Fig. 18. Stages in mining diamondiferous gravel

When the gravel for one day has been mined down to bedrock, an overburden gang is put on the following day to strip the overburden and expose the gravel alongside the excavation of the previous day, the overburden being thrown into this excavation. Good mining consists of throwing this overburden well back from the gravel, thus leaving a gutter between the overburden and gravel for drainage purposes, as a fair amount of water is encountered owing to the impervious nature of the bedrock; the water is usually carried off to a temporary sump, and pumped out by a small portable centrifugal pump, driven by a gas engine. The pump and engine are mounted on an iron framework, fitted with four wheels, and can thus be moved from one working place to another as required.

It has been found by experience that an overburden boy can excavate 8 cubic yards in a 9-hour shift, and a gravel boy 3 to 3 cubic yards in the same time; as the capacity of the washing plant is 60 to 70 cubic yards per 9 hours, this means the employment of a gang of 20 boys for the excavation of gravel, on contract, while the overburden contract is 64 cubic yards for a gang of 8 boys. The necessary lengths of the daily contracts may be obtained from the formula

,

where В is the width of the cut of paddock, in feet:

D is the average depth of the gravel (or overburden) in feet, this depth being taken from the previous cut, as generally the depths of gravel and overburden do not vary to any great extent from one cut to another;

L is the length of the contract, in feet.

It has been stated before that a width of 10-8 ft for each paddock is most convenient; this is because on a 10-8 ft width, having a frontage of 10 ft, every foot in depth will represent 4 cubic yards, which saves time and trouble in measuring up, as the yardage for each 10 ft in length can be booked at once, instead of first having to make various calculations in the office. Measurements are made daily of the gravel excavated and of the overburden stripped, and entered up in the measurement book.

Transport

For transporting the gravel excavated from the paddock to the plant, 20 to 25 wheelbarrows are necessary, provided the distance does not exceed 500 ft. When this distance is exceeded it is necessary, either to move the plant closer up to the working place, an expensive undertaking at the best of times, or else to put in truck haulage. For truck, haulage, six ordinary mine cars are required (three being loaded while three are being trammed), and a portable loading platform about 12 ft by 12 ft, just high enough to clear the top of the cars. The platform is placed as near the working place as possible, and the gravel, which is dumped on it by wheelbarrows, is loaded into the cars and trammed to the plant. At the plant the cars are tipped, the gravel being dumped into a bin, whence it is fed on to a conveyor belt for transport to the main trommel.

Milling

Diamond milling consists of washing, sizing, and jigging, a flow-sheet of the process being shown in the fig. 19. The conveyor belt is an endless rubber belt, 60 feet long by 18 inches wide and 1/4 inch thick; it has a linear speed of 90 feet per minute and is set at an inclination for conveying the gravel to the main trommel, which is situated at the top of the plant 18 to 24 feet above ground level (fig. 20).

The main trommel is 12 feet long and 3 feet in diameter, have a speed of 16 r.p.m. and a fall of 1/2 inch per foot of length from the feed to the discharge end. It consists of a cylindrical shell, the first half of which is blind and fitted with blades to disintegrate any clayey lumps in the diamondiferous gravel, before being discharged on to the second half, which consists of 5/8-inch screening. The undersize from the trommel goes to the concentrating pans, while the oversize is discharged into the washer, for further cleaning and washing. The concentrating pan has an outside diameter of 8 feet and an inside diameter of 2 feet 6 inches, the outer wall or rim being 18 inches high, and the inside rim 12 inches high. The pan is fitted with eight revolving arms, each carrying five tynes, or knives, of triangular cross-section, the points of which are fixed 1 inch above the pan bottom; these arms revolve at 13 revolutions per minute, the power being transmitted to them through a vertical shaft by bevel gears having a ratio of 4:1. The undersize from the main trammel, mixed with water, is fed into the pan, and is agitated by the revolving arms: by this means the lighter sands and tailings are discharged over a gate, fixed in the inside wall or rim, while the heavier particles, among which are the diamonds, settle on the bottom and are worked outwards to the outer rim, whence they are drawn off through a concentrate discharge pipe. Concentration is chiefly by centrifugal force.

Fig. 19. Flow-sheet of diamond plant

Fig. 20. Diamond concentrating plant. Plan and section (Not to scale)

As the tailings from the first pan contain a small percentage of diamonds, they are treated in a second pan before being discharged to waste. Actually, 90 to 97% of the diamonds recovered are obtained from the first concentrating pan.

The sizing trammel is 10 feet long by 18 inches diameter, revolving at 18 revolutions per minute. It is fitted with five screens having 1-millimetre, 2- millimeter, 2 - millimeter, 3- millimeter and 8- millimeter apertures respectively. The 1- millimeter product goes to waste, as does the + 8 millimeter, the remaining sizes being jigged.

The jigs are ordinary two-compartment Hartz jigs, the strokes of which vary from 1/4 inch to 5/8 inch, the number of strokes being from 170 to 150 per minute. Fine sands require a short stroke and a fast speed, while the heavier particles require a long stroke and a lesser speed. The diamonds and heavier sands settle on the jig bed, while the lighter tailings remain on top, whence they are scraped off and discarded. The concentrates are collected from the jig beds from two or four times per day and sent to the magnetic separator, which separates the magnetic particles from the non-magnetic in the concentrates, after which the concentrates are sent to the pickers for picking.

