The first challenge in mining is finding a mineral deposit. The second one is starting a mine. The third is extracting the mineral-bearing rock from the earth.
The fourth challenge is separating the desired mineral from the rock which it is encased in. This is the mineral separation process. This is the process of the mills.
Is Mineral Separation Needed?
Mineral separation is necessary when the mineral being mined for makes up a small portion of the rock being mined. Mineral separation has become increasingly important since the 1800s because on average, the percentage of rock mined which contains the desired minerals has been decreasing.
This is due to consumption – the deposits which had higher percentages have been mined out.
The result of this change was innovations in how to separate out the ever decreasing quantities of valued minerals from the mined rock while still being able to remain profitable.
Britannia Mine provides a lens onto these changes that occurred during the first half of the 1900s.
Britannia Begins – Mill 1
Mill 1 stands on the hillside.
The first mill was completed in 1905, but it did not process all the rock from the Mine. This is because some of the first mineral-bearing rock mined was composed of up to one third chalcopyrite – the principle mineral found at Britannia which contains copper. This rich rock was shipped directly from the Mine to the smelter, beginning in August 1905.
Where where the smelters?
The first smelter was located in Crofton, on Vancouver Island. This smelter proved to be inefficient, and was shut down in 1908 after only a few years of operation.
Following the closure of the Crofton smelter (which was owned by Britannia when it processed Britannia's ore), smelting was performed at the Tacoma (Washington) smelter.
It is the Tacoma smelter which was the primary smelter over the course of Britannia's operations.
Other smelters which were used included the Anaconda smelter in Butte, Montana (after Anaconda purchased Britannia in 1963), and the Trail Smelter.
The farthest away was in Japan. It is at this smelter that the iron pyrite (which originally was considered a waste product) was processed for its sulphur content.
A Glimpse into Britannia's Early Challenges
When Britannia came to an agreement to purchase and restart the Crofton smelter, the town of Crofton looked to once again have a bright future. In the September 20, 1905, Ladysmith Daily Ledger, an article spoke of "Seven hundred tons of ore, valued at $10,000 by a conservative estimate, was shipped last week from the Britannia copper-gold mines on Howe Sound to the Crofton Smelter." and "... the shipments will net a substantial profit after payment of the cost of mining and treatment at the smelter."
These headlines from before the smelter started processing the ore could not have predicted the challenges facing both the Mine and the Smelter in the coming years.
Some of these challenges are captured in a report from the 1911 Minister of Mines Annual Report, which summarizes those first years as follows:
Little is heard of the Britannia Mine, and it is realized by but few how much work has been going on there, very quietly, for a number of years back; the company does not advertise itself, and its stock is not one usually dealt with on the exchanges. Yet the property employed during 1911 an average of about 145 men below ground and 180 men above ground, and mined about 500 tons of ore a working-day, all of which was concentrated and shipped to Tacoma for smelting.
The output of the mine for the past year was about 118,900 tons, which contained, approximately, 46,000 oz. of silver and 8,685,000 lb. of copper. The ore as it is broken in the mine will run about 2 1/2 per cent. copper, with 1/10 oz. of silver to the per cent. of copper. This is roughly hand-sorted at the mine and sent down an aerial tramway, about three and a half miles long, to the mill at Britannia Beach, on Howe sound. The concentrates, as shipped, run about 14 percent. in copper and 1.4 oz. silver to the ton, and are shipped on scows to the Tacoma smelter.
A long description of the property, as it was first opened up, appeared in the Report of the Minister of Mines for 1900.
The ore-bodies originally exploited were exceedingly large, and were not only very low grade, but would not make a concentrate running over 7 to 8 per cent. copper; consequently, these old ore-bodies would not allow a profit with copper at almost 12 cents a pound.
This was the condition of affairs, continued under different managements, until about 1909; large tonnages were mined and every known kind of concentrator was tried, but the results were never satisfactory commercially, chiefly on account of the difficulties met with in concentration owing to the presence of iron and zinc sulphides accompanying the copper-pyrites, the whole being so intimately mixed as to necessitate fine grinding before a separation could be effected.
