Chemistry of Blending

CHEMISTRY OF BLENDING

2Cement Technology Course

Blending - what and why

Defining variability

Variability timebase

Why reduce variability?

What impact can it have on the process?


Variability

Variability is best described as lack of uniformity in a system. Most people understand the concept but fewer are able to quantify it. In order to manage variability we must have a way or method of assessment - this is usually done by reference to the standard deviation of a property.


Where x is the measured variable e.g. LSF

x is the variable mean

n is the number of observations

Standard Deviation ()

2

1

)(

n

xxi i =1 to n


Relationship between and ranges

• For a large range of data (100+ points) approximates to 1/6 of the range (highest - lowest

result)

99.7% of results should lie within 3

•For lower numbers of results, (<40) then approximates to 1/4 of the range.

95% of results should lie within 2


2.0 2.5 3.0 3.5

Typical distribution curve100

50

0

No.

of

resu

lts

Northfleet despatch SO3 = 0.15%

66% 1

95% 2

99.7% 3


Variability timebase

Perceived variability will vary according to both the type of sample taken and the frequency of measurement. As a general rule, instantaneous or “spot” samples will show a higher variability than samples accumulated over a long period. For the purposes of control, it is necessary to understand both the current variability and any underlying trends in mean data.


150

140

130

120

110

100

90

80

70

60

50

Long Term Variability

0 100 days


150

140

130

120

110

100

90

80

70

60

50

Medium Term Variability

0 100 days


150

140

130

120

110

100

90

80

70

60

50

Short term variability

0 100 days


150

140

130

120

110

100

90

80

70

60

50

Overall Variability

0 100 days


Selective Quarrying methods

Stockpile Homogenisation

Raw Meal blending silos

Mill Circuits

100.0

50.0

20.0

10.0

5.0

2.0

1.0

0.5

0.2

0.1

Var

iab

ilit

y p

erio

dic

tim

e -

hou

rs

Cumulative frequency0.0 0.2 0.4 0.6 0.8 1.0


Why reduce variability?

Variable kiln feed chemistry:-

•Reduces kiln output

•Increases kiln fuel consumption

•Reduces refractory lifetime

•Impairs quality

So - what should be controlled?


Quality factors to be controlled

Lime Saturation Factor (LSF)

Silica Ratio (SR)

Alumina ratio (AR)

Particle size (90m or 75 m residues)

Minor Elements (MgO, SO3, Na2O etc.)


Variability

What is the starting position?

LSF SR AR Size

Limestone 60-2000 0.5-5 0.5-4 2m3-dust

Shale/clay <1-40 1.5-4 1-4 15cm-dust

Sands ~0-2 5-100+ 1-5 10mm-dust


Lime Saturation Factor (LSF)

Is a measure of the Lime (CaO) component of the raw meal/raw material available for combination with the three other major components (Si, Al & Fe). A high lime saturation factor indicates excess lime while a low factor (<100%) indicates a surplus of the secondary materials.


Silica Ratio (SR)

Is the term used to describe the ratio of silica to the

other two major components (Al2O3 & Fe2O3) . In

practical terms, it describes the relative ratios of

strength giving silicate minerals to the flux minerals

in clinker.


Alumina ratio (AR)

Defines how much of the Alumina (Al2O3) present will

convert to C3A in the kiln and How much will form

C4AF. This will have an impact on factors such as

cement colour, workability and sulfate resistance.


Particle size (residues)

Although not a direct “chemical” effect, the size distribution of the various components, particularly silica particles, has a significant influence on reaction rates in the kiln. Variable particle sizes will dictate variable burning temperatures for the same degree of reaction.


Minor elements

Minor elements such as sulfur, alkalis and magnesium (MgO) can all play a part in de-stabilising the process and also these elements have potential impacts on cement quality. While few plants can significantly adjust the levels of these elements, minimising variations in them is important.


Where do we want to get to? Target variability’s

Variable Max sd

LSF 1.0

SR 0.07

AR 0.05

90 m 1%

Coal Ash 1.5%

Feed rate 2%


0 0.5 1.0 1.5 2.0 2.5 3.0

SR = 3

SR = 2

1550

1450

1350

1250

Tem

p °C

1300

1400

1500

1600

90 micron residue

Temp °C

5 10 15

20

LSF=100

LSF=90

Combinability temperature for 1% free lime155

0

1450

1350

1250

Tem

p °C

90 micron residue

2.0 2.5 3.0 3.5

Combinability temperature for 1% free lime155

0

1450

1350

1250

Tem

p °C

Combinability temperature for 1% free lime

90 92 94 96 98 100 102 LSF

1550

1450

1350

1250

Tem

p °C

Effect of Chemistry on Burning zone temperature


Predicted BZ temperature

(Actual data)1700

1200

1300

1400

1500

1600

Time


Fuel Consumption vs. LSF variability

725

750

775

800

825

850

875

900

0 1 2 3 4 5 6 7Kiln Feed LSF Std. Dev.

kca

l/kg


Kiln output penalty vs. kiln feed LSF Std. Dev.

50

60

70

80

90

100

0 1 2 3 4 5 6 7Kiln feed LSF Std Dev.

% o

f O

utp

ut


Summary

Cement manufacture starts with highly variable raw materials

Reduction of this variability to low Sd's is achieved by several methods.

Low chemical (and fineness) variability yields the highest quality and output and leads to the lowest costs.

Chemistry of Blending

Documents

Transcript of Chemistry of Blending