1. 4. Quality Control of Composition, Microstructure and properties of refractories

2.

In order to study the effect of composition and microstructure of a representative sample (ISO, 5022) of any shaped and unshaped refractories on its properties and performance under service conditions, the following successive investigations should be carried out according to the Egyptian and ISO standards :

4.1 Quantitative determination of their chemical constituents by wet- and/or X-ray fluorescence (XRF)- methods.

4.2 Qualitative and quantitative determination of their solid-phase composition by X-ray diffraction (XRD) method.

4.3Study of their microstructure and microchemistry bypolarizing microscope (PM), scanning electron microscope (SEM) and electron-probe micro-analyzer (EPMA). 3.

4.4 Determination of their technological properties:

The technological properties of refractories, that should be determined are divided as follows:

4.4.1 Densification properties.

4.4.2 Mechanical properties.

4.4.3 Thermal properties.

4.4.4 Refractory properties.

4.

4.4.1 Densification properties:

Linear Change,(%):

It is a measure of how much the refractory body will shrink or expand, when fired for the first time in manufacturing or application at a certain temperature. It is calculated according to the following equation:

X100, Where: l ois the sample original length, l tis the sample length after firing at a temperature t. 5.

Bulk Density (BD, g/cm 3 ):

It is the weight of 1 cm 3of the bulk refractory sample and calculated according to the following equation:

BD = [w 1/ (w 2 w 3 )] x liquid

Apparent porosity (AP, %):

It is the percentage of open-pores volume relative to the volume of bulk sample and calculated according to the following equation:

AP= { (w 2-w 1 ) /( (w 2-w 3 ) } x 100

6.

Water absorption (WA, %):

It is the weight percentage of water absorbed by the bulk sample relative to its dry weight and calculated according to the following equation:

WA = [(w 2 w 1 ) / w 1 ] x 1/ g liquidx 100

Where:

w 1 = weight of the dry sample

w 2= weight of the saturated sample in air

w 3 = weight of the saturated sample immersed in the liquid

g= specific gravity of the used liquid.

7.

For the determination of BD, AP and WA, at least three samples are saturated with a suitable liquid by boiling and/or evacuationfor 2 hours. This is followed by weighing the saturated samples in air (W 2 ),immersed in the liquid (W 3 ) and dry (W 1 ), after drying overnight at 110 o C. A computerized mercury Porosimeter is also used to determine bulk density and apparent porosity as well as pore-size distribution of any refractory material. This method is based on intrusion of mercury into grains of the material with < 8mm dimensions in an evacuated cell.

8.

True Density or Specific Gravity:

It is the weight of one cm 3of the fine powder of a material without any air in its open pores. It is determined using the pyknometer method according to the following equation:

True density = (g/cm) Where : W 1 Weight of dry pyknometer W 2 Weight of pyknometer + fine sample, W 3 Weight of pyknometer + fine sample +liquid, W 4 Weight of pyknometer + liquid 9.

True or total porosity, (TP, %):

It is the percentage of total pore volume of a bulk sample relative to its volume. It is calculated from the following equation:

True porosity = x 100 10.

Closedporosity, (CP,%):

The percentage of closed porosity can be calculated by subtracting the apparent porosity from the total one.

Apparent porosity is a controlling and very effective physical property. As the apparent porosity increases, bulk density, mechanical properties, thermal expansion, thermal conductivity, heat capacity and load-bearing capacity decreases, with the increase of thermal shock resistance and corrosion due to the increase of permeability of gas and liquid phases through the refractory lining, under service conditions. High quality dense bricks usually have apparent porosity up to 20 % with the highest bulk density, which varies according to their phase composition.

11.

4.4.2 Mechanical properties:

Cold- and hot- crushing strength, [CCS and HCS, kg/cm2 N/mm2 (MPa)]:

CCS and HCS are the capability of cylindrical or cubic sample to resist vertical stress (compression) up to failure at room temperature and at high temperatures, respectively. They are calculated according the following equation:

CCS or HCS= =Rate of loading: -2.0 kg/cm 2 / sec . For dense Refractories , - 0.5 kg / cm 2/ sec .For lightweight refractories 12. . Cold- and hot- modulus of rupture, (CMOR and HMOR, kg/cm2, N/mm2 (MPa):CMOR or HMOR are the capability of a rectangular bar sample to resist breaking by bending stress at room temperature or at high temperatures, respectively. They are calculated according the following equation: CMOR or HMOR. =x,[kg/cm 2 , N/mm 2(MPa)],Where: F= Stress at which the sample is broken, L= Sample length, W= Sample width, R=Sample thickness Rate of loading: - 1.5 kg / cm 2/ sec For dense Refractories, - 0.5 kg / cm 2/ sec For lightweight Refractories. 13.

.Abrasion resistance, (cm 3/ cm 2 ) :

It is a tribological property that indicates the rate of wear of the refractory lining surface due to sliding or movement of the fired material on its surface under kiln service conditions.