The original ore-bodies were contained in a formation which was called, locally, the "Jane schits," as they were best developed at the mine of that name.
About 1909, while the property was under the management of R. H. Leach, one of the lower working-tunnels was driven in to crosscut a vein, the outcrop of which was exposed on the mountain-top; this outcrop was not very promising, but it contained much less iron and zinc than did the deposits then opened up, and it also lay in a formation slightly different in character. This newer formation is known as the "Fairview schists," and is chloritic in character.
The tunnel found the vein sought and also several others upon which for the past two years development has been energetically carried out with force of from 150 to 200 men. As a result, there was developed in the newly discovered veins approximately 1,000,000 tons of 2.5 to 3 per cent. copper-ore, which was amenable to concentration, and on which an 80-per-cent. extraction is claimed to be made, the concentrates running, as already stated, 14 per cent. copper an 1.4 oz. silver to the ton.
This was the condition of affairs in November, 1911, when R. H. Leach was forced to resign the management, his health having given way under the heavy strain of the work.
Mr. Leach was succeeded in the management in November by J. W.. D. Moodie, who had formerly been in charge of the Tintic properties in Utah, controlled by the same financial group as is thr Britannia.
With the tonnage of ore mentioned as assured, the new management felt justified in an extensive system of improvements, which is now being pushed ahead with energy.
Among the more important improvements under way is a long, deep tunnel starting in 1,200 feet lower than the lowest old tunnel, and which will naturally be the outlet for all the ore; that from the upper workings being dropped down through chutes to this lower tunnel.
This new tunnel starts in from about the "transfer station" on the tram-line, almost half-way up, and will thus cut off the upper half of the tramway, which has been a source of great expense. It was originally intended to retain in service the lower stretch of the aerial tramway, but recent advice from one of the company's officers is to the effect that a surface electric tramway is now under construction from the Beach up to the portal of the new tunnel.
A new an very large storage-dam for water is nearing completion which will store water for power purposes, delivering it at the Beach under a head of 1,900 feet and delivering 1,000 horse-power.
The air-compressor for the mine is at the Beach, the compressed air being conveyed to the mine in three miles and a quarter of 8-inch pipe.
The concentrating plant is the largest in the Province, but, as it is now being somewhat revised to meet the new conditions of the recently developed ore-bodies, it will not be advisable to describe it in detail until changes have been completed.
The plan now in use consists of a first mill, situated on the hill overlooking the flats, to which the ore is delivered directly from the tramway. This first mill building, or "jig-house," is equipped with a picking-belt, from which a first separation is made, the waste and first-class ore being sorted out, the second-class going into the process, which consists mainly of sizing and jigging, on the ordinary type jig, and also on Hancock and Richards jigs.
The middlings and much fine material from this first mill - amounting to about 50 per cent. of the original feed - is carried by flume to the vanner-house located on the dock-mill.
In the vanner-house the material is received by two dewatering tanks, from which the ore goes to two 6-compartment Richards classifiers, from which it is distributed to 17 Frue vanners, 11 Wilfleys, 13 Overstroms, and 9 Johnson vanners.
Experiments have been in progress for some time with a unit of the Elmore oil process, having a capacity of 35 tons of ore per day. The results obtained are reported as being very satisfactory, and the product is certianly of high grade; the process has, however, not as yet been accepted as a part of the permanent process.
It is at the smelter that the copper was extracted from the chalcopyrite.
The vast majority of mined rock, however, required milling to separate the desired minerals from the waste rock.
This is because smelters require their feed to be fairly pure. If there is too much waste they will charge the Mine a penalty. For this reason, combined with shipping costs, when the rock is lower grade (contains lower amounts of the mineral being mined for), it is necessary to remove the waste prior to shipping.
Milling begins by physically breaking the rock down to a uniform size. This first step is referred to as crushing. In Britannia’s first years, crushing allowed for hand sorting of chalcopyrite-rich rocks with the remaining rock being further broken down and then sent to the vibrating tables.
On these tables vibration allowed the heavier, more mineral rich rocks to settle. The lighter rocks, which were waste, would ‘float’ to the top.
