Showing posts with label Bricks. Show all posts
Showing posts with label Bricks. Show all posts

September 19, 2010

Types of Insulating Refractory Bricks and Castables - their Manufacturing and Installation

No high temperature operation can go without heat management especially, in this ‘endless era’ of rising energy costs. The solution is of course, refractories and typically speaking, Insulating Refractories. The reason - it allows a furnace to reach temperature faster than without it, at the same time protects the unit’s surrounding environment from excessive heat and saves energy costs; add value to the customer’s product. 
In one of our earlier articles Insulating Refractories (Part–I), we reviewed the functions of insulating refractories, some of the fundamental technologies of high-temperature refractory insulation, mechanisms of heat transfer in industrial processes, rate of heat flow (heat loss), considerations of Thermal Conductivity in a refractory material, and how to calculate Heat Loss or Heat Transport and Thickness of Refractory Lining for a given furnace conditions etc. This article presents and discusses on the types or few qualities of insulating refractories, their manufacturing procedure including the various raw materials used, and also on the installation of insulating refractory bricks and castables.
There are several types of insulating refractories, including insulating fire brick (IFB), insulating castables, insulating pumpables, granular insulation, and ceramic fibre insulation [How effective are Insulating Refractory (Ceramic) Fibers?]. The insulating bricks may be classified mainly into two categories, one being used for the low temperatures, below 1000OC (CFI) and the other (HFI) for any temperature above 1000OC, depending on the raw material used in their manufacturing. Ceramic fibres of various compositions with corresponding application temperatures form another category of insulation.
To be a good insulating refractory brick they must have the following properties:
1. Low thermal conductivity.   
2. Mechanically strong enough to bear the load of the structure.
4. High porosity.
5. Low permeability.
6. Withstand the heat of at which they are used.
7. Must not shrink or react chemically with the material with which they are in contact during use.
It is well known fact that vacuum is the best insulator. Next to this comes the motionless air. But can we create vacuum in a brick for having insulating properties? For obvious reasons, the answer is no. This property is introduced in a brick by including a large number of air spaces in its body. The air spaces inside the brick prevent the heat from being conducted but the solid particles of which the brick is made conduct the heat. So, in order to have required insulation property in a brick a balance has to be struck between the proportion of its solid particles and air spaces. The thermal conductivity is lower if the volume of air space is larger. Importantly, the thermal conductivity of a brick does not so much depend on the size of pores as on the uniformity of size and even distribution of these pores. Hence, uniformly small sized pores distributed evenly in the whole body of the insulating brick are preferred. The brick should have enough pore space at the same time cellular in texture. This cellularity for manufacturing insulating refractory bricks can be introduced by one of the following ways:
(a) By addition of a combustible substance in the composition (mixture) of brick e.g. saw dust, paper fiber, coal dust, rice husk ash, styrophore etc. During firing this burns out leaving behind a porous structure.
(b) By using minerals which expand and open up on heating e.g. raw kyanite, some china clays.
(c) By addition of a volatile compound in the composition (mixture) of brick e.g. naphthalene.
(d) By using substances which by themselves have open texture e.g. insulating brick grog, vermiculite, ex-foliated mica, raw diatomite etc.
(e) By chemical bloating. This is generally done by using aluminium (Al) powder in combination with NaOH solution.
(f)  By aeration.
(g) By putting foaming agents in the mixture of the brick.
Amongst all these the first method is more common and easier for producing cellularity. The manufacturing of insulating refractory bricks and other insulating materials require a different approach. The low temperature insulating bricks are manufactured using granules of vermiculite, ex-foliated mica, and raw diatomite. While using any of the above raw materials, a good percentage of combustible or carbonaceous grains is used in the batch composition, which burns out during firing, leaving voids inside the texture of the brick. The high temperature insulating bricks are produced from mixtures of grains of calcined clay, raw kyanite with combustible material in the batch. Raw kyanite expands on heating by 15-18 per cent. This fact has made raw kyanite an excellent material for making insulating refractory bricks. When the bricks are fired, the kyanite expands and the bricks become porous. The addition of saw dust or any other combustible is helpful in the sense that on burning, the saw dust leaves open spaces and when kyanite expands, the expansion is borne well by these spaces and the structure is not disturbed [One complete 'Production Recipe' is given at the end of this article for manufacturing insulating refractory brick of two different compositions and properties].
