serting up a gold mill

operating costs of gold processing plant

operating costs of gold processing plant

A small gold mine flowsheet is shown below together with cost data which clearly shows the results that can be obtained when handling a low grade ore by a simple process. There are many commendable features in such a set up and no complicated machines or practices are embodied in this plan. In fact, its simplicity is appealing and the results obtained prove the practicability of this flowsheet.

The rod mill product passes over a roughing table where a high grade gold concentrate containing most of the free mineral is sent directly to a Clean-Up Pan. The middlings from the table are sent directly to an 8-cell Sub-A Flotation Machine from which a high grade concentrate is obtained. The tailings from the flotation machine are sent over a pilot concentrating table for visual examination. A middling pump and a dewaterer can be installed to take care of the middlings for regrinding, should enough value be found in them to warrant such a treatment. In this concentrating mill, however, it has been found that the middling product is of so little value that it would not be economically profitable to regrind and retreat this product. The cost data above referred to has been compiled from actual operating results and shows the cost of milling both for the old 50-ton plant and the new 100 ton concentrator, as is now in use. Note the reduction in milling costs through the use of the new flowsheet which includes an 8-cell Sub-A Flotation Machine. The savings made in lower operation costs and lower maintenance costs are noticeable and this saving was also made in the face of a 12.5% increase in wages made when the new plant was started.

Many of our so-called small gold mines or low grade deposits could well take this simple and inexpensive flowsheet for an example and be rewarded by a good profit per ton of ore treated, if the owners or operators would adhere to these principles. These are namely a simple crushing, grinding, concentrating, and flotation circuit without frill or fancy theories, but with a constant supply of ore through the mill at a nominal profit per ton, operating continually 24 hours per day. In other words, it may pay to have a lower recovery with a higher net profit than a higher recovery with an actual loss.

You will note from the above chart, the fairly high cost of marketing concentrates, but with the addition of the Clean-Up Pan a large portion of the free gold can be recovered in the form of amalgani and then reduced to bullion for shipment direct to the mint. The treatment of concentrates even of a complex nature is now standardized and should result in additional profits.

To aid in arriving at approximate operating costs, we are giving four charts with curves showing operating costs for the different standard methods oftreating ores. These charts show the costs compiled from actual operating figures secured from a number of mills operating under varying conditions. These figures show that operating costs do not increase greatly as one would naturally suppose when smaller tonnages are handled.

In studying the various factors affecting milling costs, the one point that is most noticeable is the effect of operating time on these costs. Plants thathave mechanical difficulties show costs greatly in excess of those with full operating time working under comparable conditions, and this demonstrates clearly the importance of having fool proof and ruggedly designed machinery that will run 24 hours per day. Modern milling equipment must be able to take care of overloads and handle oversize material without choke-ups and the resulting shut-downs. One must always keep in mind that after all a machine, a mill, or a process is only as strong as the weakest link.

The EXAMPLE Gold Mine Operating Manual has been prepared to provide a practical reference for operators in the EXAMPLE Gold Mines Processing Plant. This manual contains basic information about the process and equipment used in the process. Each unit is identified by name and equipment number. The function of each unit, a process and control description, and operating information is discussed. The last two sections deal with general plant safety and the definitions of terms used in the context of the processing plant.

This manual is intended as a general guide to plant operations. It is NOT intended to supercede, contradict or in any way negate manufacturers instruction manuals, which contain detailed information about each piece of equipment. In the case of uncertainty or requirement for clarification and/or further details, readers are to refer to the manufacturers manuals.

The EXAMPLE Gold Mill is designed to process 500 tonnes per day of ore. The flowsheets are conventional and consist of two stages of crushing, single stage ball mill grinding, cyanidation and a Merrill-Crowe precipitation circuit for gold and silver recovery. In addition to the cyanide circuit, gold leach residue is treated in a flotation stage to recover un-leached silver (Argentite) minerals. This circuit is followed by a small cyanidation circuit for recovery of silver (and remaining gold) from the flotation concentrate. Precipitated gold and silver are refined in an induction furnace to produce dore bullion.

The crushing plant will operate independently of the mill. In addition, whereas the mill will be primarily run from local start/stop stations, the crusher is designed to be run from its control panel. The dividing point between the two plants lies at the fine ore bin; all equipment prior to this bin is considered in the Crushing Plant and all equipment subsequent to it in the mill.

gold leaching equipment, circuits & process plants

gold leaching equipment, circuits & process plants

In Leaching for Gold, there is often a tendency to overlook or minimize the importance of the small mine. The small mine of today may develop into the large mine of tomorrow. Under proper management and financing it has as good a chance of yielding a profit as the larger property. Unfortunately large capital is seldom interested in them and they are left to the small groups who are not in a position to obtain the best engineering service. Mills are often erected without proper metallurgical tests and expensive Gold Leachingplant equipment are installed at a time when such large expenditures of capital on the surface is not justified by the underground developments. Careful metallurgical testing on the ore might have disclosed the fact that a simple method of amalgamation or concentration could have been employed and the mill built for a third the cost of a Gold Leaching plant.