The waste was then ground down into a fine sand and flushed via a flume to the Vanner Building. The building was named for the machines inside – Frue Vanners. These machines enabled recovery of the trace amounts of chalcopyrite found in the mill waste.
But Mill 1 was inefficient. The problem stemmed from the presence of sphalerite – a mineral rich in zinc – being found in the rock with the chalcopyrite. The issue was that these two minerals have very similar densities – similar enough that the separation methods in use left them mixed together.
In 1907 an extensive retooling of Mill 1 was performed to improve its efficiency, but it was not until a new technology was introduced that mineral separation became efficient at Britannia.
Enter the New Mill – Mill 2
Mill 2 completed. Circa 1916.
In 1911, the Mine was appointed a new general manager charged with making it profitable. His name: J.W.D. Moodie.
In his first years at Britannia he revamped the mining methods to improve productivity. One of the biggest changes was to the milling process.
In 1911 Britannia began testing the new separation process of froth flotation. In 1912, Britannia became the first mine in British Columbia to use flotation in a production environment. It replaced the Frue Vanners, improving the mineral recovery rates, but Mill 1 still presented a challenge of capacity.
Mill 2 addressed that issue. It was built in two phases from 1914 to 1916. When completed, it had over twice the capacity of Mill 1 (2000 tonnes per day vs 800). It took both volume and efficiency to make Britannia profitable.
As a result of the introduction of froth flotation, mineral recovery rates changed from less than 50% in 1904 to over 95% in 1917. Mill 2 was recovering 10% of the chalcopyrite with hand picking, 40% with vibratory tables, and 50% with flotation.
The Mill that Never Was
Mill 2 destroyed by fire,1921.
Mill 2 was designed for a future it never saw. The mill was planned to eventually consist of four 1000 tonne-per-day units. The first two of these units
were constructed, giving the building its 2000 tonnes-per-day capacity.
Before the third and fourth units were built, however, disaster struck. In March of 1921, a fire broke out in the building while it was shut down. When the fire reached the oil used in flotation, it spread rapidly. While the town was quick to respond to the fire, it was not able to save the mill.
A New Mill Rises to the West
Mill 3 under construction, circa 1920.
Following the loss of Mill 2, work began on the planning and construction of a new Mill, Mill 3.
It went into operation in 1923, 18 months after construction began.
In the original planning, Mill 3 replicated Mill 2’s workings, less the hand picking as there was no longer enough rock of high grade to warrant it. Shortly after completion, however, it was realized that all recovery could be accomplished with froth flotation, and the building was retooled.
Over its years of operation, Mill 3 was retooled several times. Each time, it brought changes which improved the efficiency and the processing capacity of the building. While it was designed to handle 2500 tonnes of rock per day, at its peak levels, reached before the Great Depression, it processed over 7000 tonnes per day.
Mill 3 is the mill that turned Britannia from a large mine into the highest producing mine in the British Empire. It is for this significant role that it is recognized as a National Historic Site.
It is for its unique shape, however, that it is most commonly known. Built in steps up the hillside, the building is 20 storeys tall.
Its height is reflective of how it processed the rock. It, like Mill 2, used gravity to assist in moving rock through the building. For this reason, these buildings are known as gravity-fed mills. At the time of construction, it was efficient to use gravity to move rock. As pumps became more capable of moving rock, gravity-fed mills became seen as inefficient. In the 1960s, the Mine’s general manager, Barney Greenlee, described the Mill as metallurgically efficient but mechanically inefficient – it recovered almost all of the desired minerals, but it cost more than a modern-at-the-time design would.
Today, few gravity-fed mills are still standing. Those that remain capture a short-lived but significant era in mineral separation.
Explore Mill 3 and its mineral separation process below.
Part 1: Through the Mill - The Journey of the Rock
|The mineral-bearing rock, called ore, was carried to Mill 3 by train from a mine portal at the same elevation as the top of the mill.
A train loaded with ore enters the Mill. The train travelled through the Mill and into another tunnel to allow the last cars to be emptied. After the last car was emptied, the train was reversed back into the Mine.
