Dredge Vessel Specifications

A Standard 165 ft offshore supply boat of 2500 SHP was und for the cruise. The main advantages of this type of ship are: relatively low operating costs, low freeboard, ample desk space and availability on short notice. Their main disadvantage lies in their relatively poor sea-keeping qualities, particularly with a long, heavy wire out astern. Their tendency to pitch, heave and make too much leeway contribute to a degradation of control. The direct-drive propulsion systems are not conducive to fine speed control, an important consideration at low dredging speeds.

The ideal able should have a deep, rounded bilge and a low see board to mitigate any tendency to go to leeward. At least 2500 SHP is required for the uncommon occasion when ample bollard pull is needed. The drive should be diesel-Movement of the nodules forward to the storage bins is electric or through controllable pitch screws to provide fine control at low speeds. A good bow thruster capable of continued operation over long periods would be a definite asset.

Ship Positioning By Dead Reckoning

Dead reckoning, updated by satellite navigation, was used exclusively for ship positioning while dredging. The system provided sufficient information to maintain a general position within the dredging area, usually by adjusting the ship's position after every two or three runs.

Photographs have confirmed the suspicion that the dredge does not always remain in contact with the bottom and does not always move forward during the towing period. This would appear to emphasize the extreme criticality of the speed/weight relationship while dredging. The unit must be continually moved forward throughout the operation, with enough wire out to ensure a horizontal pull, but at a minimum speed so as not to fly or bounce the dredge. The range of speed necessary to accomplish a satisfactory pull is on the order of one-half knot. The range of critical speed can be expanded by adding weight to the dredge. This will increase the speed at winch a unit can be towed without flying off the bottom. Weight can be added, however, only at the expense of a decreased payload and greatly increase handling difficulty on deck.

The most important single tool to the dredge operator is a good tensiometer. The main wire may be paid out quickly to a length slightly less than water depth, at which time pay-out, to avoid piling the wire on the bottom. The operator should supplement tensiometer information through frequent observation of the wire.

10 Tpd Average

The dredging system was not designed for the recovery of nodules in commercial quantities. With some modification, it will return a consistent 10 tpd. Although the overall average for the 1972 cruise was little more than half this amount, production was held down by a need to further develop hardware and technique. Once these were refined, the returns per haul rose sharply and peaked out at close to 10 tpd. An increase of about 15 percent in the dredge's horizontal and longitudinal dimensions and deployment from a more competent ship, are the only modifications required to consistently reach this figure. The system can be scaled up to collect greater tonnages if desired, but only to a point. A very large increase in dredge size will impose concomitant increases in wire, winch and handling requirements and may drive the cost per ton collected beyond a desirable limit.

UNIT 4

DIAMOND MINING

PRE-READING

Exercise 1.

1. What is the difference between lode and placer deposits?

2. Do you know in what parts of the world gold deposits are found?

Exercise 2. Match the words and their synonyms:

weathering stream accumulate gem profitable

creek collect erosion beneficial precious stone

READING

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

Text 1. Geological and Geographical Characteristics of Gold Mines

Gold deposits are found in different types of rocks and geologic formations, falling into two mining categories: lode (primary) and placer (secondary). Lode deposits are contained within surrounding rock whereas placer deposits are dust particles contained in streams and stream beds. Geographically, gold can be found on all seven continents, and the world's oceans also contain vast amounts of gold.

Lode deposits are the result of magna, high temperature and high pressure that thrust liquid gold up from Earth's crust. Cooling water seeping through the metamorphic rocks hardens the gold, resulting in ore deposits that run through sedimentary rocks as veins. Lode deposits are found in old rocks, older than 2.5 billion years, belonging to the Archanean geologic period and near sea floor volcanoes. Typically, granite, basalts and komatites are found along with lode deposits.

Placer deposits are concentrations of gold transported from enclosed rocks through erosion and gravity. Gold is resistant to weathering, but flakes and dust are easily transported via streams that surround gold-bearing geologic formations. Placer deposits accumulate in stream beds and in sand and gravel, also known as "black sands," and are heavier than other types of minerals. Other minerals found in black sands include magetite, cassitente, monazite, ilmenite, chromite, platnium metals and sometime gem stones.