By taking advantage of the fact that gold is one of the heaviest metals known and readily forms an amalgam with mercury, an effective but simple and inexpensive plant can be built for most small gold mines. Usually the major percentage of the gold values are in the native or metallic state and are free at commercial fineness of grinding and can be recovered by some combination of amalgamation and concentration.

Plate amalgamation, where the gold values are caught and held in the quicksilver film on a copper plate is the only step required for a commercial recovery on some few ores. In most cases a portion of the gold is filmed so that it does not amalgamate readily or is contained in ores with other minerals that also amalgamate or foul the quicksilver sufficiently to destroy its effectiveness for gold recovery. Here a form of selective concentration such as the Mineral Jigs and blanket tables, is used to concentrate the gold values in a small bulk of high grade concentrates for treatment in an amalgamation barrel or other amalgamator, where the gold is amalgamated and recovered as bullion.

The advantages of these simple plants are many and are not only attractive to the proved small mine but also to those under development. Within recent years many of our well known mines have been developed and brought into large scale production from revenue secured from a small milling plant operating on development ore.

A study of a large number of mills using amalgamation and concentration has disclosed bullion recoveries ranging from 60 per cent to 90 per cent and total recoveries, including concentrates, from 85 per cent to 97 per cent. The average bullion recovery will be about 70 per cent and very often this is of utmost importance as geographic location makes the shipping of the concentrate to a smelting plant undesirable.

While cyanidation is usually favored for treating gold ores to get maximum recovery of the values in bullion form, nevertheless, the fact that an amalgamation plantcan be built for approximately one- third of a complete Gold Leaching mill, together with the lower operating costs of the simpler plant, partially offsets the lower recovery. It is customary to impound the tailings from the amalgamation plant and these are cheaply treated when mine developments have justified the erection of the more complete Gold Leachingplant. An amalgamation and concentration plant can be operating intermittently without sacrificing efficiency, and this allows the operation of the plant for only one or two shifts per day to keep the peak power requirements at a minimum as mine compressors can be operated or the hoisting done while the mill is not in operation. The fact that 60 to 90 per cent of the values can be recovered by amalgamation will usually supply sufficient revenue from the mill to pay for development charges andbuild a reserve for the construction of the complete Gold Leaching plant.

With reasonable care in the design and construction of the original amalgamation and concentration plant all of the equipment can be utilized in the later complete Gold Leaching mill. By using standard equipment it is possible to add the Gold Leaching equipment following the already installed amalgamation and concentration units as these are an essential part of the completed plant.

Other advantages of these simple and inexpensive amalgamation and concentration plants are that they can be successfully operated with unskilled labor as no chemical knowledge or previous experience is necessary. Even flotation has been simplified through the use of Sub-A Flotation Cells; this addition of flotation means no marked increase in milling costs, but often a large increase in recovery due to the saving of extremely fine mineral values.

It is interesting to note the numerous dividend paying gold properties, particularly those in Eastern Canada, which have followed the treatment methods shown in the following flowsheets during the development stage and they have gradually added to the equipment as the profits and ore developments warranted. The use of standard proved equipment eliminates the biggest element of chance, and from this nucleus a more efficient and complete plant can be acquired as the flexibility of the equipment permits the change from one flowsheet to another.

We are giving five typical flowsheets used in treating gold ores and are describing the possible applications of these flowsheets, together with their fields of usefulness, and while in each case there is a similarity in equipment, you will note the changes necessary for various type ores. In each case we have endeavored to show the simplest possible plant for best results on each type of ore and to show the improvements that can be made to further increase recoveries at slight additional cost.

This flowsheet is the lowest in price, and can be used on what are commonly termed as free milling gold ores where a high percentage of the values are free and where these values are unlocked at reasonably coarse grinding.This flowsheet is often used for treating high grade pockets. The ball mill is in open circuit and the size of the product to amalgamation plates is controlled by a Spiral Screen on the ball mill discharge. The concentrating table also functions as a classifier and the middling is returned as oversize product for further grinding.

