Refractory(المواد المقاومة للحرارة)

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Refractory

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This article is about heat resistance. For other uses, see Refractory (disambiguation).

A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that made them applicable for structures, or as components of systems, that are exposed to environments above 1000 °F (800K, 500 °C)".^[1]

Refractory materials are used in linings for furnaces, kilns, incinerators and reactors. They are also used to make crucibles.

Contents

[edit] REFRACTORY MATERIALS

Refractory materials must be chemically and physically stable at high temperatures. Depending on the operating environment, they need to be resistant to thermal shock, be chemically inert, and/or have specific ranges of thermal conductivity and of the coefficient of thermal expansion.

The oxides of aluminium (alumina), silicon (silica) and magnesium (magnesia) are the most important materials used in the manufacturing of refractories. Another oxide usually found in refractories is the oxide of calcium (lime). Fireclays are also widely used in the manufacture of refractories.

Refractories must be chosen according to the conditions they will face. Some applications require special refractory materials. Zirconia is used when the material must withstand extremely high temperatures. Silicon carbide and carbon are two other refractory materials used in some very severe temperature conditions, but they cannot be used in contact with oxygen, as they will oxidize and burn.

[edit] CLASSIFICATION OF REFRACTORY MATERIALS

Refractories can be classified on the basis of chemical composition, method of manufacture, physical form or according to their applications.

• BASED ON CHEMICAL COMPOSITION

1. Acidic refractories : These are used in areas where slag and atmosphere are acidic. They are stable to acids but attacked by alkalis. The main raw materials belongs to the RO₂ group, ex- silica (SiO₂), zirconia (ZrO₂) etc.

2. Neutral Refractories: These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases. The main raw materials belongs to, but not confined to, R₂O₃ group. The common examples of these materials are alumina (Al₂O₃), chromia ( Cr₂O₃) and carbon.

3. Basic refractories: These are used on areas where slags and atmosphere are basic, stable to alkaline materials but reacts with acids. The main raw materials belongs to the RO group to which magnesia (MgO) is a very common example. Other examples includes dolomite and chrome-magnesia.

• BASED ON METHOD OF MANUFACTURE:

1. Dry press process

2. Fused cast

3. hand molded

4. Formed (Normal, fired or chemically bonded)

5. Un-formed (Monolithic- Plastic, ramming and gunning mass, castables)

• BASED ON PHYSICAL FORM:

1. Shaped: These have fixed size and shapes. These may be further divided into standard shapes and specially shapes. Standard shapes have dimension that are conformed by most refractory manufacturers and are generally applicable to kilns or furnaces of the same types. Special shapes are specifically made for particular kilns or furnaces.

2. Unshaped: These are without definite form and are only given shape upon application. These types are better known as monolithic refractories. The common examples are plastic masses, Ramming masses, castables, gunning masses, fettling mix, mortars etc.

[edit] INTRODUCTION OF DOLOMITE REFRACTORIES

The modernization and the expansion of steel and cement industries has necessity the use of a number of sophisticated refractory and dolomite is one of them. Dolomite refractories are generally preferred by steel and cement makers because of the following inherent advantages-

1) It provides cost effective refractory lining

2) It helps in achieving cleaner steel

3) It is found environment friendly

[edit] DIFFERENT TYPES OF DOLOMITE PRODUCTS

Dolomite products are divided into two parts according to the shape: 1)Bricks 2)Monolithic

Bricks are divided into two parts: a) Burnt bricks

1.pure dolomite bricks

2.Mag-Dolo bricks

b) Un-burnt bricks

1.Dolo pitch bonded bricks

2.Dolo resin bonded bricks

3.Dolo chemical bonded bricks

c) Monolithic

1.Dolo fettling mass

2.Dolo ramming mass

[edit] RAW MATERIAL USED FOR MANUFACTURING OF DOLOMITE REFRACTORY

The raw material for the manufacturing of the dolomite bricks is dolomite itself. This mineral is a double carbonate of calcium and magnesium: CaCO₃.MgCO₃.

