During the last ten years great strides have been made, on the Mesabi range, in the practice of beneficiating low grade iron ore material. By beneficiation is meant all methods of removing impurities, and raising the iron content to a point where it can be sold in open market, the principal impurities being silica and moisture. The general processes to which low grade iron ores are amenable are as follows:
(a) Drying; removes hydroscopic or atmospheric moisture.
(b) Calcining; removes carbon dioxide from iron carbonate, molecular water from hydrated hematites, and atmospheric moisture.
(c) Roasting; removes sulphur, carbon dioxide, molecular water and atmospheric moisture.
(d) Agglomeration; primarily for the purpose of preparing finely divided material for blast furnace; briquetting and sintering.
(a) Screen sizing; removes rock and sand.
(b) Classification; removes sand by means of currents of water of varying velocities.
(c) Log washing; removes fine sand.
(d) Jigging; removes larger particles of impurities than is possible by log washing. Certain types of jigs remove fine sand.
(e) Reciprocating tables; recover fine iron particles from sand discarded by above processes.
(f) Magnetic separation; applicable to the commercial separation of the magnetic oxide of iron from gangue material. From a scientific standpoint it is possible to separate certain hematites and limonites from their gangue.
(g) Miscellaneous processes; comprise dry concentration, electro-static separation and other processes.
The economic features that bear upon the commercial success of any process of beneficiation are:
- Character and size of ore body, and the proportion of merchantable or direct shipping ore to ore that can be treated.
- Results of treatment tests, recovery, etc.
- Cost of mining.
- Cost of beneficiation.
- Cost of marketing.
- Grade and market value of product.
The profit per ton that can be made by beneficiation will be small, and it is only by handling large tonnages that the proposition becomes financially attractive.
Various estimates of the exhaustion period for Lake Superior merchantable ores run from thirty to fifty years, but there is an almost inexhaustible supply of low grade ores awaiting development.
At this time four of the above methods of beneficiation, i. e., Drying, Screening, Log Washing, and Table Concentration are in use, and a plant for a fifth process, magnetic concentration, is now under construction.
Drying plants may be seen at the Brunt mine, at Mountain Iron, and at the Whiteside mine, at Buhl, the latter being inactive at this time.
Crushing and screening plants are to be found at the Leonidas mine, Eveleth; Morris, Albany, the Buffalo-Susquehanna, the Warren and Webb mines at Hibbing.
The washing plants are located at the Leonidas mine, Eveleth; Webb mine, Hibbing; Hawkins, La Rue, Quinn-Harrison,
Crosby, Shada and York at Nashwauk; the Draper, Majorca; Hill Annex at Calumet, Patrick at Pengilly, Danube at Bovey, and the Trout Lake concentrator at Coleraine.
The Brunt Ore Drying Plant. In 1910, M. A. Hanna & Co. built an experimental drying plant at the Hollister mine, Crystal Falls, Mich. This was the first plant of this nature built in the iron districts of Lake Superior. The ore to be dried was of very painty nature, being high in alumina, and the moisture content was in the neighborhood of 18 to 20 per cent. The capacity of this plant was approximately twenty tons per hour. The type of dryer used was what is known as the Cummers Patent Dryer. Considerable tonnage was handled through this plant with satisfactory results. By eliminating a part of the moisture the percentages of the other ingredients of the ore are raised in proportion to the amount of moisture abstracted. It was found by drying the iron ore that such grades as were a few points below the required iron content could be raised to grades that would be accepted as merchantable ore—in other words, taken out of the lean ore class. Many difficulties were encountered at the Hollister plant in the mechanical handling of the materials. As iron ore is of an extremely abrasive nature, the wear and tear on conveyors, elevators, chutes and any other apparatus with which it came in contact was very excessive. This plant was finally dismantled, but not until the ore at the property had been shipped.
In 1911, the above company began building a dryer of much larger proportions at the Brunt mine, Mountain Iron, Minn. The capacity of this plant was to be 120 tons per hour. The first year of its operation only two units were installed, each having a capacity of forty tons per hour. These also were the Cummers Dryers, and were designed especially for iron ore. A great many mechanical difficulties were enconutered in this installation, due to the stickiness and the high moisture content of the ore to be handled. After the first year’s, operation it was found necessary to practically tear down the whole plant and rebuild it along different lines, as the experience gained in the first year’s operation proved that the equipment was entirely inadequate to cope with the situation. The wet ore bins were changed; the feeders required an entirely different arrangement. To take the place of conveyor belt, a bucket conveyor was installed, and the whole plant was changed over from steam driven equipment to individual electric motor drives. This same year two more dryer units were installed. These were of the Ruggles-Cole type. Each of these machines had a capacity of twenty tons per hour. Again an endless number of mechanical troubles were encountered with this equipment, the gear drives, etc., being entirely too light for the work imposed upon them. These were enlarged and rebuilt. The wear and tear on this equipment was very severe, and these dryers were practically rebuilt throughout with the exception of the outside shells. At this time large dust collectors were installed for collecting the dust which was carried over by the fans from the dryers. The theory was that the ore should be dried down to a moisture content of 4 or 5 per cent. The first cargoes shipped to the lower lake ports gave a great deal of trouble due to the dust which was given off during the handling, and finally the moisture content of dried ore was increased to between 8 and 11 per cent. When the ore was dried to this moisture content very little dust was given off from the dryers; consequently the use of dust collectors was discontinued, the fans discharging directly to the atmosphere through short stacks. The continuous bucket conveyor was finally discarded on account of the high cost of upkeep and delays caused by breakage and repairs. This was taken out and replaced by continuous bucket elevators which are giving very satisfactory results to date.