Flowsheet BB has a Mineral Jig and amalgamator in addition to the equipment shown for Flowsheet AA, and is used for an inexpensive plant where values are coarse but minerals are coated or filmed, and will not amalgamate readily on plates. The jig recovers the rusty values in a high grade concentrate for forcedamalgamation treatment in the Amalgamator. Onthe ores where this flowsheet is applicable, blankets, corduroy, or Gold Matting are usually substituted for amalgamation plates and their concentrate also is treated in the amalgamator with the jig product.

This flowsheet with the ball mill in closed circuit with a classifier, and with the jig in this circuit, will give the highest recovery possible for amalgamation and gravity concentration. The addition of the classifier allows finer grinding and the efficiency of the jig is greatly increased by using it in the closed grinding circuit. This flowsheet not only improves recoveries on ores as described in the previous flowsheets, but is also useful where the minerals are fine and where metallic values are in auriferous sulphides as well as in the free state in the gangue.

The addition of flotation to Flowsheet CC brings recovery to the highest point in Flowsheet DD as flotation recovers the slime values that are normally lost where gravity concentration only is used. The values that can be amalgamated are secured in bullion form from the high grade jig and table concentrates, and the remaining values are recovered in the flotation concentrate. This flowsheet is also necessary where a minor percentage of the gold values are present as metallics at commercial fineness of grinding or where the minerals are friable and easily slimed in fine grinding such as galena or the various telluride minerals.

The addition of flotation does not increase greatly the first cost of the plant, nor does it increase the operating expenses more than a few cents per ton. In a great many cases the additional recovery made by flotation means the difference between operating at a profit and at a loss. Flotation is responsible for the success of many small mining properties today.

Where the isolated location of the mill makes shipping of concentrates prohibitive, many properties store their product until they are justified in installing a complete treatment plant on the ground; current expenses are thus paid through bullion recovered by amalgamation ahead of flotation.

The equipment in this flowsheet is identical to that of DD. Here the ability of the Sub-A Flotation Machine to effectively handle a coarse feed is capitalized on to allow the handling of greatly increased tonnages. The ball mill discharge passes in open circuit over the jig, amalgamation plates or blanket tables and the flotationmachine. A middling product is returned from theconcentrating table and is dewatered in the classifier and returned for regrinding. On tailings, dumps, or low grade ores where it is necessary to handle a larger tonnage, this flowsheet is very effective, and while the recoveries would not be as high as in Flowsheet DD, the loss in recovery is more than offset by the greatly increased tonnage handled and the resultant lower milling cost. With this flowsheet a coarse tailing can be discarded, but slime losses are entirely eliminated as these, together with the granular minerals, are recovered in the flotation machine.

This flexibility of flowsheet is possible only where the Sub-A Flotation Machine is used. The (Selective) Mineral Jig is a valuable addition here as the excessive dilution would make it impossible to use any other type of gravity concentration device ahead of flotation. The change from Flowsheet DD to Flowsheet EE can be very easily made to accommodate changes in ore and to allow greater profits from the treatment of any type gold ore encountered.

No two ores are exactly alike. What method of treatment will give you the greatest net profit in milling your ore? This can be determined by proper metallurgical tests. They will show the recoveries which may be obtained by various methods of treatment; and the type and cost of equipment required, and the operating cost for each method are then easily established.

Ore tests are conducted on the basis of obtaining the simplest possible flowsheet, using standard, proved equipment. Also, as you will note in the flowsheets shown, this fundamental principle is always followed: Recover the mineral as soon as it is free.

A study of a large number of mills using amalgamation and concentration has disclosed bullion recoveries ranging from 60 per cent to 90 per cent and total recoveries, including concentrates, from 85 per cent to 97 per cent. The average bullion recovery will be about 70 per cent and very often this is of utmost importance as geographic location makes the shipping of the concentrate to a smelting plant undesirable.

While cyanidation is usually favored for treating gold ores to get maximum recovery of the values in bullion form, nevertheless, the fact that an amalgamation plant can be built for approximately one-third of a complete cyanide mill, together with the lower operating costs of the simpler plant, partially offsets the lower recovery. It is customary to impound the tailings from the amalgamation plant and these are cheaply treated when mine developments have justified the erection of the more complete cyanide plant. An amalgamation and concentration plant can be operating intermittently without sacrificing efficiency, and this allows the operation of the plant for only one or two shifts per day to keep the peak power requirements at a minimum as mine compressors can be operated or the hoisting done while the mill is not in operation. The fact that 60 to 80 per cent of the values can be recovered by amalgamation will usually supply sufficient revenue from the mill to pay for development charges and build a reserve for the construction of the complete cyanide plant.