It is measured by many methods depending on oscillating of hard bodies, e.g. corundum balls on a ground sample surface under certain temperature, pressure and time. The rate of wear of the refractory sample may be calculated as the loss in weight and/or volume as well as linear wear, wear depth and coefficient. When measured as loss in volume, the unit of abrasion resistance can be expressed in this case as cm 3 / cm 2 . It must not exceed 0.25 cm 3 /cm 2 .

14.

Important Notes :

. Abrasion resistance is a very important property for the refractories used in the blast furnace and in the direct-reduction (Midrex) furnaces, especially in their upper parts. This is mainly due to the abrasive power of the raw materials and their reaction products.

. Refractories rich in quartz, corundum, mullite,spinel and silicon carbide with the lowest apparent porosity always show distinguished abrasion resistance.

15.

4.4.3Thermal properties:

.Reversible linear thermal expansion, (RLTE):

It is the percentage of expansion of a rod sample on firing up to 1000 or 1500 C as a function of firing temperature. It is calculated with the coefficient of thermal expansion according to the following equations:

Percentage of thermal expansion at t (oC) =x 100Coefficient of thermal expansion. at t (oC) =Where:L o = original sample length at ambient temperature, L t = sample length at the testing temperature, t, t o = ambient temperature, t=testing temperature. ( o C -1 ) 16. Figure 1: Reversible Linear Thermal Expansion of Some Refractories 17.

Important Notes:

. The expansion coefficient of any furnaces steel shell is higher than that of the refractory bricks, but its temperature is vice versa. Therefore, the linear thermal expansion of the brick lining would be greater than the shell. This yields higher compressive stresses, which lead to spalling of kiln brick-lining, if there are no sufficient expansion joints.

. The expansion joints, usually made of card-boards or ceramic fibers, are designed to absorb about half of the bricks thermal expansion (1.2-1.4% for basic bricks and 0.6-0.8% for alumino-silicate types at 1000 o C, respectively ).

18.

. It is better to have large number of expansion joints with smaller dimensions than smaller number with bigger dimensions.

. If expansion is made too wide, the risk of brick falling would be high.

. During kiln shut down, it is possible to find opened joints between bricks, since its thermal expansion is reversible.

19.

Thermal shock resistance (TSR) orspalling resistance, (SR) :

It is measured as number of cycles that 2" cubic samples can resist without cracking and/or disintegration, after repeating cycles of 15 min. sudden heating at 1000C followed by 15 min. sudden cooling in air or in water as one cycle.

It is also measured as loss in CCS or MOR of the quenched samples as a function of number of certain thermal shock cycles, e.g. 5, 10, 15, .... etc.

The thermal shock resistance of the refractory kiln-lining is improved by its lower thermal expansion as well as higher thermal conductivity.

20.

.Thermal conductivity, (W/m. o K) :

Two techniques are used :

.First:Direct measurement of the heat-flow according to the following equation :

K = (q / A) .( X / t)

Where:

q = rate of heat flow (W),

X= distance between the samples hot and cold faces, i.e. thickness (m),

t = t 1- t 2temperature of the samples hot and cold faces, ( o K),

A = area of the sample surface (m 2 ) .

21.

.Second: A comparative method of sample heat-flow rate parallel to a calibrated sample:

K=K.

K s= thermal conductivity of the test sample (S), K K c = thermal conductivity of the calibrated sample, t 1= temperature of the test sample hot face, t 2= temperature of the test sample cold face and the calibratedsample hot face,t 3= temperature of the calibrated sample cold face. 22. Figure 2: Thermal Conductivity Curves of Some Refractories 23.

Important Notes :

.For fire clay and high-alumina bricks and castables, thermal conductivity increases with increasing lining temperature, while it is the reverse for basic magnesite and dolomite bricks.

. The basic types generally have higher thermal conductivity coefficient (>3.0 kcal/m. h /C), at 1000C, than the former types(1.0 1.5 kcal/m.h/C).

24.

.The steel kiln shell thermal conductivity is about 40 k cal/m.h/C. Therefore, the higher the thermal conductivity of kiln lining, the higher are kiln-shell temperature as well as its overheating and heat losses.

.As an example, if kiln shell temperature increased from 200C to 315C, then the heat loss from kiln shell at open space area will rise from 4000 k cal/m 2 .h to 8330 k cal/m 2 .h.

25.

4.4.4Refractory properties :

.Permanent linear change (PLC ato C, %) :

PLC is the percentage of expansion or contraction occurs after re-firing shaped or unshaped refractory 2" cube samples for>5 hours at a temperature, at which they will be applied. It should not exceed 1.0% to confirm its degree of volume stability under service conditions.

26.

Refractoriness,(cone fusion test, cone No. or oC) :

It is the final melting temperature of the refractory materials.