The ore that is being dried at the Brunt mine contains from 16 to 22 per cent moisture and is reduced to about 9 or 10 per cent moisture content. The furnace men are very well pleased with the structure of the ore after it has been dried, as in drying it nodulizes or rolls into small pellets, which makes a very satisfactory furnace ore. At the same time that the dryer was being built at the Brunt mine, the Shenango Furnace Company installed a large plant at the Whiteside mine, at, Buhl. This plant consisted of four large Ruggles-Cole type dryers, each having a capacity of thirty tons per hour. The location for the Whiteside dryers was ideal as the ore was transferred through the plant entirely by gravity, no conveyors, elevators or other ore handling equipment being necessary outside of bins and
feeders. The Whiteside plant experienced considerable difficulty similar to the Ruggles-Cole units at the Brunt, the gear drives, etc., being too light in design to stand up under the severe work. A new driving mechanism was finally installed at this
plant and some very satisfactory results were obtained. At the present time this plant is not in operation.
In iron ores which have a natural iron content of 45 to 48 per cent, and a moisture content of 14 to 20 per cent, an abstraction of 2 per cent moisture will increase the iron content approximately 1 per cent. Thus, if from an iron ore containing 48 per cent iron, as mined, 6 per cent moisture be abstracted, it yields a product of approximately 51 per cent iron. Considerable money has been spent in developing the drying of iron ore, and the results have been very satisfactory.
Butler Brothers, contractors, have a drying plant under construction at the present time at the Lamberton mine, at Hibbing.
Trout Lake Concentrator. The first attempt to wash Mesabi ores was made in 1901-2, when a carload of ore was sent from the Arcturus property to Cedartown, Ga. The results justified further investigation, and in 1903-4 a small experimental plant was built at the Holman mine. In 1905, the Oliver Iron Mining Company became interested, and after exhaustive investigations conducted during the following four years erected the present Trout Lake concentrator. The work was commenced in April, 1909, and the plant was ready for use in 1910. It is located on the east side of Trout Lake, readily accessible from all directions. The mill building is of heavy steel construction throughout, 255 feet long, 162 feet wide, and 124 feet high, enclosed with corrugated iron. The approach to the mill is an earth fill, some 4,000 feet long, containing several million cubic yards of stripping from the Canisteo and Walker pits. It has a maximum height of 125 feet and was planned to accommodate four tracks. A steel trestle, 650 feet long, connects it with the mill. At the opposite end of the mill, 300 feet of additional steel work is in place and is now being used for tail track; it can be utilized for an addition to the mill if need arises.
The power and water for the mill are supplied from a power and pumping plant 7,000 feet distant on the shore of Trout Lake. The pump has a capacity of 500,000 gallons per hour. The water
is pumped direct to a 100,000-gallon supply tank at the mill. All the machinery in the mill is electrically driven. A 1,250 k.v.a. direct-connected generator transmits electric power at 6,600 volts to the mill, where it is stepped down to 440 volts.
The total amount of steel in the mill building, approach, tail-track and power plant structures is approximately 7,000 tons. The total cost of mill and equipment, according to the published report of the United States Steel Corporation, approximates $1,500,000.
The concentrating machinery is arranged in live units, each complete and capable of independent operation. This was done in order to keep the machines within reasonable size, to be able to handle separately the ores from different properties at the same time, and to increase the capacity. The tail-track already mentioned is long enough for seven additional units. The crude ore from the different mines is hauled to the mill over the big fill approach and trestle and dumped into the bins at the top of the mill. Each unit has a separate bin of 450 to 500 tons capacity. The ore is sluiced out from the bottoms of the bins by a hydraulic jet, and descends by gravity through the different machines. The crude ore tracks are 90 feet above the concentrate tracks. The ore is handled entirely by gravity; there is no elevating machinery to get out of order, other than the sand pumps necessary to lift the table concentrates to dewatering tanks over the concentrate bins.
Each unit consists of the following equipment, listed in the order in which the crude ore passes through it. Reference to the unit flow-sheet (Fig. 100) will help to an understanding of the process.
Flow Sheet of Trout Lake Concentrator
One crude ore bin and one bar-grizzly.
One 20-foot conical revolving trommel with 2-inch openings.