With reasonable care in the design and construction of the original amalgamation and concentration plant all of the equipment can be utilized in the later complete cyanide mill. By using standard equipment it is possible to add the cyanide equipment following the already installed amalgamation and concentration units as these are an essential part of the completed plant.

Other advantages of these simple and inexpensive amalgamation and concentration plants are that they can be successfully operated with unskilled labor as no chemical knowledge or previous experience is necessary. Gold ore bodies can be accurately sampled by milling all of the ore from mine development work and the errors resulting from ordinary sampling methods can be entirely eliminated.

It is interesting to note the numerous dividend paying gold properties, particularly those in Eastern Canada, which have followed the treatment methods shown in the following flowsheets during the development stage and they have gradually added to the equipment as the profits and ore developments warranted. The use of standard proved equipment eliminates the biggest element of chance, and from this nucleus a more efficient and complete plant can be acquired as the flexibility of the equipment permits the change from one flowsheet to another.

We are giving four typical flowsheets used in treating gold ores and are describing the possible applications of these flowsheets, together with their fields of usefulness, and while in each case there is a similarity in equipment, you will note the changes necessary for various type ores. In each case we have endeavoured to show the simplest possible plant for best results on each type of ore and to show the improvements that can be made to further increase recoveries at slight additional cost.

This flowsheet is the lowest in price, and can be used on what are commonly termed as free milling gold ores where a high percentage of the values are free and where these values are unlocked at reasonably coarse grinding. This flowsheet is often used for treating high grade pockets. The ball mill is in open circuit and the size of the product to amalgamation plates is controlled by a Spiral Screen on the ball mill discharge. The concentrating table also functions as a classifier and the middling is returned as oversize product for further grinding.

Flowsheet BB has a Mineral Jig and amalgamator in addition to the equipment shown for Flowsheet AA, and is used for an inexpensive plant where values are coarse but minerals are coated or filmed, and will not amalgamate readily on plates. The jig recovers the rusty values in a high grade concentrate for forced amalgamation treatment in the Amalgamator. On the ores where this flowsheet is applicable, blankets, corduroy, or Gold Matting are usually substituted for amalgamation plates and their concentrate also is treated in the amalgamator with the jig product.

This flowsheet with the ball mill in closed circuit with a classifier, and with the jig in this circuit, will give the highest recovery possible for amalgamation and gravity concentration. The addition of the classifier allows finer grinding and the efficiency of the jig is greatly increased by using it in the closed grinding circuit. This flowsheet not only improves recoveries on ores as described in the previous flowsheets, but is alo useful where the minerals are fine and where metallic values are in auriferous sulphides as well as in the free state in the gangue.

The addition of flotation to Flowsheet CC brings recovery to the highest point in Flowsheet DD as flotation recovers the slime values that are normally lost where gravity concentration only is used. The values that can be amalgamated are secured in bullion form from the high grade jig and table concentrates, and the remaining values are recovered in the flotation concentrate. This flowsheet is also necessary where a minor percentage of the gold values are present as metallics at commercial fineness of grinding or where the minerals are friable and easily slimed in fine grinding such as galena or the various telluride minerals.

The addition of flotation does not increase greatly the first cost of the plant, nor does it increase the operating expenses more than a few cents per ton. In a great many cases the additional recovery made by flotation means the difference between operating at a profit and at a loss. Flotation is responsible for the success of many small mining properties today.

Where the isolated location of the mill makes shipping of concentrates prohibitive, many properties store their product until they are justified in installing a complete treatment plant on the ground; current expenses are thus paid through bullion recovered by amalgamation ahead of flotation. The equipment in this flowsheet is identical to that of DD. Here the ability of the Flotation Machine to handle a coarse feed is capitalized on to allow the handling of greatly increased tonnages. The ball mill discharge passes in open circuit over the jig, amalgamation plates or blanket tables and the flotation machine. A middling product is returned from the concentrating table and is dewatered in the classifier and returned for regrinding. On tailings, dumps, or low grade ores where it is necessary to handle a larger tonnage, this flowsheet is very effective, and while the recoveries would not be as high as in Flowsheet DD, the loss in recovery is more than offset by the greatly increased tonnage handled and the resultant lower milling cost. With this flowsheet a coarse tailing can be discarded, but slime losses are entirely eliminated as these, together with the granular minerals, are recovered in the flotation machine.

This flexibility of flowsheet is possible only where the standard Sub-A Type Flotation Machine is used. The Mineral Jig is a valuable addition here as the excessive dilution would make it impossible to use any other type of gravity concentration device ahead of flotation. The change from Flowsheet DD to Flowsheet EE can be very easily made to accommodate changes in ore and to allow greater profits from the treatment of any type gold ore encountered.

