after that of the Sullivan Concentrator.
In this paper I will not detail treatment nor equipment in any of our plants but will describe special features in, or in connection with, all of them. I will then deal with flotation reagents, describing our use of each of them, particularly where differing from general practice. I will conclude with comments on special subjects related to differential flotation
Sullivan Concentrator
The Sullivan is a massive sulfide ore with a specific gravity of 4.35. The principal minerals are galena, marmatite, pyrrhotite and pyrite.
The insoluble content is approximately 6 per cent. The minerals are finely crystalline and their mixture is intimate.
Successful differential flotation of Sullivan ore commenced in the spring of 1920, in the pilot concentrator at Trail. During 1920, 193,804 tons were treated by this process. This operation continued without interruption and with steady growth. For the first 7 months of 1923, which concluded the pilot plant operation, approximately 1100 tons was treated daily. I believe the Sunnyside plant of the United States
Smelting & Refining Co. was the only successful and large scale operation practicing differential flotation, prior to 1920 in North America. The Sullivan ore, because of its high iron content, was quite a different problem from that at Sunnyside. The entire tonnage of Sullivan ore had to be separated into, or accounted for by three sulfide concentrate products—lead concentrate, zinc concentrate and iron concentrate.
The choice and .arrangement of equipment, and the original flow sheet for the Sullivan Concentrator were controlled by the results of the exhaustive pilot mill tests. These covered a continuous period of 3½ years and involved the treatment of 1,076,368 tons of Sullivan ore.
The Sullivan Concentrator, near Kimberley, B. C., commenced operation in August, 1923. It has been described in the technical press in the past, and while a great number of changes have been, made in treatment since commencement, I will not detail them except to show the present flow sheet. (Fig. 2.)
Treatment
Relative to the Sullivan treatment the following features should be noted in the flow sheet:
- Crushing in rolls in series in open circuit. We crush thus, from 2½ in. to “2 per cent. +½ in. and 50 per cent. —4 mesh,” at the rate of 250 tons per hr. The ore is friable, does not pack, and has a moisture content such that dust is not a nuisance. It is possible that closed circuit with screens on the secondary rolls would be more efficient; but certainly the present operation is most simple and satisfactory.
- Three-stage, fine-grinding to flotation feed with classification in each stage, is practiced. (Flotation feed was 5.1 per cent. +150 mesh and 86.9 per cent. —200 mesh, for April, 1927.)
- We use Wilfley pumps throughout. There are no elevators in the mill.
- Note the separate plant for treatment of zinc flotation middling with no return to the rougher circuit.
- We practice double-cleaning on both lead and zinc concentrate The lead cleaner tailing is returned to the original grind, and the zinc cleaner tailing is treated in the zinc middling plant. (See preceding paragraph.)
Power
Normal power is hydro-electric; 550-volt, 60-cycle current is use throughout the plant. To protect against interruption in normal power supply, three 1500-kw. condensing steam turbines are installed for use with the boiler plant. Normal heat for process is from live steam, but when turbines are in use the warm condenser water is used as a source, of heat.
Recovery
Very satisfactory recoveries are made from feed, 11.0 per cent, lead and 7.5 per cent, zinc in the form of lead concentrate containing 69 per cent, lead, 4.5 per cent, zinc, and zinc concentrate containing 3.0 per cent, lead and 49 per cent. zinc. (The pure zinc mineral in Sullivan ore is “marmatite,” containing approximately 54.5 per cent, zinc and 10 per cent. iron. It is apparent, therefore, that 49 per cent, concentrate is high grade for this particular ore.)
Until the end of 1926 there had been concentrated by differential flotation, 4,371,818 tons of Sullivan ore, and at the present time 3500 tons is being milled daily.
St. Eugene Concentrator, Moyie, B. C.
This plant (Fig. 3) was built during 1925 and 1926 for the treatment of gravity mill tailing from former operation. The tailing had been run into Moyie lake and extended from the shore several hundred feet and to a maximum depth of approximately 100 ft. The satisfactory use of a-suction dredge by the Calumet & Hecla Co. for reclaiming
tailing from Lake Linden persuaded us to adopt similar equipment. A- 10-in. Morris suction dredge was installed which discharged into a shore pond for dewatering, over which pond and adjoining mill bin a derrick with clam shell was operated. The reclaiming procedure and equip¬ment have proved most satisfactory.