Acid insulation bricks can be made similarly with crushed quartzite, fireclay and saw dust in batch. The use of combustible material may be eliminated by adopting the foaming technique during forming the shapes. In rice growing countries, the rice husk ash is a cheap and important insulating raw material suitable for use at a fairly high temperature 1500OC. A small percentage of plastic clay as bond is used in both low temperature and high temperature insulating bricks. The firings of insulated bricks are carried out at a temperature depending on the raw materials used as well as on the temperature of their application. The firing temperature should be preferably higher than the temperature of their application. Naphthalene is also used produced cellularity. It is mixed with fireclay and insulating grog in powder form and pressed into bricks. On firing naphthalene volatilizes leaving a cellular mass. Sometimes aluminium powder is used with NaOH to produce chemical bloating or froth. Chemicals like ammonium sulphate, ammonium chloride, ammonium nitrate, calcium phosphate, phosphoric acid, and sulphuric acid are also used for manufacturing insulating refractories. But these are generally used in manufacturing basic insulating refractories like magnesite. Sulphuric acid acts in green state. CO2 is expelled leaving the body of the brick porous. Other substances like ammonium chloride decompose on heating. The Cl2 mixed with water vapour acts with magnesite and CO2 is expelled.
Some Drawbacks of Insulating Fire Bricks (IFB)                         
Generally saw dust is used in the batch composition for manufacturing of insulating fire bricks which gives porous structure to the brick after firing. Although porosity decreases thermal conductivity and density of the brick, it also degrades the mechanical strength of the brick as compared to a dense refractory firebrick. The porosity also makes IFB more susceptible to chemical attack by gases, fumes, slags etc. The porosity in IFB or any other insulating refractories creates a large amount of free surface area. Since chemical attack starts at surfaces, porosity leads to poor chemical resistance as compared to dense refractories of similar compositions. Liquids such as slags, molten glass etc. at high temperatures can penetrate porous bricks easily, making insulating fire bricks unsuitable for direct contact with such liquids or gases.
The poor strength of IFB due to their high porosity can pose structural design problems. In addition, insulating fire bricks often suffer from thermal spalling problems, particularly in an environment of rapidly changing temperature. Since these bricks are good insulators, a substantial temperature gradient will occur between the hot and the cold face of each brick. The hot face will expand more than the cold face. The thermal gradient thus, gives rise to a mechanical stress in the body of the brick. Since, insulating fire bricks are not very strong, the surface can be spalled off by these stresses, especially if the temperature changes frequently.
Installation               
The procedure of installation of insulating fire bricks is same as dense brick. “Techniques of Installation of Fiber Refractory Linings” and “Techniques of Adding Insulation over the Existing Refractory Linings” have been discussed in a separate post. Insulating fire bricks are used as the hot-face refractory materials in ceramic kilns and many heat-treating furnaces. They can not be used on the hot-face when severe temperature or operating conditions exist. But insulating fire bricks are often used backup insulation in such circumstances. When used as backup insulation, it is important that the interface temperature between the working face of the furnace and the backup insulating brick is known so that the proper grade of these insulating fire brick (IFB) can be selected. Similarly, insulating castable refractories are monolithic refractory mixes into which a large amount of porosity has been introduced. The method of manufacturing is much the same as described above for insulating bricks. One way is to put saw dust or any other combustible material in the aggregate to make this porous when it is fired. Then the aggregate is crushed and sized and mixed with more conventional bonding chemicals to prepare the castable. Another approach is to put foaming agents (as mentioned above) in the mix, which are activated when water is added for installation. By this porosity is introduced in the matrix instead of the aggregate. Installation of insulating castable refractories is almost the same as for a dense refractory castable but again with due attention to the mechanical weakness of these castables in design of the system. Insulating refractory castables may be poured into an intervening space deliberately left between the steel shell and a free-standing wall of working refractory bricks. This technique ensures an excellent fit between any irregularities in the brickwork and irregularities that are bound to occur in the surrounding shell. Some poured-in backup insulation is simply made of loose, porous, granular fired refractories. This, of course, has no mechanical strength at all. Its use tends to be limited small furnaces whose brickwork is entirely self-supporting and to a number of other similar situations in relatively small vessels. Typically mineral fiber or other low-duty refractory materials used as backup insulation generally degrade over time, allowing heat to channel through. Insulating pumpables are refractory materials which provide quick and easy refractory lining repair. Common insulating pumpable applications include – re-insulating hot-spot in utility boilers, industrial furnaces and kilns, sealing around burner blocks and flues, and placement between fiber modules that have shrunk excessively. And more recent addition to these is the Insulating Foams that are cast with different cellular configurations.
Manufacturing and Composition Recipe
Materials
Volume  %
Firing Temp / ST (Shrinkage %)
Tentative Properties
China Clay (Ball Mill Fines)
Saw Dust (Fines)
Raw Kyanite (Fines)
45
32
23

