COMPLETE SPECIFICATION
FIELD OF THE INVENTION
rhe present invention relates to a process of producing an aggregate called "LOW DENSITY AGGREGATE" from bottom clinkers of travelling grate coai fired boilers along with ash from Circulating Fluidized Bed Combustion (CFBC) boilers of thermal power plants. The end result i.e. the light weight aggregate being an eco-friendly product will replace the natural stone chips/ aggregates and sands.
BACKGROUND OF THE INVENTION
It is well known that the travelling grate coal fired boilers use a wide range of coal with varied fusion temperature and sizes. Due to improper combustion, the travelling grate boilers generate bottom clinkers with high un-burnt carbon. This bottom clinker with high un-burnt carbon is not useful for cement / fly ash brick plant or any other ash utilization processes. The present invention is to utilize the un-burnt carbon present in the bottom clinkers of the travelling grate boilers in producing aggregate. I'urther, the objective of the present invention is to create an useful product out of the bottom clinkers coupled with ash from CFBC boilers with binding materials. This aggregate is a homogeneous mixture of the ash 1 rem CFBC boilers and the bottom clinkers from the travelling grate boilers mixed with a binding agent called bentonite and water and then sintered at 1200 to 1400 degree centigrade.
The current generation of bottom clinkers as well as the ash from coal fired thermal power plants in the country is quite alarming. Disposal of both bottom clinkers and ash is a big concern for power houses. Development of suitable process for gainful utilization of ash as well as bottom clinkers on a large scale has been actively under consideration by all.
2

I
.^7ie key purpose of the present invention is to produce aggregate from bottom clinkers of travelling grate coal fired boilers with higher percentage of un-burnt carbon and ash from circulating fluidized bed combustion boilers of thermal power plants. Both of these are waste materials and their disposal is an environmental problem.
The un-burnt carbon left in bottom clinkers and in ash are possible sources of energ}^ Travelling grate system with best of its operation will still be left vvitii un-burnt carbon varying between 8 to 14%. This results in pure loss of depleting resources. The aim of the present invention is to utilize the residual carbon of the bottom clinkers and convert the same into useful heat energy. The un-burnt carbon when burnt in the process not only provides heat source but also creates void necessary for usefulness of product aggregate.
The present invention is to utilize the higher amount of un-burnt carbon content iwnilable in the bottom clinkers with other form of ash generated in the sintering
process.
'The final result i.e. the aggregate thus formed is lower in density compared to natural stone chips. The aggregate is useful for several applications in the construction industry like construction of buildings, roads, pavements, pre fab structures, embankments, etc. It can also be used as filler for low lying areas, mines, etc. It is also useful as an absorbent media in waste water treatment plants and protecting layer for covering ash ponds in place of ncitural stone aggregate & sand.
SUMMARY OF THE INVENTION
The manufacturing process of the aggregate requires grinding of bottom clinkers with higher percentage of un-burnt carbon (> 8 %) in a grinding mill to a size less than 40 micron. The main object of the present invention is to utilize ash of any kind which has a certain percentage of un-burnt carbon to make aggregates. This process requires 5 to 7% of un-burnt carbon which
3