Approximately 650 tons is treated daily. Concentration treatment consists of fine grinding and differential flotation. Splendid recoveries are obtained from feed containing approximately 2.5 per cent, lead and 3.5 per cent, zinc in the form of lead concentrate containing 73 per cent, lead, 3.5 per cent, zinc, and zinc concentrate containing 1.5 per cent, lead and 52 per cent. zinc.
Tadanac (Customs) Concentrator, Trail, B. C.
The first unit of this plant was built in 1917 for concentration experiments on Sullivan ore. Gravity work and the Horwood process were thoroughly investigated but were abandoned. A wet magnetic concentration method was developed and a 150-ton plant built adjacent to the flotation unit. The wet magnetic plant was later extended to 600 tons daily capacity. In the spring of 1919 some success was met with in laboratory differential flotation separation. Many difficulties were experienced and investigations forced before commercial trial in the spring of 1920. Operation on Sullivan ore until August, 1923, has been described previously in this paper.
This plant (Fig. 4) commenced operation as a customs concentrator in the summer of 1925. During 1926, 81,215 tons of customs ore from 39 different mines was milled, and present operation is about on the same scale. The flotation practice and control are similar to that at the Sullivan and St. Eugene mills, and the business is conducted in a manner similar to that of the customs mills in the Salt Lake Valley.
Flotation Reagents
A list of our “more or less” standard flotation reagents follows:
Soda ash, copper sulfate, cyanide, cresylic acid, water gas tar, coal tar creosote, xanthate, sodium bichromate, steam. In addition, or as substitutes, we use at times on certain ores one of the following: Zinc sulfate, sodium dibasic phosphate, fumol.
Soda ash is used in the three mills as a conditioning and alkaline reagent. Caustic soda and lime have been found unsuitable in our operations, to date. The quantity of soda-necessary for good work varies with the character of the ore being treated, our range being from 1 to 10 lb. per ton. The demand is usually constant with each ore. Tailing solution can generally be returned for soda and heat saving together with thickener overflows and filtrates into the circuit without noticeable fouling.
Copper sulfate is used in the zinc flotation feed in quantities ranging from 0.2 to 1.5 lb. per ton as demanded by the ore being treated. The demand is usually constant for each ore.
Cyanide is used as a zinc-iron depressant. On some ores as little as 0.05 lb. per ton is sufficient, whereas on others up to 0.3 lb. per ton or even higher is desirable. For retreatment of flotation zinc middling high cyanide up to 1.5 lb. per ton is often advantageous making possible a lead float from the zinc flotation middling. In all cases the cyanide consumption of the ore is greatly in excess of the amount used. There are cases where disadvantages from the use of cyanide more than offset the advantages, such as: (a) Gold ores or base metal ores containing gold where, solution loss of gold results; (b) cyanide has a depressing effect upon silver when associated with gray copper (tetrahedrite) and since high silver recovery in the lead concentrate is desirable, cyanide as a reagent is often objectionable with ores of this nature.
Cyanide, patented by Sheridan and Griswold, as-a zinc and iron depressant, was adopted for Sullivan ore treatment and has been in continuous commercial use since Jan. 8, 1923, which I believe was prior to its regular use by any other plant. It was of course discovered, and its merit established, in the Timber Butte laboratories, and its use by us has been under license.
We use cresylic acid and water gas tar for practically all of our ores. While we still use a small quantity of coal tar creosote a mixture of cresylic acid and water gas tar could be satisfactorily substituted. To date we have been unsuccessful in supplanting the above oils at the Sullivan Concentrator by xanthate or other reagents to advantage in spite of the accumulation of evidence elsewhere in favor of the more recent chemical reagents. Our preference for the old oils is due to their giving at least equal, if not better, results and to their being less sensitive to slightly changing conditions.
Xanthate is used only to a negligible extent in the zinc middling retreatment plant at the Sullivan Concentrator. On the St. Eugene and customs ores it is a valuable and important reagent as used in the usual way.
Sodium bichromate is used by us only at the Sullivan Concentrator. It may have particular application to this ore because of its high iron content; 0.1 lb. per ton is used in the zinc flotation feed. Bichromate has a stabilizing effect upon the zinc float and tends to offset the effect upon zinc flotation of varying lead flotation conditions. Those intimate with differential flotation plant control know that lead flotation conditions have to be changed from time to time and that these changes often have an effect on the ensuing zinc flotation. Bichromate tends to nullify these effects. It is believed also that it enhances slightly zinc concentrate grade and zinc recovery.