Raw Dolomite composition

MgO 21 %

CaO 28 %

Al₂O₃ 0.3 %

SiO₂ 0.4 %

Fe₂O₃ 0.3 %

L.O.I 47 %

For refractory purposes the dolomite ideally contains 45.65% MgCO₃ and 54.35% CaCO₃. It must contain at least 40% MgCO₃, and silica, iron, alumina should be low.

EFFECTS OF SILICA:-

Silica % should be low. Higher % of silica will result in the problem of dusting.

EFFECT OF IRON:-

Iron % should be low because it gives black spots in manufactured brick.

EFFECT OF ALUMINA:-

Low % of alumina is desirable because if its amount is high it can reduce the refractoriness.

[edit] ADDITIVES AND BINDERS USED

• ADDITIVES USED:-

1. Resin powder – for hardening

2. Carbon black – to increase the carbon % in pitch bond

3. Mill scale – for sintering

4. Stearic acid – Burnt quality

5. Easter oil

6. LDO

• BINDERS:-

1. Wax – for burnt bricks

2. hard pitch & resin liquid – for un-burnt bricks

[edit] PROCESS OF SINTERING

The sintering of raw dolomite is done in rotary kiln in temperature of about 1800 0 C for 3 hours.

PURPOSE OF SINTERING

1) To increase the hydration resistance.

2) To increase the bulk density of raw dolomite from 2.89 gm/cc to 3.22 gm/cc

3) To achieve the grain porosity of lower than 4.0 %

4) To achieve the thermodynamic stability of CaO and MgO

The sintering is done by rotary kiln. The feed size of raw dolomite in rotary kiln is 5 mm to 25 mm. Working temperature in sintering zone is 1850 to 1900 0 C.

[edit] CHEMICAL COMPOSITION OF SINTERED DOLOMITE

CaO 56.4%

MgO 41.5%

SiO₂ 0.8%

Fe₂O₃ 0.6%

Al₂O₃ 0.6%

[edit] CRUSHING AND GRINDING

crishing and grinding of sintered dolomite can be done by jaw crusher, impact mill, ball mill etc.

[edit] SEIVING

Seiving or Screening is the important process in the production.The sizes of materials used are:

coarse 3-5 mm

medium 1.6-3 mm & 0.3-1.6 mm

fine 0-0.3 mm

dust < 0.2 mm

[edit] MIXING

Different size of particles are collected from silos in batching car and forwarded to mixer machines with addition of binders. Various mixer machines are used in dolomite department. For mixing of batch materials, mullar mixer is used.

MIXING SEQUENCE FOR BURNT BRICKS:-

Coarse

↓ mixing

Binders (wax)

↓ mixing

Fines + Dust + Additives (stearic acid)

↓ mixing

Total mixing time 10-15 minutes

MIXING SEQUENCE FOR UNBURNT BRICKS:-

• FOR PITCH BONDED BRICKS

Coarse

↓ mixing

Binders (molten pitch)

↓ mixing

Fines + Dust + Additives (graphite)

↓ mixing

Total mixing time 13-15 minutes

• FOR RESIN BINDED

Coarse

↓ mixing

Binders (liquid resin)

↓ mixing

Fines + Dust + Graphite Flaky type

↓ mixing

Resin Powder

↓ mixing

Total mixing time: 23-25 minutes.

[edit] PRESSING

The pressing is done in hyraullic pressing machine. After pressing the bricks are checked by the inspector whether there is some defects such as corner breakage, segregation, size defect, cracks etc. or not.

Then the bricks are set in the kiln car. Then the kiln car is transferred through the transfer car and pushes inside the kiln for firing or tempering.

[edit] FIRING OR TEMPERING

Burnt dolomite bricks are fired in dolomite tunnel kiln at a temperature of 1650 to 1600 °C. The word tempering means heating up of bricks at relatively low temperature. It is done for pitch and resin bonded bricks to prevent the oxidation of binders. In this tempering chamber, producer gas is fired in the combustion chamber and then the heat is transferred to the brick i.e. the brick is heated indirectly.