The 5 Gold Leaching Equipment Flowsheets illustrated above indicate the equipment essential for small cyanide mills of five different tonnages. These flowsheets are all similar with equipment sized for the tonnages shown. They are typical flowsheets for continuous counter-current decantation cyanidation plus a Mineral Jig in the grinding circuit with provisions for amalgamation of the jig concentrates.

The Mineral Jig and Amalgamation Unit have a definite place in cyanide plants as the coarse and granular gold can be readily recovered which may not be completely dissolved by the cyanide solution during the treatment time given to the pulp. The cyanide process has the advantage of producing precious metals in bullion form with the highest net return from those gold and silver ores amenable to cyanidation. The counter current decantation washing circuit has been found to be a most economical method for removing dissolved precious metals. Washing Tray Thickeners require the minimum floor space and capital costs. In counter current decantation wash water and barren solution are added in the last thickener units and flow counter to pulp flows, becoming enriched and are finally passed to clarification and precipitation where precious metals are precipitated and recovered.

The above flowsheets illustrate a method of increasing both capacity and recovery in a small gold plant by several stages. This is typical of the Pay As You Grow method of increasing capacity and profits essential in so many small operations. Because each ore has its own individual characteristics it is wise to first start with reliable test data. This is just as important in developing a flowsheet for a small mill as it is for a large plant.

Gold Flowsheet No. 1 shows a typical simple mill for the recovery of gold by amalgamation and by concentrating tables. However, on many ores such a flowsheet gives high losses of both fine gold and sulfide minerals.

Gold Flowsheet No. 3 indicates the addition of a required mill, classifier and extra Sub-A Flotation cells to provide for an increase in capacity and improvement in recoveries by regrinding of middling products.

Gold Flowsheet No. 4 shows an increase in flotation capacity to further improve recovery. The additions as illustrated allow an operation to be started on limited capital and gradually to be expanded as conditions warrant.

** Extracted from Memorandum Series No. 47, by C. S. Parsons, Engineer, Ore Dressing and Metallurgical Division, Mines Branch, Department of Mines, Ottawa. Published by permission of the Director, Mines Branch.

Source: This article is a reproduction of an excerpt of In the Public Domain documents held in 911Metallurgy Corps private library.[/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]

the explosion that changed milling | a taste of general mills : a taste of general mills

the explosion that changed milling | a taste of general mills : a taste of general mills

The evening of May 2, 1878 at the Washburn A mill the largest mill of its time in the U.S. began as any other as the day shift crews left the Minneapolis mill and the smaller night crew clocked in.

Flame and smoke in dense volumes leaped hundreds of feet heavenward, and the word went from lip to lip, almost with the rapidity of lightning, that the Washburn mill, which has long and justly been the pride of Minneapolis, had exploded and was destroyed It was a night of horror in Minneapolis. St. Paul Globe May 4, 1878.

His immediate concern was for the families of the employees and others in the area who were killed or injured by the explosion. Washburn initiated a fund to help them and was eventually revealed to be the funds most generous contributor. He also made sure the workers displaced by the loss of the mill could continue working in another mill.

But, determined to prevent a similar disaster in the future, Washburn requested a demonstration of a new device that hed heard would successfully deal with the millstone exhaust and prevent flour dust from clouding the air.

It was engineer William de la Barre who hauled the machine from Philadelphia to Minneapolis to show Washburn and business partner John Crosby its merits. The two leaders were completely satisfied, and asked for it to be immediately installed in the C mill, and eventually in their other mills.

Washburn also shared these innovations with his competitors. Once the devices eventually called dust collectors were installed in other Minneapolis mills, the flour milling process and industry became much safer.

By mid-1880, the new A mill was fully functioning. Built with state-of the-art steel rollers and then middlings purifiers (an invention that made flour whiter), capacity was increased and costs were lowered.

The A Mill continued to grind wheat until 1965 when General Mills closed the facility. A fire in 1991 destroyed much of the building and all the remaining original equipment. The Minnesota Historical Society began to renovate the property in 2001, which was already listed as a National Historic Landmark.

Editors note: Reports suggest that the mill explosion prompted a concern in Cadwallader Washburn about the children of workers injured or killed in industrial accidents. As a result, after his death in 1882, Washburns will established an orphanage the Washburn Memorial Orphan Asylum.

Over the years, the organization changed names several times. From the Washburn Foster Home Placement Agency to the Washburn Memorial Clinic, and then from the Washburn Child Guidance Center to its name today the Washburn Center for Children. It is the leading provider of services for children with social, emotional and behavioral problems in the Twin Cities.

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