Gold deposits are found in Northeast Canada, Brazil, Russia, the Congo, Egypt, Indonesia, Kazahstan and Australia. The world's oceans contain vast amounts of gold within sea floor sedimentary rock. The advantages of undersea mining include large deposits and unaffected landmass and populations. Furthermore, cyanide pollution will not be a factor because deposits are large enough to be recovered without acidic leaching; however, ocean floor mining techniques have not yet been developed to mine gold profitably from the seabed. The ocean floor around Papau New Guinea will likely be the first place gold mining exploration occurs once land deposits are fully exploited.

Politics play a role in mining available gold from across the globe. As of 2011, political stability in Canada, the United States and Australia are conducive to profitable gold mining. South Africa, on the other hand, provides a significant amount of gold, but political instability creates uncertainties regarding long-term supplies. (http://pubs.er.usgs.gov/)

COMPREHENSION CHECK

Exercise 4. Are the statements true or false? Correct the false statement using the facts from text1.

1. Lode deposits are dust particles contained in streams and stream beds whereas placer deposits are contained within surrounding rock.

2. Gold can be found on all seven continents and in the world's oceans.

3. Granite, basalts and komatites are found along with lode deposits.

4. Sand and gravel are also known as "black sands".

5. The world's oceans contain small amounts of gold within sea floor sedimentary rock.

VOCABULARY

barrow - тачка

plank - планка, доска

gutter - водосток, желоб, кювет, лоток

impervious - водонепроницаемый

sum -зумпф, отстойник, углубление

paddock -площадка вокруг ствола, шурф с прямоугольным сечением

yardage -производительность в ярдах

trommel -дражный грохот, барабанный классификатор

trem -вагонетка, тележка

tremmed -откатываемый

sizing - сортировка по крупности, сухая классификация, грохочение

jigging - отсадка, мокрое обогащение

discharge -разгрузка

lump - кусочек, зерно, фракция

pan -рештак, желоб, лоток, шлюз

bevel -скос, наклон, откос

picker -сортировщик, грохот, кирка

blade -лезвие, лопасть

gang - бригада

stroke - ход

commence - начинать

settle - осаждаться

rim - обод, край

Exercise 5. Find the meaning of the word combinations in the dictionary and try to remember them:

alluvial diamonds; average value; mining operations; preliminary work; to clear of; suitable roads; necessary plant; to excavate a cut (a paddock); to expose the gravel; to load into wheelbarrows; to make a track; to mine down to bedrock; overburden gang; to pump out water; truck haulage; diamond milling; conveyor belt; feed end; discharge end; to be fitted with blades; concentrating pans; to treat tailings; magnetic separator; to consume wash water.

Exercise 6. Fill in the blanks with one suitable word:

1. The area is first cleared of bush, all trees being cut down, and suitable ... made over which machinery and material may be transported.

2. The necessary plant is brought over to the site, in sections and ...

3. A "key" cut is made; this consists of excavating a cut or paddook in the overburden, 12 to 15 feet wide, for the full length of the area thus exposing ...

4. When all the above operations are completed, mining may commence, by excavating the gravel with ... .

5. For the ... a track is made along the edge of the gravel exposed.

6. The water is usually carried off to a temporary … .

7. Diamond milling consists of ... ... and … .

READING

Exercise 7. Read the text below looking for the main ideas concerning mining of alluvial diamonds.

Text 2. ALLUVIAL DIAMOND MINING

Mining Methods

When an area has been prospected and its average value obtained and it has been decided to start mining operations, the following preliminary work is done:

1. The area is first cleared of all bush, all trees being cut down, and suitable roads made over which machinery and material may be transported.

2. The necessary plant is brought over to the site, in sections.

3. A "key cut" is made; this consists of excavating a cut or paddock in the overburden, 12 to 15 feet wide, for the full length of the area, thus exposing the gravel. All overburden is thrown on one side of the cut and the walls are kept vertical (fig. 18).

When all the above operations are completed, mining may commence, by excavating the gravel with picks and shovels, down to bedrock, and loading it into wheelbarrows for transport to the plant, which should not be more than 500 feet from the gravel paddock, at any time, as this is about the limit for wheelbarrow transport. For the barrows, a track is made along the edge of the gravel exposed, consisting of planks, 12 inches wide by 12 inches thick by 12 feet long, laid end on to each other, and extending from the working place to the plant. Both the overburden and gravel in the key cut are excavated on day’s pay, working a 9-hour shift, and the depths of each are noted for reference for contract work in the future. These depths vary from 3 to 6 feet for overburden and from 2 to 6 feet for gravel. Also it should be noted that all successive cuts are from 10 to 12 feet in width, a width of 10-8 feet being most convenient for measuring-up purposes, as will be explained later.