[edit] APPLICATION AREA OF DOLOMITE BRICKS

• AOD CONVERTER

Dolomite refractory behave excellently in AOD vessel and it has been found to be cost effective. AOD vessels are generally lined with DBMC refractory. During reduction process chrome is leached out from the refractory lining. Besides this, high basicity slag destroys the chrome-spinel in the mag-chrome refractory. Dolomite being free from chrome and containing high CaO, relatively unaffected under such condition.

• STEEL INDUSTRIES

The stringent operating conditions prevailing at the customers end has thrust upon the steel manufacturers on the development of high quality steel with improved properties. It is in this scenario stringent quality requirements have been placed on the steel manufacturers. In the presence of certain elements in the steel matrix affects its end properties significantly. Hence there have been rapid strides in the steel making technology to produce cleaner steel. The most beneficial effect on steel quality is obtained by precise control of oxygen, carbon, sulphur and level of alloying elements in steel. The refractory manufacturers had to keep pace with the changing technology in steel industries. The success of any steel manufacturing process is directly depended in the performance of refractory lining. Secondary refining processes after primary melting in EAF or BOF have gained important over years. Today world over, the most common routes for steel production are the EAF/AOD/LF, EAF/LF and BOF/LF.

• ELECTRIC ARC FURNACE:-

The burnt, direct bonded dolomite bricks are being used in the working hearth of arc furnace and slide walls below slag zone. They have excellent corrosion resistance towards steel and slag at elevated temperatures. Dolomite fettling masses are extensively in used for fettling of working hearth of EAF in between heats to take care of uneven erosion during operation.

• BASIC OXYGEN FURNACE

The sever wear areas are the slag line, the trunnion areas, the bottom, the charging pad and the tap hole areas. The burnt, the pitch impregnated dolomite back up lining is standard practice.

• CEMENT ROTARY KILN:-

The burnt direct bonded dolomite bricks are being used in the burning zone of cement rotary kiln. They do not react with alkalis for their high basicity. At 1400 ºC, Dolomite is highly resistant to redox reaction because of the direct bonded between MgO and CaO. It favors the formation of coating, which is required to obtain high performance from the refractory. There is no environmental pollution with dolomite refractory.

• LADLE FURNACE

The major areas of application of dolomite refractory are in steel ladles and secondary ladle furnace. pitch bonded, resin bonded and fired dolomite bricks are used in steel ladle application i.e. pitch bonded dolomite bricks are used in metal line, mag-carbon bricks are used in slag line and resin bonded dolomite bricks are used in impact pad and bottom.

• VOD LADLE

De-carburization under vacuum allows production of high chrome and nickel alloy steel with extremely low carbon contents i.e. less than 0.02 %. Reduction and Desulphurization periods favors the use of dolomite lining and ceramic-ally direct bonded quality is virtually and industrial standard in all European plants.

[edit] Refractory anchorage

Anti-abrasion refractory attached to pipes, chambers, etc, will require anchorage systems such as wire formed anchors or hexmetal to support the refractory linings. The anchorage used for refractory on roofs and vertical walls are more critical as they are to support the weight of refractory even at the elevated temperatures and operating conditions. The commonly used anchorages have circular or rectangular cross-section. Circular cross-section are used for low thickness refractory and they support relatively less weight per unit area; whereas the rectangular cross-section is used for high thickness refractory and can support higher weight of refractory per unit area. The number of anchors to be used depend on the operating conditions and the refractory materials. The choice of anchors shape, numbers and size has significant impact on the useful life of the refractory.

[edit] Refractory heat-up

Usually, refractories require special heat-up techniques to ensure that their performance will be attained as designed, and to avoid thermal shock and drying stresses until the operational status is achieved.

[edit] See also

[edit] .

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