It is measured by heating triangular cones of refractory samples up to complete fusion in comparison with standard cones having definite final melting temperatures.

The results are given as number (s) of the standard cones, which are melted along with the test sample cone or their equivalent temperatures in C.

27.

Load-bearing capacity, [Refractoriness under load (RUL):

It exhibits the relationship between linear changes occur in a 2cylindrical samples during its heating up to a given temperature under a constant load up to 10% subsidence(Failure) as a function of firing temperature and/or time. The used constant loads are: 2 kg/cm2 for dense refractories and 0.5 or 1 kg/cm2 for lightweight refractories.

Two techniques are used:

28.

A-Rising temperature test, [To, T0.5 (Ta), T1, T2, T3, . T10 (Te), oC] :

The temperatures corresponding to maximum expansion (To), as well as to beginning of subsidence (at 0.5-0.6 % subsidence; T 0.5or T a ) and at subsidence of 1% (T 1 ), 2% (T 2 ), 3% (T 3 ) . and 10 % (T 10or T e ) were derived from the RUL curves.

These curves show the linear changes of the test sample as a function of firing temperature at a constant rate ( < 5 o C/min.) up to 10% subsidence(Failure).

29.

B-Maintaining temperature test (rate of subsidence or creep rate mm/hour) :

Creep rate is determined by plotting subsidence percent of the test sample, at constant temperature (Ta) and load, as a function of time in hours. To determine the rate of subsidence (creep rate), the slope of the obtained curve is calculated as mm / hour .

The load bearing capacity of refractory materials is represented by temperature at which it starts to soften under constant pressure (Ta) as well as the rate of its creep under constant load and temperature (Ta).

30. Figure 3: RUL Curves of Some Refractories 31.

.Corrosion resistance :

This test is done to determine the rate of corrosion (wear) of refractory materials due to the physical and chemical action of the gas, liquid and solid phases in contact with them at service conditions. There are many methods used for studying corrosion resistance of refractory materials. Some of these methods are summarized in the following:

A-Lab tests :

a- Determination of refractoriness and phase composition of fired clinker / refractory powder mixtures (20:80 or 50:50) are used.

b- Following the rate of wear of refractory rods dipped or rotated in molten clinker.

c- Determination of rate of clinker corrosion on the surface and the degree of penetration of the clinker / refractory reaction product into the refractory, by using the Pill test.

32.

B- Simulative tests:

These tests are carried out at conditions similar to those, at which the refractory is exposed in service. The best way to carry out these tests is to apply the test refractory samples in lining a pilot-plant furnace under service conditions similar to those of the industrial kiln.

Factors affecting corrosion resistance of the refractory materials:

a- Chemical constitution of the clinker and refractory materials.

b- Phase composition of the refractory material.

c- Phase arrangement, i.e. microstructure of the refractory body.

d- Firing temperature.

e- Atmosphere inside the furnace.

f- Mechanical effects.

33.

Due to the basic nature of the kiln feed, which have nearly 64%CaO, 20-30% ofabasic liquid phase is developed in the fired batches inside the kiln hot zones. These batches also contain variable amounts of cycled and condensated vapours of alkali sulfates and chlorides, which are chemically very reactive.

The chemical constitution of the refractory lining plays a decisive role in determining its capability to resist different chemical attacks and to retain its physical and refractory characteristics under service conditions.

The following Figure No. 4 exhibits an EPMA-line scans, indicating the changes occurred in the chemical & phase composition and densification parameters at the interface of a magnesia-chrome brick / cement-clinker in the clinkering zone.

34. Figure 4: EPMA-Line Scans Showing the Changes Occurred in the Chemical & Phase Composition and Densification Parameters of a Magnesia-Chrome Lining / Cement-Clinker Interface in the Clinkering Zone. 35. Specific Gravity & Prosity using Pycnometer 36. Specific Gravity using Pycnometer W 1(weight of Pycnometer ) W 2(weight of Pycnometer + Sample ) W 4(wt of Pycnometer+ Sample+ water ) W 3(wt of Pycnometer+ Sample+kerosene ) 37. Apparent Porosity D(Dry weight of specimen ) S(Suspended weight ) W(Saturated Weight ) True porosity == [( B) /]100 Apparent Porosity 38. Sample Calculation

Apparent porosity (P)% =

Specific Gravity=

39. Solved example for : Density and Porosity- calculations 40. Silicon carbide particles are compacted and fired at a high temperature to produce a strong ceramic shape. The specific gravity of SiC is 3.2 g/cm 3 .The ceramic shape subsequently is weighed when dry (360 g), after soaking in water (385 g), and while suspended in water (224 g). Calculate the apparent porosity, the true porosity, and the fraction of the pore volume that is closed. Example 14.4 SOLUTION 41. Example 14.4 SOLUTION (Continued) The closed-pore percentage is the true porosity minus the apparent porosity, or 30 - 15.5 = 14.5%. Thus: 42.