1200OC / 2hrs
(1%)
Al2O3 = 40%   
Fe2O3 = 2%
Service Temp = 1400OC (max)
BD = 1.1 gm / cc
PCE = 32 ½
Apparent Porosity = 58%
CCS = 40 kg/cm2
Thermal Conductivity at 600OC H/F =         0.45 K.Cal/m/hrOC  

China Clay (Ball Mill Fines)
Saw Dust (Fines)
Insulating Grog (0 - 3mm)
55
35
10
1220OC / 2hrs
(1%)
Al2O3 = 30%   
Fe2O3 = 2%
Service Temp = 1300OC (max)
BD = 0.8 gm / cc
PCE = 30
Apparent Porosity = 70%
CCS = 15 kg/cm2
Thermal Conductivity at 600OC H/F =         0.35 K.Cal/m/hrOC  

  
production Process       
(a) Saw dust (containing 30% moisture max.) is screened through a Rotary screen (2mm). (b) Dry mixture is made. Materials are added by Volume per cent as per composition (e.g., here it is 8 boxes China Clay + 5 boxes Saw Dust + 1 box Insulating Grog = Total 14 boxes. If we calculate the same by weight then it comes about - China Clay 64%, Saw Dust 28% depending on its Moisture%, Insulating Grog 8%). (c) This dry mix is Pug Milled adding only water & kept in a Bunker under a plastic cover to avoid rapid drying. (d) Showering of water is done over this mix for 10-12 days. For accountability starting & last date of showering should be marked on the respective Bunker wall. (e) After 10-12 days of showering the same mixture is remixed in a Muller Mixer after adding some organic bond. After this final mixing, the Mixture is taken for moulding into clots as per the required size (provision in mould size should be kept for firing-shrinkage & final cutting). (f) After floor drying, clots are fired in a batch type kiln at about 1200-1220OC / 2hrs or as mentioned above. (g) Fired clots after cutting & little bit finishing are ready for packing and despatch.                     

March 18, 2010

Insulating Refractories (Part - I)