One picking-belt, to receive and convey the trommel oversize to the concentrate bin. (The taconite chunks are picked off the belt by hand and dropped into a rock bin.)
Two 25-foot log washers, one on either side of the trommel to treat the trommel “throughs,” discharging a product into the concentrate bin.
Two chip-screens, one for either “log,” receiving “log” overflow
Two settling tanks, receiving chip-screen “throughs.”
Two 18-foot “turbos” (small log washers to treat fines), receiving the tank settlings, discharging a concentrate directly into the concentrate bin.
Four settling tanks receiving the overflow from turbo and first settling tanks.
Twenty Overstrom tables, arranged in two rows of ten each, to treat the settlings from the four settling tanks just mentioned.
Eight Frenier spiral sand pumps (four primary and four relay), to pump the table concentrates to dewatering tanks from which they discharge directly into the concentrate bin.
The mill product thus consists of belt product, log-, turbo-, and table-concentrates. Each unit has its own concentrate bin of ninety tons capacity. Tracks for ore cars run directly under the bins, a separate track serving each bin. The cars are stored in a yard south of the mill, from which they can be spotted under the bins by releasing the brakes, as the grade from the yard is down towards the mill. The tailings consist of chip-screen discharge, No. 2 and No. 3 settling-tank overflow and table tailings. They are collected by launders in the mill basement and discharged into a concrete launder outside the table addition to mill. This launder discharges into Trout Lake, some 2,000 feet distant. The amount of water used per unit is about 1,500 gallons per minute, or 90,000 gallons per hour, approximately 360 gallons per ton of concentrates.
The rock picked off the belt and bar-grizzly is drawn from the rock pockets or bins into a rock car and hauled by an electric motor to a stockpile east of the mill. One motor and car serves all five units. One 100-h.p. electric motor drives the trommel, picking belt, logs and turbos; one 15-h.p. motor drives the tables and chip-screens; one 20 h.p. motor runs the sand pumps in the basement of the mill.
The mill building is commodious and the machinery is conveniently arranged. Safety devices, machine guards, and protecting railings are installed throughout for the safety of employees.
In 1910, the mill was operated on two eleven-hour shifts. In 1911, this was cut down to two ten-hour shifts. The average capacity is nearly 400 tons of crude ore per hour per unit, the maximum being 900 tons. The largest tonnage washed in one season of six months was over 4,000,000 tons of crude ore. The mill can not be operated in freezing weather, and its operating season coincides with the ore-shipping season. Nearly 100 men are employed on the day shift, and 75 to 80 on the night shift.
The average grade of crude ore varies greatly, depending on the character of material being treated and the local conditions at the time of mining. The concentrate product varies within wide limits, depending upon the character and class of ore, just as the grade of “direct-shipping” ores varies greatly. This is not a matter of degree of concentration produced by the mill, but rather the amount of concentration and enrichment produced by nature.
The results obtained vary widely for different ore bodies and even for different layers in the same ore body. The quantity of rock sorted out in the pit is so variable that the mere statement of mill recovery might give a very misleading idea of the total percentage of recovery from these low grade ore bodies. It may be authoritatively said that the mill has satisfactorily solved the problem of economical handling of western Mesabi ore bodies, a most important achievement in the line of conservation of our national resources.
The several smaller washing plants listed above consist of two, one, or one-half unit installations similar to that of the Trout Lake plant.
The Concentration of the East Mesabi Magnetites. The last method of treatment to be considered, and the last to make its appearance in the Lake Superior district, is magnetic concentration. This method of beneficiating iron ores has been in use for many years in New-York State and Canada, and some foreign countries, but, until recently, it has not been used in this district.
The whole process of magnetic concentration as applied to the eastern Mesabi magnetites is a good illustration of the manner in which the various types of machines can be made to work together so as to produce a high-grade furnace product from an ore material containing only 25 per cent of iron in the form of magnetite. The hard rock is first crushed to about three-inch size and is then passed over a magnetic cobber. The field strength of this cobber is so adjusted that all of the coarse material containing no magnetic iron is discarded as tailing. The concentrate from this cobber is still too low grade to be useful, and is, therefore, crushed again to two-inch size. This material is passed over a second cobber and the worthless gangue again discarded. This process of crushing, cobbing and discarding worthless material continues until the product has been reduced to about ¼-inch size. When this stage has been reached, approximately one-half the ore has been discarded as tailing and the other half contains practically all of the magnetic oxide that was originally present in the rock. This ¼-inch material, however, still contains too much gangue to be considered a desirable furnace product. It is, therefore, ground wet in ball mills until it will all pass a 100-mesh screen. This fine material is concentrated by magnetic log-washers in which the final separation is made. The concentrate produced by these machines is then dewatered by the use of continuous filters in the tank of which the fuel for sintering is mixed. The filter cake is conveyed directly to the sintering plant, where the ore is agglomerated. After being sintered the ore is screened in order to remove any fine material, and only the clean coarse sinter is shipped to the furnaces.