is present in the homogeneous mixture. The bottom clinkers and ash are stacked separately and later blended as per their composition in a blending silo. The proportion of each component depends upon the un-burnt carbon content in the different forms of ashes. The homogeneous ash mixture is mixed with bentonite powder (less than 1 %) to form a powdery mix. The powdery mix is fed to a disc type pelletizer along with water to form ball like pellets of various sizes below 14 mm. The pellets thus formed are sintered between 1200 to 1400 degrees centigrade to get the aggregate.
A brief description of Bentonite powder is as follows:
Bentonite powder is prepared from bentonite clay, a naturally occurring sodium substance and works as a binding agent.
Chemical Equation of Bentonite : A1203 4Si02 H20
Chemical name : Naturally occurring hydrated aluminum silicate
l^h) sical properties :
Bulk density (Tapped) : 37 to 40 lbs / ft 3 (592.6 to 640.6 kg per cu. meter)
Specific gravity : 2.4
Colour: Light Cream
fet another objective of the present invention is the adoption of easy and low processing routes ivhich when translated, results in complete utilization of bottom clinkers and ash of thermal power plants.
Due to the fusion during burning in the travelling grate coal fired boilers bottom clinkers are usually formed. These hot clinkers are dropped in a water pond for quenching and subsequently collected in the form of wet Jumps. For their utilization the bottom clinkers have to be dried and ground. This is done with the help of high pressure vertical roller grinding mills. The bottom clinkers are ground over the rotary grinding table with the help of grinding rollers which pressurize the bottom clinkers by liydraulic forces via hydraulic cylinders. Generally grinding takes place in
4

between the mating surface of the rotating grinding table and freely moving rollers. The dynamic classifier present at the top of the mill body separates the coarse particles from the fine particles. The ground product is sucked by an Induced Draught (ID) fan. The fine particles pass through the classifier and are collected in the hopper whereas the coarse particles fall back on the grinding table again for regrinding. The ground bottom clinkers are then transported to the blending silo where the process of dry mixing with ash is performed.
The production of aggregate in this process basically requires three types of raw materials and water in limited quantity. The raw materials are ash from CFBC boilers (un-burnt carbon 2% to 4%), ground bottom clinkers (un-burnt carbon 12% to 14%) and bentonite which acts as a binding agent. The ideal requirement of un-burnt carbon in ash mixture is 6% to 8%. In order to achieve the un-burnt carbon percentage of 6% to 8%, suitable blending of bottom clinkers having higher un-burnt carbon and ash from CFBC boilers with lower un-burnt carbon is required to be carried out. The blended ash with un-burnt carbon of 6% to 8% is then mixed with Bentonite (0.5 to 0.75 % by volume).
the blended ash and bentonite is mixed in a blender. This homogeneous mixture is then fed to a disc type pelletizer along with controlled water. By fid ding controlled amount of water to a specially designed pelletizing pan, wet round pellets are formed. The wet round pellets (also called green pellets) thus formed are fed on to the green pellet conveyor. The green pellet conveyor takes the green pellets to spreader conveyor and the spreader conveyor spreads the green pellets on the sintering strand. The green pellets are thereafter heated under the sinter strand at a temperature ranging between 1200 to 1400 degrees centigrade. This entire process produces a hard, fused, porous aggregate. The sinter strand has three zones viz. (a) the firing section (b) the soaking section and (c) the cooling section.
In the sinter strand, the top layers of the green pellets are fired by burners. hiitiaily the burning starts with the help of a liquid fuel. Once the un-burnt c arbon present in the pellets catches the fire, the sintering process continues
5

and the supply of liquid fuel is stopped when a stable bed temperature is
achieved.
The present invention helps to avoid use of costly fuel used in sintering process during burning by utilizing the un-bumt carbon content in the bottom clinkers.
The burning red hot pellets move out of the firing zone to the soaking zone. In the soaking zone, the top layer fire is sucked dovv^n by the reverse draught from the ID fan. Due to this process, the entire thickness of the sintered pellets on the strand gets uniform^ly hot. The pellets now move into the cooling zone from the soaking zone. Forced natural air is blown onto the pellets for quick cooling in the cooling zone.
Fiy the time the pellets reach the end of the strand, the temperature is bi ought to 350 to 400 degrees centigrade. Due to the intense heat, the pellets tend to stick to each other. Therefore, for the purpose of separating the pellets, the cooled pellets are dropped onto a Breaker. The breaker then separates the pellets and the pellets are passed onto Steel Belt Pan Conveyors. The pellets formed are round in shape and the size ranges from 14 mm down to fines. These are then carried by conveyors to the Finishing Section.
An installed ID fan sucks air from the sintering strand. The air passes through the dust collection chamber and then through bag filters before it is let out into the atmosphere through a chimney. The dust that is collected in the dust collection chamber is taken back and fed back into the silos through pressurized disposal pump (dense phase) for recycling.
riie cooled down pellets are then conveyed to a vibro-sieving machine. The sieving machine segregates pellets of different sizes and channelizes different sizes to different conveyors for storing in different Finished Product Feeder Silos.
The shape of the final product is round or elliptical with varying sizes of 2 mm to 14 mm. The burning of un-burnt carbon during the process of sintering gives rise to porous nature of pellets which also
6