Insulating refractories are thermal barriers that keep in the heat and save energy. Furnaces used for melting, heat treatment, heat regeneration or for any other purpose demand maximum heat conservation so as to minimize heat losses for maximum heat efficiencies and minimum fuel consumption as well as high production as a result of maintaining high working temperatures. As the cost of energy has increased, the role of insulating refractories has become more important. Not too long ago, energy costs were low and stable, while the costs of insulating materials and, particularly, installation labour were moving northwards. Those circumstances dictated the use of minimal insulation. The situation is quite different now. The use of considerable quantity of refractories is socially and economically justified. With today’s energy costs at such higher levels has come the development of a wide range of new insulating refractory materials and technology of high-temperature insulation which are capable to restrict the escape of heat even at a much elevated temperature. Instead of going direct into the discussion of insulating refractories, their types, raw materials, manufacturing, properties and applications etc., here we will first review some of the fundamental technology of high-temperature insulation.     
The function of insulating refractory is to reduce the rate of heat flow (heat loss). Although it is not possible to totally prevent the flow of heat energy when there exists a temperature differential between two points, but it can be retarded. There are three mechanisms of heat transfer that we must understand. These are conduction, convection, and radiation. We must consider all these three mechanisms when we study the overall conductivity of a given material.
Heat transfer by Conduction occurs via the transfer of energy from atom to atom (or molecule to molecule) in a material. Atoms vibrate faster in higher temperature as they possess more energy. This energy will be passed to the adjacent atoms having lower energy. Since atoms and solids are bonded to one another and are in close contact, conduction in solids is higher than in liquids. Metals, especially, have high rates of conduction because both the atoms and their electrons conduct the electrons much more rapidly. Liquids generally have lower conduction rates than solids because of their lack of regular structure and strong bonding. Gases have much lower rates of conduction since their molecules exist at much lower concentrations and are in relatively infrequent contact. So, within metals, dense ceramics, and dense refractories Conductivity is the main mechanism of heat transfer.   
Energy transfer by Convection relies on the mass movement of a fluid. The moving fluid may be either a liquid or a gas. Convection does occur horizontally; but it depends on the gravitational force of the earth. Again, in case of dense refractory bricks heat transfer through this process can not happen since there is no fluid for convection.
Radiation process of heat transfer does not require the presence of any material. Radiation occurs most readily through empty space. The sun radiates energy through space to earth. Similarly all hot bodies radiate heat, and if they are hot enough they also radiate visible light which we call as glow.
When one studies heat transfer mechanisms in industrial processes, all three modes of heat (or energy) transfer must be considered. In a high temperature furnace or kiln, for example, energy is transferred from the heat source i.e. a burner to the material being heated and to the surrounding furnace refractory walls by all the three processes. The amount of energy transferred by radiation increases dramatically as the temperature increases. It is the dominant heat transfer mechanism at high temperatures. The load and the refractories of the furnace wall absorb energy, get hot, and re-radiate energy. The moving gases within the carry heat with them and transfer it when they come in contact with cooler solid. A small amount of gas conduction occurs, and conduction is the main process of transferring energy or heat from the surface of the solid or liquid load to its own interior.
One of the prime roles of a refractory is to withstand the effects of heat usually in a hostile environment. That is why for the selection of refractory and its designing Thermal Conductivity is one property which one has to consider. Usually one would like to have a refractory with low thermal conductivity so that heat may be more effectively contained within a furnace or kiln. Sometimes, however refractories and materials having high thermal conductivity are desired. For example, a protective muffle in certain ceramic kilns is designed to prevent combustion gases from reaching the ceramic ware. It must transfer as much heat to the ware as possible, so conductive ceramic materials like silicon carbide are often used for muffles.       
Since insulation refractories find application in processes involving thermal energy, an understanding of thermal properties especially, thermal conductivity of these refractories is quite important. Thermal Conductivity of a refractory material, k, is a measure of the amount of heat that it will allow to pass under certain conditions. Thermal conductivity can be defined as the quantity of heat transmitted through a material in unit time, per unit temperature gradient along the direction of flow and unit cross sectional area. First, let us understand the material conditions affecting this thermal property of a refractory brick whether it is insulating or normal brick, and then the most common method used to measure (or calculate) the same. While there are many factors affecting the thermal conductivity of refractories, some of the most important are [Reference: J.E. Burke, Progress in Ceramic Science, Vol. 2, Ed., Pergamon Press, Chapter 4, 1962]: 
1. Temperature
2. Complexity of structure (crystal and microstructure)
3. Defects (impurities, solid-solution, and stoichiometry)
Temperature dependence of thermal conductivity for several materials graph
                  Fig: Temperature dependence of thermal conductivity for several materials
The temperature dependence of thermal conductivity of several materials is shown in the adjacent figure. In general, the thermal conductivity is expected to decrease with increasing temperature when the temperature exceeds the Debye temperature. The Debye temperature is a characteristic temperature for a given material and may be below or above room temperature. The structural features such as, anisotropic arrangement of ions, relative mass difference between anion and cation, pores, and grain boundaries etc. do affect thermal conductivity of a material. Spinel (MgAl2O4) for instance, has a thermal conductivity lower than that for either MgO or Al2O3. Another example is reducing the thermal conductivity of a solid by introducing porosity and this is the most common technique of manufacturing insulating refractories.
Fortunately for us, the thermal conductivity of a refractory material is ordinarily measured in such a way as to account for all of the heat transfer processes that happen to be operating in that material. We do not have to unscramble them or deal with tem separately, for most ordinary purposes. Once that property is known for each material in the vessel, some very sophisticated calculations can be performed to find out where the heat goes in a given operation. In the next following lines we will discuss only the simplest of these calculations. This will be enough to enable you or someone to select among various insulating refractories and also to measure what will be the refractory lining thickness.
Imagine a large flat slab or wall of refractory, whose hot face (hot side), is at some fixed temperature, Th. Its cold face (cold side) perhaps in contact with a steel shell, is at some lower temperature, Tc. We will call the thickness of the refractory X. Let us assume that the heat is supplied to the hot face at some fixed rate by process fluids, and that heat is removed from the cold face (may be by the steel shell and the air outside it) at exactly the same rate. Two things then follow: (a) heat flows through the refractory at exactly the same rate as well and (b) temperatures Th and Tc do not change with time. This is called Steady State situation. If we call some amount of heat H flows in time interval t then the rate of heat-flow Q would be H / t. If you think about it, you will understand that this rate of heat-flow or heat transport has to be proportional to the area of refractory wall, A, through which heat is flowing. One mathematical equation connects all of these things at once is:
Refractory Lining Technology

 where, k is the value of thermal conductivity.


