Здавалка
Главная | Обратная связь

Exercise 2. Make a summarizing translation of the text.



Text 6

 

Metalworking processes

Metals are shaped by processes such as casting, forging, flow forming, rolling, extrusion, sintering, metalworking, machining and fabrication. With casting, molten metal is poured into a shaped mould. With forging, a red-hot billet is hammered into shape. With rolling, a billet is passed through successively narrower rollers to create a sheet. With extrusion, a hot and malleable metal is forced under pressure through a die, which shapes it before it cools. With sintering, a powdered metal is heated in a non-oxidizing environment after being compressed into a die. With machining, lathes, milling machines, and drills cut the cold metal to shape. With fabrication, sheets of metal are cut with guillotines or gas cutters and bent into shape.

Cold working processes, where the product’s shape is altered by rolling, fabrication or other processes while the product is cold, can increase the strength of the product by a process called work hardening. Work hardening creates microscopic defects in the metal, which resist further changes of shape.

Various forms of casting exist in industry and academia. These include sand casting, investment casting (also called the “lost wax process”), die casting and continuous casting.

Heat treatment

Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion. Common heat treatment processes include annealing, precipitation strengthening, quenching, and tempering. The annealing process softens the metal by allowing recovery of cold work and grain growth. Quenching can be used to harden alloy steels, or in precipitation hardenable alloys, to trap dissolved solute atoms in solution. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties.

Often, mechanical and thermal treatments are combined in what is known as thermo-mechanical treatments for better properties and more efficient processing of materials. These processes are common to high alloy special steels, super alloys and titanium alloys.

Plating

Electroplating is a common surface-treatment technique. It involves bonding a thin layer of another metal such as gold, silver, chromium or zinc to the surface of the product. It is used to reduce corrosion as well as to improve the product's aesthetic appearance.

Thermal spraying

Thermal spraying techniques are another popular finishing option, and often have better high temperature properties than electroplated coatings.

Microstructure

Metallurgists study the microscopic and macroscopic properties using metallography, a technique invented by Henry Clifton Sorby. In metallography, an alloy of interest is ground flat and polished to a mirror finish. The sample can then be etched to reveal the microstructure and macrostructure of the metal. The sample is then examined in an optical or electron microscope, and the image contrast provides details on the composition, mechanical properties, and processing history.

Crystallography, often using diffraction of x-rays or electrons, is another valuable tool available to the modern metallurgist. Crystallography allows identification of unknown materials and reveals the crystal structure of the sample. Quantitative crystallography can be used to calculate the amount of phases present as well as the degree of strain to which a sample has been subjected.

Leaching (metallurgy)

Leaching is a widely used extractive metallurgy technique which converts metals into soluble salts in aqueous media. Compared to pyrometallurgical operations, leaching is easier to perform and much less harmful, because no gaseous pollution occurs. The only drawback of leaching is its lower efficiency caused by the low temperatures of the operation, which dramatically affect chemical reaction rates.

There are a variety of leaching processes, usually classified by the types of reagents used in the operation. The reagents required depend on the ores or pretreated material to be processed. A typical feed for leaching is either oxide or sulfide.

For material in oxide form, a simple acid leaching reaction can be illustrated by the zinc oxide leaching reaction:

ZnO + H2SO4 → ZnSO4 + H2O

In this reaction solid ZnO dissolves, forming soluble zinc sulfate.

In many cases other reagents are used to leach oxides. For example, in the metallurgy of aluminium, aluminium oxide is subject to leaching by alkali solutions:

Al2O3 + 3H2O + 2NaOH → 2NaAl(OH)4

Leaching of sulfides is a more complex process due to the refractory nature of sulfide ores. It often involves the use of pressurized vessels, called autoclaves. A good example of the autoclave leach process can be found in the metallurgy of zinc. It is best described by the following chemical reaction:

2ZnS + O2 + 2H2SO4 → 2ZnSO4 + 2H2O + 2S

This reaction proceeds at temperatures above the boiling point of water, thus creating a vapor pressure inside the vessel. Oxygen is injected under pressure, making the total pressure in the autoclave more than 0.6 MPa.

The leaching of precious metals such as gold can be carried out with cyanide or ozone under mild conditions.

 

Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in several processing industries. Historically this was first used in the mining industry.

History

William Haynes in 1869 patented a process for separating sulfide and gangue minerals using oil and called it bulk-oil flotation.

The first successful commercial flotation process for mineral sulphides was invented by Frank Elmore who worked on the development with his brother, Stanley. The Glasdir copper mine at Llanellyd, near Dolgellau, in North Wales was bought in 1896 by the Elmore brothers in conjunction with their father, William Elmore. In 1897, the Elmore brothers installed the world's first industrial size commercial flotation process for mineral beneficiation at the Glasdir copper mine. The process was not froth flotation but used oil to agglomerate pulverized sulphides and buoy them to the surface, and was patented in 1898 with a description of the process published in 1903 in the Engineering and Mining Journal. By this time they had recognized the importance of air bubbles in assisting the oil to carry away the mineral particles. The Elmores had formed a company known as the Ore Concentration Syndicate Ltd to promote the commercial use of the process worldwide. However developments elsewhere, particularly in Australia by Minerals Separation Ltd, led to decades of hard fought legal battles and litigations which, ultimately, were lost as the process was superseded by more advanced techniques.

The modern froth flotation process was independently invented the early 1900s in Australia by C.V Potter and around the same time by G.D Delprat. Initially, naturally occurring chemicals such as fatty acids and oils were used as flotation reagents in a large quantity to increase the hydrophobicity of the valuable minerals. Since then, the process has been adapted and applied to a wide variety of materials to be separated, and additional collector agents, including surfactants and synthetic compounds have been adopted for various applications.

In the 1960s the froth flotation technique was adapted for deinking recycled paper.

 

Words to know:

1. forging

2. flow forming

3. rolling

4. extrusion

5. sintering

6. metalworking

7. machining

8. fabrication

9. shaped mould

10. red-hot billet

11. to hammer

12. a die

13. lathe

14. milling machine

15. guillotine

16. work hardening

17. investment casting

18. die casting

19. continuous casting

20. annealing

21. precipitation strengthening

22. quenching

23. tempering.

24. dissolved solute atoms

25. plating

26. surface-treatment technique

27. thermal spraying

28. finishing

29. crystallography,

30. diffraction

31. aqueous media

32. froth flotation

33. hydrophobic

34. hydrophilic

35. buoy

36. fatty acids

37. surfactants

Exercise 1. Explain the meaning of the following words and word combinations:

Guillotines, resistance to corrosion, aesthetic appearance, microscopic and macroscopic properties, optical or electron microscope, diffraction of X-rays, autoclave, hydrophobicity, deinking recycled paper.







©2015 arhivinfo.ru Все права принадлежат авторам размещенных материалов.