reduces its weight per volume and hence, is called as Low Density Aggregates or LDA in short. These pellets can be used for various kinds of concrete application replacing the use of stone chips and the fines or below 2 mm can be used as a replacement for sand.
CHARACTERISTICS OF BOTTOM CLINKERS / ASH REQUIRED FOR MANUFACTURE OF LDA
Main characteristics of bottom clinkers required for producing LDA should
generally be as follows:
1 Grain size of the bottom clinkers after grinding:
Percentage passing through the sieve:
1.00 mm 100% |
0.50 mm >99%
0.25 mm >98% |
0.125 mm >97%
0.063 mm |>80% 1
2. Mass density of dry ground bottom clinkers: 2.1 - 2.25 gr/cm3
3. Loss of ignition: > or = 12 % on mass
Main Characteristics of ash from CFBC boiler required for producing LDA should generally be as follows:
1. Grain size of the ash :
Percentage passing through the sieve:
1.00 mm I 100% j
0.50 mm >99% |
0.25 mm |>98% J
0.125 mm j>97% I
-'■ ■ I ----- -■- -^
0.063 mm !>80% ;
7

2. Mass density of dry ash: 2.1 - 2.25 gr/cm3
3. Loss of ignition: < = 2 % on mass
CHEMICAL COMPOSITION (all % are on mass)
Element/ Standard Element/ [Standard
Compound Compound
I
Si02 43-65% CaO 2.4-4.2%
Fe203 6-12% A1203 18-24%
MgO 2-5.5% S03 0.4-0.8%
K20 <3.16 % Ti02 <0.90 %
Na20 <0.66% |P205 <0.36 %
1
Sr ;<0.5% Ba I < 0.04%
Mn <0.04% Zn I < 0.03%
Ni <0.02% |Li |<0.02%
Cr <0.02% |Cu <71mg/Kg
<71mg/ 1
Pb Kg I Co j<30mg/Kgi
< 13 mp^ / I !
^ Ka 1^^ <8mg/Kg
:.-^g ___] !
Ag <2mg/Kgjj^^ |<2mg/Kg
As <30mg/ |g^ |<10mg/Kg
-"^O : J
<250mg/ I ' :Kg _^ _,_ _.__ __!
8

STANDARD FOR LDA
UK 13055 - 1, 1977, European Standard coarse aggregate for concrete and
mortar.
NEN 3543, Course aggregate for LDA concrete.
TECHNICAL ACCEPTABILITY OF THE PRODUCT FOR CIVIL CONSTRUCTION USAGE OF THE PRODUCT IN EUROPEAN COUNTRIES
Similar products are already in use in the European Union in accordance with EN 13055 for which the corresponding standards are available in Europe and the Middle East.
Low Density Aggregate can be used in construction of bridges, skyscrapers, mass concrete, roads, etc.
ADVANTAGES OF USING LDA
The LDA pellet has glass like smooth and dense surface with close pore structure, which determines the pellet strength to a large extent. Use of LDA in construction is highly advantageous both due to technology and cost factors as the concrete made out of LDA has higher strength and is lighter than concrete made out of stone aggregates which makes the structure made out of LDA lighter, stronger and cost effective.
It is an ideal material for Ready Mix Concrete (RMC) Plants.
LDA concrete can be used effectively in Ready Mix Concrete (RMC) plants where in-situ concrete manufacturing in a large scale is difficult and not advantageous due to space constraint and environmental hazards. Being light in weight the transportation cost of LDA will be cheaper than that of conventional stone chips/gravel in the project specific locations. Additionally, LDA concrete will be easier to pump as compared to concrete made with conventional aggregates.
10