To use this equation, we will adopt a set of English units that engineers in the fields of processing and refractories are familiar with. The unit of heat energy, the BTU (British thermal unit), is defined as the amount of heat that will raise the temperature of 1 pound of water by exactly 1OF. The unit of time will be hour (hr). We shall take units of area A in square feet (ft2), the thickness X in inches (in.) and temperature in OF. Clearly if the situation described by A, X, Th, and Tc is held fixed but different materials are studied, the rate of heat transport (Q or H/t) will be proportional to the k (thermal conductivity) of each material. Since k is a property of each material, we can get different values for the rate of heat transport by choosing different materials or mixtures of them. Thermal conductivities i.e. values of k for different materials are measured in the laboratory and published. We can use them in calculations with the above equation. Only we need to make sure that the units of k are (BTU.in)/(ft2.OF.hr).
In fact, k is numerically equal to the rate of heat transport when the slab area (here, area of the refractory or furnace wall) is exactly 1 ft2 and the temperature gradient is exactly 1OF/in. The table below lists some of the typical values of thermal conductivity (k) for different solid materials: some metals, some ordinary “working” refractories, some insulating and some highly conducting refractories. Given below are some examples of how to calculate Heat Loss or Heat Transport and Thickness of Refractory Lining:      
Suppose we have a furnace lined with Superduty refractory brick, and the total wall area of this furnace is 1350 ft2 and also suppose the refractory lining thickness is 12 inch. Say, the process we are conducting in this furnace keeps its hot-face temperature at 3000OF. With thermocouples we find that the cold-face is at a steady temperature 600OF. Then, what will be the rate of heat loss through all the walls of this furnace ?    
We find from the table given below that k for Superduty brick is 9.5. Then by putting all the given numbers into our heat transfer equation mentioned above we get the rate of heat flow (heat loss) Q as per -
Refractory Lining Technology

Refractory Lining Technology





It will be instructive to check here as how much less refractory it would take to match this heat loss keeping all the conditions same if we used, say, an insulating refractory firebrick whose thermal conductivity (k) value is 3.0, also taken from the table below. Suppose that this insulating brick can survive at 3000OF, to make the question reasonable. Here we will find out the required thickness of the insulating brick lining for which we first rearrange the heat transfer equation to be explicit in X so that we can solve it for the refractory thickness. Then by putting all the given numbers into the equation except 3.0 for k, we get -
Refractory Lining Technology
That is 3.8 inch of insulating firebrick has the same heat transfer resistance as 12 inch of conventional Superduty refractory firebrick ! We would be na├»ve to replace the one refractory by the other until we learn more; but the effectiveness of insulating refractories in containing heat is impressive. If we were to keep the refractory lining thickness at 12 in. for example, and solve our heat transfer equation with k = 3.0, we would find that the total rate of heat loss is only 810,000 BTU/hr., instead of 2,565,000 BTU/hr. Now imagine how much thousands of dollars we could save per month in fuel costs !     
However, on practical ground or real - life, calculations are never this simple for numerous reasons. For one thing, the value of thermal conductivity itself changes with temperature as the relative contributions of conduction, convection and radiation change. The second complication we will mention here is that in most cases the refractory lining of a furnace or kiln is done with several refractory layers of varying qualities:
1. A working face of refractory layer or, interior layer of refractory lining that is exposed to the process;
2. The refractory lining between the furnace or kiln shell and working lining, often referred to as the Safety Lining or Insulating Lining. Insulating linings are used to limit heat loss and to maintain the vessel (furnace) shell temperatures at reasonable levels.
Such refractory lining arrangements definitely complicate the heat transfer calculations. But even with the simple introduction about insulating refractories what we have given above, you can appreciate that a process operator can intelligently design a refractory lining that will endure its use temperature and chemistry, and at the same time meet the restrictions on refractory lining thickness or on heat loss that are specified for the situation.
In our next post Insulating Refractories (Part - II) we will look at the different types of insulating refractories and their manufacturing etc.                      
      Table :  Typical Thermal Conductivity Values
Refractories / Materials
k (BTU.in/ft2.OF.hr)
Metals (dense solid)
Copper
Aluminium
Gold
Silver
304 Stainless Steel
310 Stainless Steel
1020 Carbon Steel