Also visible in this picture from 1938 is the construction of the addition to the Mill which housed the cyclone air filtration system.
|Once inside the Mill, the train cars dumped the ore through the tracks into large holding bins below.
||The top floor of the Mill today.|
||What remains of the receiving bins today. The farthest bin still stands to its original height. In the front bins can be seen ore left behind
at the time of the Mine's closure in 1974. The ore has cemented together. This is an issue the Mine faced when the Mill was running. If
it happened, it required a small amount of explosives be placed in the bin to break the rock apart, allowing it to flow out the bottom.
|The rock passed out the bottom of the receiving bins onto a conveyor belt which carried to the primary crushing area. As it was moved, an electromagnet captured any shards of steel (worn off of drill bits) for recycling. The rock then dropped down to the primary crushing floor.|
|The first major floor in the Mill held the cone crushers. These machines ground the rock from the six-inch diameter pieces entering the Mill
to three-inch pieces.
||The primary crushing floor today.|
After crushing the rock was moved, via conveyor belt, to the secondary receiving bins. From these bins, the rock would pass into the grinding circuit. Grinding was performed in ball mills (also called tube mills). As these mills spun, the steel balls and rock inside them tumbled over one another. As they did, the balls broke the rock down into a sand-like texture. It is also in these mills that the sulphide minerals that needed to be separated from the waste were wetted - the minerals were rendered hydrophobic - a property needed to enable their separation from the waste.
|Grinding was performed over three floors. The lower two are visible in this picture.|
||One of the few ball mills still remaining in the Mill today.|
|When grinding was complete, the rock was fed into the flotation cells. Within these cells, the hydrophobic minerals would adhere to bubbles.
The bubbles then lifted the minerals to the surface, where they flowed out of the cells and were washed into thickening tanks.
||The flotation cells today.|
||The crushing area of the Mill was dusty. When legislation required better air quality, the Mill had a cyclone air filtration system installed to remove the dust from the air. The cyclone is one of the few pieces of machinery still in the top area of the Mill.|
|The cyclone is located near the top of skip line. The skip moved equipment in and out of the building.
||The view from above. The top of the skip is on the primary crushing floor.|
|On the bottom level of the Mill, the froth entered into thickening tanks. Within these tanks, the bubbles carrying the minerals broke, allowing the minerals to settle out. A rack at the bottom of each tank then moved the minerals, called mineral concentrates, into the pump which moved them onto the drying area.|
|The dryers removed water from the concentrates to prepare them for shipping to the smelters. From here, it would travel via conveyor belt to the waterfront for shipping.|
|Copper concentrate was stored in the Concentrate Shed, located on the wharf. From here it was loaded onto ships for transportation to the smelter.|
Explore Mill 3 Further:
A walk through Mill 3: Listen to the sounds of the Mill on a walk-through, from top to bottom.
The Concentrator: Visit our virtual museum on Mill 3.
Part 2: Froth Flotation - a Focus on Innovation
It could be said that flotation came to have the same significance to mining that mining has always had to society. Flotation is a technology which most people are unaware of, yet it is a technology that fundamentally shapes our lives because it enables us to meet our mineral needs.
Flotation is a process for the separation of finely ground solids from one another.
Development of flotation began in the mid-1800s, but its first step into becoming a viable mineral separation process occured in 1889 at Broken Hill, Australia.
During the 1800s, several companies were working on what became known as flotation to solve a major issue - the industrial revolution had caused a surge in the demand for metals such as copper. Prior to the revolution, copper demand could be met by mining native copper (copper found in its metallic form). Now, new sources of copper were needed. Copper sulphide minerals were the answer, of which chalcopyrite is the most abundant (it remains the primary source of copper today).
Zinc (found at Broken Hill), and lead were the other two metals which were the primary focus of flotation at the time.
In British Columbia, Rossland was the first mine to experiment with the flotation process, but Britannia was the first to put into a production environment. That first flotation process used at Britannia was the Elmore oil flotation process - a process with which the first experiments using Britannia's ore were conducted in 1902.
