FIELD OF THE INVENTION
The present invention relates in general relates to decreasing the energy consumption during the grinding of steel making converter slag, and more particularly, to a process based on microwave pre-treatment for decreasing high grinding energy requirement of steel making converter slag.
BACKGROUND OF THE INVENTION
Steel making converter slag is a solid waste generated in the steel plant. It is rich in dicalcium silicate (C2S) and tricalcium silicate (C3S), which are the main hydration agents in cement. Based on its mineral compositions, it has great potential for application as supplementary cementitious materials of cement and concrete. It can also be used as a CaO bearing source as supplement to limestone flux in sinter and pellet making. For its use as supplementary cementitious material, it needs to be grounded to finer size to improve its hydration activity. To use this slag as flux, it needs to be grounded to finer size to improve its reactivity with other mineral constituents. But, the grindability of steel slag is relatively worse than that of other mineral admixtures, viz., fly ash and granulated blast-furnace slag and other fluxing agents like limestone. Its poor grindability is attributed to the presence of large amount of iron oxides and solid solution of other metal oxides, viz. CaO, MgO and MnO, which form a dense structure in the steel slag. This high grinding energy requirement of steel making slag is one of the main hindrances to use it either in cement making or as flux in sintering or pelletizing. Thus, clearly there lies a need to decrease its grinding energy to promote its use in industrial processes.
Conventionally, there have been attempts made to reduce the grinding energy of steels by using grinding aids like glycerol, but their addition increases the cost of grinding. For example, in US4386963A to Lange, is disclosed salts formed by reacting aromatic group-containing carboxylic acids with amines can improve the efficiency in the grinding of granular blast furnace slag by 3.9 – 15%. However, the shortcoming is the unwanted chemical compound left in the slag which needs to be further processed. In another example,

LU85784A1 to Piret discloses a method for the treatment of steelworks slag, wherein the slag is subjected to an operation of grinding mixed with an aqueous liquid (grinding technique called pulp mill). For the comparison of energy efficiency, the sample is subjected to 45 minutes of grinding, the percentage of refusals to 0.2 mm is 55% in dry grinding and rejection rate to 0.2 mm is 7.5% in pulp grinding. But the drawback of said method lies in the presence of moisture in the slag. In another instance, CN103693886A to Zhang, teaches the slag compound activating grinding aid comprises (weight %: 5-10 percent of water glass, 5-10 percent of sodium carbonate, 3-8 percent of sodium abietate, 12-20 percent of betaine mother liquor, 7-12 percent of non-ionic surface active agent, 2-10 percent of molasses, 5-10 percent of water-soluble polymer and the balance of water) which increases the specific surface area by 23.8%. Although the amount added is very less (0.03%) but it will lead to additional costs and further processing. In US005977224A to cheung is disclosed pre-treatment of slag with 0.002 to 0.2 weight percent of polyacrylic acid in aqueous with an ethylenically monomers which form the grinding aid followed by grinding with an enhanced roll press method. This leads to decrease in additional water till 4% with 260% enhancement in cohesion leading increase in grinding efficiency. However, the major drawback of this method is additional drying energy has to be given at the end of the process. Moreover, ground material in roll press will not have uniformity in product as compared to the ball mill. Further, in WO2000039046A1 to Lopes is illustrated that silicon containing grinding aides such as sliane, cyclic silaxane, siliconate etc. dispersed into the hydrocarbon derived solvents have shown the improved grinding efficiency of the slag upto 42% by reducing energy consumption during grinding process and also the better compression strength over concrete.
The shortcomings associated with the aforesaid disclosed prior arts and the others in the conventional art are either additional costs of grinding aids or change in the chemical properties of the ground product due to addition of grinding aids. There is therefore a need to provide a process or method that overcomes the aforesaid limitations of these conventional practices used to