2500
900 - 1500
2060
2900
113
96
360
Dense Refractories
Silica Brick
Superduty Brick
Periclase
High Alumina
Chrome - magnesite
Zirconia

13
9.5
20 - 50
10 - 40
14
5
Insulating Refractories
Insulating firebrick 2800
Insulating firebrick 2600
Insulating firebrick 2300
Ceramic Fiber Blanket 4 pcf (lb/ft3)
Ceramic Fiber Blanket 8 pcf (lb/ft3)
Vacuum formed board
Backup insulation

2.5 - 3.0
2.0 - 2.5
0.9 - 1.3
0.6 - 3.0
0.35 - 2.0
0.4 - 1.5
0.3 - 1.0
Conducting Refractories
Silicon Carbide
Baked Carbon
Graphite

100 - 200
300 - 800
500 - 1200

June 14, 2009

MgO-C (Magnesia Carbon) Refractory Bricks : Granulometry

MgO-C (Magnesia carbon) refractories or Carbon containing Magnesite refractories have been extensively used by steel makers in ladles that are containers for the secondary treatment of steel. MgO-C refractory bricks are widely used in slag lines of BOF (Basic Oxygen Furnace) because of their superior wear resistance. The service life of Magnesia-Carbon refractories used in BOFs have been pushed quite significantly (largely due to slag splashing and gunning improvements) even as the service conditions have become more severe due to the increased operating temperature required for continuous casting and the need to produce cleaner steel.


Selection of raw materials, their grading and grain size distribution (Granulometry) and composition together play a very important role in the development of various physical properties, microstructure and thermo-mechanical properties of MgO-C refractory bricks. Various different types of MgO (Magnesite) grains provide different levels of corrosion resistance. It has been found that Magnesia-carbon bricks having 3 mm particle size show better wear resistance and other characteristics as compared to the bricks with 5 mm size grains. The graphite flakes used in these bricks impart -


=> High thermal conductivity

=> Good thermal shock resistance

=> Low thermal expansion

=> Non-wettability by liquid slag

=> Low corrosion rates by slags


Graphite contents of typical bricks range from 4 - 35% natural flake graphite. Since oxygen affinity of carbon is very high so different kinds of antioxidant minerals are used (in fines or superfines) in order to protect refractory material against chemical corrosion. The REDOX reactions in magnesia carbon can be reduced by selection of high purity magnesite, large crystal size and use of graphite with low impurities. Slag corrosion resistance of MgO-C refractories can be improved by use of magnesite grains with less reactivity i.e. fused magnesite grains of high Bulk Density (BD) and high purity.


The above are some of the reasons which explain how selection of various raw materials can affect the performance of magnesia-carbon bricks. More on this aspect and the compositions of Magnesia-carbon refractory bricks will be discussed in a separate post. Here, our topic is Granulometry i.e. overall grading and the grain size distribution, suitable for the best performance of MgO-C bricks. Grading and the grain size distribution are important as these are directly related with the following properties of Magnesia-carbon bricks:


=> Porosity

=> Mechanical strength

=> Spalling resistance

=> Microstructure and phase development

=> Wear resistance


From the experience of various trials and performances it has been found that 0 - 4 mm grading is the best for MgO-C bricks for all general applications and also for different shapes like Tap Hole Blocks, Sleeves, etc. (except Slide Gate refractories which will be different). The size distribution of the press mixture (powder) for MgO-C bricks with different Graphite percentages as they should be are given in the following table:

Grain size

IS Mesh No

BS Mesh No

Graphite < 12.5%


Graphite > 12.5%


> 2 mm;

<= 4 mm

+ 200#

+ 8#

20 - 25%

25 - 30%

> 0.5 mm;

<= 2 mm

+ 50#

+ 30#

35 - 40%

33 - 38%

> 0.2 mm;

<= 0.5 mm

+ 20#

+ 72#

10 - 15%

12 - 17%

<0.2 mm

- 20#

- 72#

25 - 30%

20 - 25%