TECHNICAL DATA FOR LDA
: Size ranges from 2 mm to 14 mm. Shape is
Size / Shape spherical. Can be crushed, washed and
I graded for specialised applications.
Density Bulk density 700 -800 kg/ m^. |
Fire resistance Class 1
Water Absorption (after 30 15% approx. i minutes)
I Water Absorption (after 24 :18% approx. I
I hours) [
\ Crushing Strength ; >4 N/mm2
j Authority Conforms to BS EN 13055
USAGE OF THE PRODUCT
STRUCTURAL LIGHT WEIGHT CONCRETE
LDA is specifically designed and manufactured to meet all the requirements of modern construction practice while overcoming the prime disadvantage of concrete - its weight. Structural concrete can be produced with an effective reduction in dead weight over normal weight concrete as this reduction in dead weight would mean that considerable savings can be made in foundation and reinforcement costs. It allows for longer cantilevers and slimmer general sections, enabling buildings to be created that would not be possible with traditional materials. Architects and structural engineers are thus able to enjoy significantly greater opportunities when considering design options.
LDA can be used in structural concrete in a wide range of important construction projects where its lighter weight can be an important factor in achieving the required design solution such as
11

high-rise commercial complex, important developments infrastructural projects in major cities and sports stadias, etc.
Typical concrete densities
Fresh Wet I Oven Dry
Strength Class o ! „, , ^v
LC20/22 - LC45/50 1910-2000 1600-1800 Features / benefits / applications:
1. 25% lighter structural concrete over natural aggregate concrete
2. Same level of structural strength as of natural aggregate concrete.
3. Reduces coefficient of thermal expansion by 1/3 that of normal concrete.
4. Easier placing and compacting than normal weight concrete.
5. Improved insulating properties and thermal conductivity can be reduced by 25%.
6. Improves fire resistance
FLOOR AND ROOF SCREEDS
LDA floor and roof screeds offer a wide range of benefits including cost and weight savings benefits in addition to considerably enhanced thermal and sound insulation.
LDA screeds have been specified for and included in a variety of projects as also their distinct and specific advantages have been utilized in many types of building, ranging from prestigious commercial construction and civic developments to educational institutions and hospitals.
12

Typical roof insulation K values
Name of construction , . ,
materials
Dense Concrete (2300 kg/m^) \ 1-63
LDA Concrete (1750 kg/m^) | 0.80
LDA Base Coat 0.29 - 0.40
Sand / Cement (2100 kg/m^) 1-40 |
Insulation Board I 0.027-0.037
Asphalt (1700 kg/m^) I 0.50
f'eatures/ benefits / applications in screed systems:
1. Lighter in weight by 50% over normal sand: cement screeds
2. Usage in base coat produces material with very low shrinkage, typically 0.04%
3. Quick drying and curing process of the screed
4. Greater depths of base coat with no upper depth limitation
5. improves the insulation capabilities of the screed system
6. Reduces airborne sound
'7. Usage in under floor heating system maximizes the efficiency of the system
FRE-CAST CONCRETE
LDA is used extensively in a variety of precast products, such as lintels, kerbs, panels, stairs and bespoke units as its weight saving
13