With Mill 2, the Minerals Separation Company process was licenced and implemented. Britannia was the first in British Columbia to implement this technology developed in Australia. One of the advances of this flotation process was the use of forced air to create the bubbles needed to lift the minerals. This approach allowed for a uniform bubble size, which provided better recovery rates.
With Mills 1 and 2, Britannia demonstrated an ability to adopt and adapt technology developed elsewhere. With Mill 3, Britannia developed its own unique implementation of froth flotation to meet the unique qualities of the Britannia ore.
Before this occurred however, there as one more significant adoption of technology within the Mill - selective flotation. This new advancement in mineral separation allowed what was formerly one part of the waste material to become a marketable product. This advancement was noted in the Mining and Industrial Record, 1926.
' A notable achievement in the utilization of a formerly waste material is the success of A.C. Munro, superintendent of the Britannia Mill, in producing by selective flotation a high-grade iron pyrite from the ore gangue. This pyrite is being sold to pulp and chemical plants for the manufacture of sulphuric acid, and will partially replace the importation of Texas Gulf and Japanese sulphur.'
The next step was the development of a flotation system that was optimized to recovery of pyrite, chalcopyrite, and galena. This system was named the Britannia Deep Cell process, named so because of the deep-for-the-tme deep cells.
The process consisted of two stages. In the first stage, the sulphide minerals (galena, chalcopyrite, pyrite) were separated from the waste rock in the deep cells (eight feet deep). The mineral-bearing material was then ground finer in ball mills before going through shallow cells (three feet deep) which separated the chalcopyrite and galena from the pyrite.
Galena was separated from the chalcopyrite based on their differences in densities in the Lead Plant before the separated end products, called concentrates, were moved to the waterfront for storage and shipping.
Zinc, found in sphalerite, was recovered in a separate grinding and flotation circuit.
How Flotation Works
The principles behind flotation are similar to what happens when you take a bubble bath.
In taking a bath:
You have a tub of water. In flotation we have a flotation cell.
You have bubble bath, which changes the surface tension of the water, allowing for the formation of a thick layer of bubbles. In flotation we have a frothing agent.
You have oil on your skin. Oil repels water. In flotation, we have chemicals which do the same.
When you take a bath, the oil on your skin coats the dirt. When the oil-coated dirt comes off your skin and enters the water it readily binds to the air bubbles you make by splashing around and floats to the surface. In flotation, the chemical coated minerals readily bind onto the air bubbles created and float to the surface.
Flotation works due to the differences in affinity between the ground particles for attaching to an air bubble. This affinity is manipulated through the use of chemicals which alter the surface properties of the minerals desired, making them more prone to binding to the air bubbles.
In its most basic application, flotation consists of the following steps:
1. The rock is ground to a fine material in water. Grinding in water prevents the fresh surfaces from oxidizing, which will inhibit the flotation process. Surfactant chemicals are then added to modify the surfaces of the specific minerals.
2. Reagents are then added to the ground rock, called a pulp. The reagents bind to the minerals to be recovered, making them water repellent (hydrophobic).
3. In flotation cells, a frothing reagent is added to the water. It helps establish a stable froth at the surface by lowering the surface tension of the water.
4. The pulp is pumped into the flotation cells. Air is added to the cell to form bubbles by either low pressure injection or agitation.
5. The treated minerals bind onto the air bubbles and are lifted to the surface of the cells.
6. The mineral-rich froth is removed from the cells.
Flotation works because of the differences in surfaces of the materials. Hydrophobic surfaces bind to the air bubbles readily while the hydrophilic surfaces do not. By manipulating which materials have hydrophobic surfaces, flotation is able to achieve up to 98% recovery rates.
Flotation continues to play a critical role in mining, allowing for the mining of ore bodies that would otherwise not be economical. In the US, for example, from 1936 to 1960, the average grade of copper ore treated dropped from 1.57% to 0.72% copper. (Mining Engineering Handbook, second edition). Today, the grade can be substantially lower, yet still successfully mined.
This is possible because of ongoing advancements in flotation.
These advancements have also allowed flotation to become part of other industries. It has been adapted to the removal of ink from paper, recovery of bitumen from tar sands, and separation of wheat hulls from kernels, for example (Mining Engineering Handbook, second edition).