Heat
We heat zinc flotation feed to 30° C. on most ores including Sullivan ore. Splendid zinc results, however, are secured on St. Eugene ore when floated cold, and this is normal practice.
Zinc Sulfate
Sustained investigation of this reagent in conjunction with cyanide with most of our ores shows little, if any, advantage over the use of cyanide singly. This may be due to the usual high iron content of our ores. We have exceptions, however, among the customs ores where the combination is desirable. The use of zinc sulfate without cyanide as a zinc depressant is of particular interest on two of our customs lead- zinc ores. These contain some gold. Cyanide is objectionable. Zinc sulfate is more effective as a zinc depressant than cyanide alone or than the cyanide-zinc sulfate combination. Incidental to the use of zinc sulfate alone as above there is noticeable a considerable increase in oil consumption.
Sodium dibasic phosphate is used for certain customs ores as an alternative for soda ash. Its use in place of soda probably slightly increases silver recovery in the lead concentrate.
Fumol is used as a frother for the iron float following zinc flotation on certain customs ores.
Comments on Special Subjects
Effect of Grinding Conditions on Differential Flotations—It is generally known that in the usual closed circuit ball mill grinding with desliming classifiers such as “Dorrs,” the most efficient classification, results with a dilute overflow. It is also true that the more dilute the overflow the finer the solids contained therein, that differential flotation of lead-zinc ores usually requires finer grinding than common practice for copper ores, and that, ordinarily, to get the most efficient classification and fineness of overflow desired, the pulp density of that overflow is lower than desirable for lead machine feed. The usual practice has been to strike a compromise.
The case cited above has particular application to Sullivan ore milling. The mineral mixture is so intimate that grinding to 90 per cent. —200 mesh should be practiced for mineral liberation, and hence most efficient concentration. To secure this, classifier overflow density should approximate 25 to 30 per cent, solids. With an ore of specific gravity such as Sullivan (4.35) desirable pulp density to lead flotation is 45 per cent, solids.
We are installing the first unit of what we believe to be a novel provision to make possible ideal conditions during both fine grinding and flotation operations. The classifier overflow will be of density desirable for classification. This will be sent to Genter vacuum thickeners in which an open weave filter medium will be used. A dirty water filtrate will result but the filter rate will be very high. The filtrate will be returned to the classifier for dilution purposes and the spigot will be controlled at the density desired for lead flotation. This will go to a surge tank and thence to flotation.
Proper Treatment of Middling Products.—It has been our experience that we can return lead middling to the original grind, even to the extent of a high percentage of the total lead circulating, and maintain good results. Not so with zinc middling, particularly if zinc content of feed is high. It is our opinion that the ideal disposal for middling is to separate units for retreatment of middling products, producing therefrom their own concentrate and final tailing products. We consider concentrate products from middling plants can satisfactorily join rough concentrate products from the main circuits as feed to cleaners.
Indexes of Efficiency.—It is always desirable to have some key figure that correlates variables and gives an immediate picture of over all results. We use a number of these throughout our different operations. One of these has particular application to lead-zinc-iron ores treated by differential flotation. We conceived and applied it and during recent years its use has spread with us and its honesty is phenomenal. We call this “the three metal recovery sum.” It is the sum of the “percentage of total lead recovered in the lead concentrate,” plus the “percentage of total zinc recovered in the zinc concentrate,” plus the “percentage of total iron recovered in the iron concentrate.” You will see that the ideal or perfect “three metal recovery sum” is “300.” The closer the approach to this figure the more efficient the metallurgy. It considers grade as well as recovery. If grade of one concentrate is sacrificed to enhance recovery, recovery of one of the other metals pays for it and “the three metal recovery sum” shows it up. By dividing it by three a “per cent, metallurgical efficiency” figure can be secured.
Use of the Microscope in Differential Flotation.—The vanning plaque has lost most of its usefulness in recent flotation practice. As a general thing the zinc flotation looks after the “clean-up” of the lead in the final tailing. This is not always so of the zinc, however. The microscope in the mill for use by the millmen is of incalculable value.
At the Sullivan Concentrator, the zinc flotation operators are required to report at 1-hr. intervals their estimate of the zinc content of the final tailing. Their average for the shift is checked again with the shift assays the following day. Marvelous skill has been achieved by them, and it is a fact that they can judge final zinc tailing to 0.1 per cent. zinc. Their monthly average estimate rarely exceeds 0.05 per cent, zinc difference from control assay.