properties can lead to significant advantages in production techniques and logistics. Larger panel sizes may reduce the number of joints required thus, speeding up construction and reducing the cost of jointing and moulding. Wall and floor panels are more resistant to accidental loading and absorb more energy in the elastic range than natural aggregate concrete. Additionally, the thermal properties are improved when compared with natural aggregate concrete. These benefits allow for significant overall cost savings to be made and thus make LDA the natural alternative.
Typical concrete densities
Fresh Wet Oven Dry
Strength Class ,^ / 3v /T^ / 3^
^ (Kg/m ) (K^m )
i ^
LC20/22 - LC45/60 1910 - 2000 1600 - 1800
'i ' -
! __^ _
Features / Benefits / Applications:
1. Weight of the concrete can be reduced by around 25%
2. Logistical advantages since due to reduction in weight of pre cast units more units can be transported in one vehicle
3. Improves the accuracy of fixing with reduction in tool wear
4. Larger panels can be cast, crane capacities can be reduced
5. Enhanced design capabilities
6. Manual handling of flags and kerbs, which are traditionally laid by hand, due to reduction in weight becomes easier
7. Due to reduction in weight the designer can take advantage of using larger pre cast units, thereby reducing the cost of casting and jointing, thus speeding up construction
14

LAND DRAINAGE
LDA is an ideal medium for land drainage applications as its round shape and regular grading gives excellent hydraulic conductivity which has been used to construct sports turf pitches, golf course greens and is frequently used in slit trenching, especially with "Shelton Trenching System" machines.
Comparison of hydraulic conductivity
Material I mm/hour
LDA 6 to 14 mm 93,6001
LDA 4 to 8 mm 60,000 j
Gravel (medium) 110,536 |
Features/benefits/applications:
1. Ensures a stable material within the ground.
2. Being half the weight of natural aggregate, twice the volume can be transported which is a great logistic advantage
3. Improves hydraulic conductivity many times over gravel
4. Using LDA aggregate provides material that is inert to natural chemicals found in the ground
5. Reduces the risk of injury during manual handling due to half the weight of normal aggregates,
6. Less damage to the surface and reduces normal wear and tear on placing machinery
7. No further compaction will occur during service because of the rounded nature of the particles, full compaction occur during placing
15

OTHER APPLICATIONS
Due to its particular and varied attributes, LDA is extremely versatile and can be used in a wide range of other applications also.
Main advantage of the present invention:
1. Gainful utilization of bottom clinkers from travelling grate boilers and ash from CFBC boilers.
2. Utilization of un-burnt carbon in the bottom clinkers
3. Cost effective process as it avoid use of fuel oil / coal.
4. Ash with very less un-burnt carbon content can be used.
5. Save the use of natural oil / coal resources
6. Save natural stone quarries from depletion.
7. Reduce environment pollution by use of bottom clinkers and ash and also due to reduced use of stone crushing for stone chips.
BRIEF DESCRIPTION OF THE DRAWING:
I igiire 1 is a block diagram showing the entire process of the invention.
DESCRIPTION OF THE INVENTION:
A preferred embodiment of the present invention is now described in detail below by referring to the accompanying drawing.
;\s shown in Figure l(i), the invention comprises a first stage of extraction of ash from the Circulating Fluidized Bed Combustion boiler used in thermal power plants and transferring the same into a silo.
16

ihe second stage is shown in Figure l(ii) and is a simultaneous process as shown in Figure l(i). This comprises of removal of bottom clinkers from the Travelling Grate Coal fired Boiler to a subsequent stage shown in Figure 1
(iii).
riie third stage is shown in Figure l(iii) which comprises of a grinding mill in order to crush the bottom clinkers which have a higher percentage of un¬burn t carbon (> 8 %) in a grinding mill to a size less than 40 micron.
The fourth stage as shown in Figure l(iv) which is essentially a blending silo which mixes the ash from the Circulating Fluidized Bed Combustion Boiler and the bottom clinker after it is treated by the Grinding Mill to form ti Jlomogenous ash mixture.
fhe fifth stage as shown in Figure l(v) where Bentonite which is a powdery material and is naturally available is added to the homogenous mixture in a mixer along with water in limited quantities in order to form a slurry mix as bentonite and water act as binding agents.
The sixth stage as shown in Figure l(vi) where the slurry mixture is moved onto a disc type pelletizer where further water if required is sprinkled on the mixture.
The seventh stage as shown in Figure l(vii) where this slurry mix is transported onto a sinter strand where the slurry mixture is heated to a temperature between 1200 to 1400 degrees centigrade with the help of
17

