thetechnologicalpropertiesofrefractory-110403091140-phpapp02

$Page 1: thetechnologicalpropertiesofrefractory-110403091140-phpapp02$
7/29/2019 thetechnologicalpropertiesofrefractory-110403091140-phpapp02

http://slidepdf.com/reader/full/thetechnologicalpropertiesofrefractory-110403091140-phpapp02 1/42

1

4. Quality Control of Composition,

Microstructure and properties of

refractories



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.

4.3 Study of their microstructure and microchemistry by

polarizing microscope (PM), scanning electron microscope (SEM) and

electron-probe micro-analyzer (EPMA).



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.



4.4.1 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:

o

ot

l

ll X 100,

Where:

l o is the sample original length,

l t is the sample length after firing at a temperature t.



Bulk Density (BD, g/cm3):

It is the weight of 1 cm3 of the bulk refractory

sample and calculated according to the followingequation:

BD = [w1 / (w2 – w3)] 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= { (w2

- w1) /( (w

2- w

3) } x 100



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 = [(w2 – w1) / w1] x 1/ gliquid x 100

Where:

w1 = weight of the dry sample

w2 = weight of the saturated sample in air

w3 = weight of the saturated sample immersed in the liquid

g = specific gravity of the used liquid.



For the determination of BD, AP and WA, at least

three samples are saturated with a suitable liquid by

boiling and/or evacuation for 2 hours. This is followed

by weighing the saturated samples in air (W2),

immersed in the liquid (W3) and dry (W1), after drying

overnight at 110oC. 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.



True Density or Specific Gravity:

It is the weight of one cm3 of the fine powder of a

material without any air in its open pores. It is determinedusing the pyknometer method according to the following

equation:

)()(

).(

4314

12

wwww

gww

True density = (g/cm )

3

Where:

W1 Weight of dry pyknometer

1

W2 Weight of pyknometer + fine sample,

W3 Weight of pyknometer + fine sample + liquid,

W4 Weight of pyknometer + liquid



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 =

densiytrue

densiybulk

.

.1 x 100



Closed porosity, (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.



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 = area

stress=

22.,

.,

mmor cm

N or kg

Rate of loading:

- 2.0 kg/cm2 / sec …. For dense Refractories,

- 0.5 kg / cm2 / sec …. For lightweight refractories



. Cold- and hot- modulus of rupture, (CMOR andHMOR, kg/cm2, N/mm2 (MPa):

CMOR or HMOR are the capability of a rectangular bar sample toresist breaking by bending stress at room temperature or at hightemperatures, respectively. They are calculated according thefollowing equation:

CMOR or HMOR. =2

3

x 2.

.

RW

LS

, [kg/cm2

, N/mm2

(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 / cm2 / sec … For dense Refractories,

- 0.5 kg / cm2 / sec … For lightweight Refractories.



. Abrasion resistance, (cm3 / cm2):

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. Whenmeasured as loss in volume, the unit of abrasion resistance can be

expressed in this case as cm3/ cm2. It must not exceed 0.25 cm3/cm2.



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 andsilicon carbide with the lowest apparent porosity always

show distinguished abrasion resistance.



4.4.3 Thermal 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) =o

ot

L

L L x 100

Coefficient of thermal expansion. at t (oC) =)( oo

ot

t t L

L L

Where:

Lo = original sample length at ambient temperature,

Lt = sample length at the testing temperature, t,

to = ambient temperature,

t = testing temperature.

)1-Co(



Figure 1: Reversible Linear Thermal Expansion of Some Refractories



Important Notes:

. The expansion coefficient of any furnace’s 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 1000oC,

respectively ).



. 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.



Thermal shock resistance (TSR) or spalling 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

1000°C 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.



. Thermal conductivity, (W/m.oK):

Two techniques are used:

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

K = (q / A) . (ΔX / Δt)

Where:

q = rate of heat flow (W),

ΔX = distance between the sample’s hot and cold faces, i.e. thickness (m),

Δt = t1 - t2 temperature of the sample’s hot and cold faces, (oK),

A = area of the sample surface (m2).



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

calibrated sample:

K = K . s c

32

21

t t

t t

sampleS of thickness

sampleC of thickness

,,,

,,,

Ks = thermal conductivity of the test sample (S),K

Kc = thermal conductivity of the calibrated sample,

t1 = temperature of the test sample hot face,

t2

= temperature of the test sample cold face and the calibratedsample hot face,

t3 = temperature of the calibrated sample cold face.



Figure 2: Thermal Conductivity Curves of Some Refractories



Important Notes:

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

magnesite and dolomite bricks.

. The basic types generally have higher thermalconductivity coefficient (>3.0 kcal/m. h /ºC), at

1000ºC, than the former types (1.0 – 1.5kcal/m.h/ºC).



. The steel kiln shell thermal conductivity is about 40 k cal/m.h/ºC. 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 200ºC to 315ºC,then the heat loss from kiln shell at open space area will rise from 4000 k

cal/m2.h to 8330 k cal/m2.h.



4.4.4 Refractory properties:

. Permanent linear change (PLC at oC, %):

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.



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.



Load-bearing capacity, [Refractoriness under load

(RUL):

It exhibits the relationship between linear

changes occur in a 2’’cylindrical 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 lightweightrefractories.

Two techniques are used:



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; T0.5 or

Ta) and at subsidence of 1% (T1), 2% (T2), 3% (T3) …. and 10 % (T10

or Te) 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 (<5oC/min.) up to

10% subsidence (Failure).



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).



Figure 3: RUL Curves of Some Refractories



. 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 moltenclinker.

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.



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 toapply the test refractory samples in lining a pilot-plant furnace under serviceconditions similar to those of the industrial kiln.

Factors affecting corrosion resistance of the refractorymaterials:

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.



Due to the basic nature of the kiln feed, which have nearly 64%CaO, 20-

30% of a basic 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.



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 & Prosityusing Pycnometer



Specific Gravity using Pycnometer

W1 (weight of Pycnometer ) W2 (weight of Pycnometer + Sample )

W4 (wt of Pycnometer+ Sample+ water ) W3 (wt of Pycnometer+ Sample+kerosene)

A t P it



Apparent Porosity

D (Dry weight of specimen )S (Suspended weight )

W (Saturated Weight )

True porosity =

= [(ρ – B ) / ρ ]100100

S W

DW

Apparent Porosity



Sample Calculation

Apparent porosity (P)% =

Specific Gravity =

100

S W

DW

)()( 2314

12

W W W W

W W

100

S W

DW



39

Solved example for :

Density – and

Porosity- calculations



Silicon carbide particles are compacted and fired at ahigh temperature to produce a strong ceramic shape.The specific gravity of SiC is 3.2 g/cm3.

The ceramic shape subsequently is weighed when dry(360 g), after soaking in water (385 g), and whilesuspended in water (224 g). Calculate the apparentporosity, the true porosity, and the fraction of the porevolume that is closed.

Example 14.4 SOLUTION



Example 14.4 SOLUTION (Continued)

The closed-pore percentage is the true porosity minus theapparent porosity, or 30 - 15.5 = 14.5%. Thus:



Documents

thetechnologicalpropertiesofrefractory-110403091140-phpapp02