The washer is a hollow iron-cylinder, 12 feet long 3 feet diameter, revolving at 35 revolutions per minute. It is fitted inside with blades to break up any clay balls, etc., that remain in the oversize discharged from the main trommel. At the discharge end of the washer, a 5/8- inch screen is fitted +5/8-inch product is sent to the waste dump, if clean enough, otherwise it is trammed to the wearing dump and afterwards retreated. The -5/8-lnch product is returned by the conveyor belt. The whole plant is usually driven by a portable stream engine of about 35 to 40 horsepower, using wood for fuel.

Wash water, at the rate of 4,000 to 5,000 gallons per hour, is consumed by each plant, and is generally pumped up from a main pumping station, into two iron tanks situated at the top of the plant; the waste water from the plant is drained into a sump whence it is also pumped back to the tanks by means of a small centrifugal pump fug.21 .22). (http://www.minelinks.com/)

Fig. 21. Double-drum Scrapehauler

Fig. 22. Three-drum type of Scrapehauler

COMPREHENSION CHECK

Exercise 8. Complete the sentences using the information from text 2 and your own experience:

1. A "key cut" consists of ...

2. For the barrows a track is made along ...

3. It has been found by experience that an overburden boy can excavate 8 cubic yards in а 9 hour shift and a gravel boy ....

4. When the distance from the paddock to the plant is more than 500 feet it is necessary …

5. Diamond milling consists of ...

6. The conveyor belt is ...

7. The oversize is discharged into ...

Exercise 9. Find the English equivalents of the words and word combinations in the text:

разработка алмазов; разведка местности; расчистка местности; прокладывать дороги; основной разрез; на всю длину; обнажить породу; завершить горно-подготовительные работы; грузить на тачки; бригада; сточная канава; насос; вагонетки; грохот; сортировка по крупности; отсадка; промывка; слишком крупные (мелкие) куски породы; рештак; хвосты ; дробление алмазов; подготовительные работы; откачивать воду; требовать меньшей (большей) скорости; быть оснащённым (оборудованным); лошадиная сила; прорезать (делать разрез).

LANGUAGE REVIEW

Exercise 10. Read the text about the history of surveying. For each line, underline the wrong word in the text and write the correct word in the space.

The history of surveying is very longly. Before the 3rd century BS the Chinese possessed some form of magnetic compass. By the 2nd century BS the Greeks used the astrolabe, an instrument for measure the altitudes of stars below the horizon. The Romans acquired Egyptian surveying instruments, to which they added the water level and the plane table. By the early 17th century, the English mathematics Edmund Gunter developed a Surveying Chain, which marked out standard measures of distant.

__long_______ _____________ _____________ _____________ _____________ _____________ _____________ _____________ _____________ _____________

Exercise 11. Complete the questions with words from the box:

when how long how many how much what who why

1. __When____ did they begin to use these measuring instruments?

2. ___________ money did the spend on the new loading equipment?

3. ___________ the output forecast?

4. ___________ new mining enterprises were opened recently?

5. ___________ is it necessary to clear the area before mining operations?

6. ___________ invented the first surveying instruments?

SPEAKING

Exercise 12. Discuss the questions with the partner.

1. What are the three main stages of preliminary work?

2. How is the mining commenced?

3. Describe the process of haulage.

4. What is drainage?

5. What stages does milling consist of?

6. What is sizing?

Exercise 13. Give a short description of a diamond concentration plant.

UNIT 5

SLUICING

PRE-READING

Exercise 1.

1. Mining is a harmful process. Do you know the main environmental risks of gold mining?

2. What can be done to lower the harmful effect of mines?

Exercise 2. Match the words and their meanings:

harm abandoned in-situ benefit species

deserted in place damage kinds profit

READING

Exercise 3. Read the text and learn more about environmental effects of mining gold.

Text 1. Environmental Effects of Mining Gold

Ironically, mining to discover beauty in nature harms nature in the process. Abandoned buildings, dumping sites and disposal areas pose environmental risks and safety risks for humans. The loss of habitats and the contamination of ecosystems threaten wildlife and plant populations.

Significance

According to the Environmental Protection Agency (EPA), there are over 500,000 abandoned mines in the United States, of which about 40 percent are hard rock mines. The four broad categories of mines are underground mining, surface mining in pits, dredging and in-situ solution mining. In contrast to excavating or removing ore, in-situ mining uses a solution to extract the gold from the surrounding hard rock material. Gold mining uses a combination of these including panning and sluicing. Sluicing is large volume panning where waters are conducted through a slough to remove sand and other material from the gold ore.

In-Situ Mining Impacts

While all mining activities can have a negative impact, in-situ solution mining is especially detrimental because cyanide is used to recover gold. Large-scale mining operations in the 1950s began using cyanide to leach gold - separating gold from unwanted material. For the mining companies, these processes have economic benefits in allowing lower-grade gold to be mined. However, the environmental risks escalate with the increased threat of leakage or spills.