burning agents. The burning agents have a Hmited role to play as the bottom clinkers have a higher percentage of unbumt carbon and can be used in the sintering process.
I he eighth stage as shown in Figure l(viii) wherein the sintered pelletized ball shaped product is clustered together due to the heating. These clustered products are then spread out on a strand and air is pressured to cool the product.
riie ninth stage as shown in Figure l(ix) where the cooled clustered pellets fire forced through a breaker to form the end product namely the Low Density Aggregate to size of less than 14 mm.
18

CLAIMS
1. A method of producing low density aggregate from bottom clinkers of the travelling grate coal fired boilers along with ash from Circulating Fluidized Bed Combustion (CFBC) boilers of thermal power plants mixed with Bentonite and water.
2. A method in accordance with Claim 1 which comprises of the following steps: (a) grinding of bottom clinkers with higher percentage of un-burnt carbon (> 8 %) in a grinding mill to a size less than 40 micron.; (b) stacking the bottom clinkers and ash in separate silos; (c) mixing the bottom clinkers and ash in a blending silo as per their composition; (d) mixing the homogenous mixture with bentonite powder (less than 1 %) to form a powdery mixture; (e) feeding the mix to a disc type pelletizer with controlled amount of water to form ball like pellets; (f) sintering the pellets between 1200 to 1400 degree centigrade to get lumps of aggregate; (g) cooling the said
18

lumps by forcing natural air; (h) crushing the cooled lumps by a crusher to form round pellets of less than 14 mm size.
3. A method in accordance with Claim 2 to form low density aggregate wherein the bottom clinkers having a higher percentage of un-burnt carbon is achieved from travelling grate coal fired boilers, the said clinkers are wet and usually in lumps due to fusion during burning, therefore, the said clinker have to be dried and ground on rotary grinding tables with the help of grinding rollers which are pressurized by hyadraulic forces via hydraulic cylinders which is then stacked in a silo; another raw material namely ash from circulating fluidized bed combustion boilers of thermal power plants is stacked in another silo.
4. A method in accordance with Claim 3 above to form low density aggregate wherein a dry mixing of the bottom clinkers (12% to 14% un-burnt carbon) with ash (2% to 4% un-burnt carbon) and bentonite (0.5% to 0.75% of the total volume of mixture) which is a binding agent is performed in a blending silo.
5. A method in accordance with Claim 4 above to form low density aggregate wherein the blended homogenous mixture is fed to a disc type pelletizer to form round pellets by adding controlled amount of
water.
6. A method in accordance with Claim 5 above wherein the said pellets
are poured onto a green pellet conveyor which conveys the pellets to
a spreader conveyor thus exposing the pellets to a process of
sintering.
7. A method in accordance with Claim 6 above wherein the green
pellets are treated with temperature ranging between 1200 to 1400
degrees centigrade on a sinter strand which results in hard fused,
19

porous aggregate; simultaneously the top layers of the said porous aggregate which consists of un-burnt carbon is also fired on the sinter strand with the help of liquid fuel and as a result of this firing the un-burnt carbon catches fire therefore, the usage of liquid fuel is limited to the extent of maintaining a stable temperature on the strand.
8. A method in accordance with Claim 7 above wherein the top layer fire in the heating zone is sucked down by a reverse draught from an ID fan which ensures that the wet pellets on the strand are uniformly heated, thus preparing the pellets to move into the cooling zone where the pellets are cooled by forcing natural air on the pellets.
9. A method in accordance with Claim 8 above wherein the pellets are passed onto a Steel Belt Pan Conveyor, where the pellets formed become round in shape and range 14 mm and below in size, such pellets are then carried by conveyors to the finishing section where it is stacked or loaded on containers for further usage. *
20