decrease the energy consumption. Our present invention substantially decreases grinding energy of steel making converter slag without changing the chemistry of ground product and very cost effectively.
OBJECTS OF THE INVENTION
An object of the invention is to overcome the aforementioned and other drawbacks existing in prior processes for decreasing high grinding energy requirement of steel making converter slag.
Another object of the present invention is to propose a novel method involving microwave pre-treatment for decreasing high grinding energy requirement of steel making converter slag.
Yet another object of the present invention is to propose a novel method for decreasing grinding energy of steel making converter slag without changing the chemical properties of the ground product.
Still another object of the present invention is to propose a novel method for cost-effectively decreasing grinding energy of steel making converter slag.
Further object of the present invention is to propose a novel method for decreasing grinding energy of steel making converter slag without addition of grinding aids.
These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.
SUMMARY OF THE INVENTION
The present application discloses a method for decreasing the energy consumption of steel making converter slag by pre-treating the slag with micro wave energy before grinding to selectively heat or react the microwave-

active and micro-inactive phases present in the slag. In an aspect, the method includes the step of preparing a steel making converter slag sample. Further, in a preferred aspect, the method involves pre-treating steel making converter slag with the microwave energy for a pre-determined duration. Furthermore, in an aspect, the method includes grinding the pre-treated steel making converter slag. In an aspect, the pre-treatment duration is preferably in the range of 10 to 30 mins. In another aspect, the pre-treatment of the steel making converter slag sample is performed at a microwave frequency of around 2.45 GHz and power in the range of 900 watts to 1100 watts.
In another embodiment of the invention, steel making converter slag is grounded to a size of less than 106 microns using techniques comprising ball milling.
The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings and presented experimental data.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The above brief description, as well as further objects, features and advantages, of the present invention can be fully appreciated by reference to the following detailed description. These features of the present invention will become more apparent upon reference to the drawings, wherein:
Fig. 1a: Illustrates a microstructure of steel making converter slag before microwave pre-treatment.
Fig. 1b: Illustrates the microstructure of steel making converter slag after microwave pre-treatment for 30 minutes according to an embodiment of the present invention.

Fig. 2: Illustrates relationship between the effects of microwave pre-treatment time on the grindability of steel making converter slag.
Fig. 3: Illustrates relationship between effects of microwave pre-treatment time on the work index (BWI) of steel making converter slag.
DETAILED DESCRIPTION OF THE INVENTION
Although the disclosure hereof is detailed and exact to enable those skilled in the art to practice the invention, the physical embodiments herein disclosed merely exemplify the invention which may be embodied in other specific structure. While the preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims.
It will be apparent, however, to one of ordinary skill in the art that the present invention may be practiced without specific details of the well known methods and techniques. Thus, the specific details set forth are merely exemplary. The specific details may be varied from and still be contemplated to be within the scope of the present invention. The features/steps discussed in an embodiment may be implemented in another embodiment.
Moreover, occasional references to the conventional methods for decreasing high grinding energy requirement of steel making converter slag in order to better distinguish the present inventive disclosure discussed later in greater detail. Few of the details pertaining to said methods are well known in the art and therefore, are described herein only in the detail required to fully disclose the present invention while unnecessarily obscuring the present invention.
Improving upon and addressing the problems discussed at length above (background), in the present disclosure an improved process for decreasing high grinding energy requirement of steel making converter slag as would also become clearer to the knowledgeable in the art with the particulars of the aforesaid unique steps and experimental results being described below in

greater detail. The present invention will be described in detail below with reference to embodiments as shown in the drawings and presented in the experimental results.
In an embodiment of the present invention, the energy required to grind the steel making converter slag is successfully decreased by pretreating or exposing the slag to microwave energy. Steel making converter slag rich in dicalcium silicate (C2S) and tricalcium silicate (C3S) and its grindability is less due to the presence of large amount of iron oxides and solid solution of other metal oxides, viz. CaO, MgO and MnO, which form a dense structure in the steel slag. This high grinding energy requirement of steel making slag is one of the main hindrances to use it either in cement making or as flux in sintering or pelletizing.
Further, Microwave energy is a non-ionizing electromagnetic radiation with frequencies in the range of 300 MHz to 300 GHz. Microwaves cause molecular motion by migration of ionic species and/or rotation of dipolar species. Microwave heating a material depends to a great extent on its ‘dissipation’ factor, which is the ratio of dielectric loss or ‘loss’ factor to dielectric constant of the material. The dielectric constant is a measure of the ability of the material to retard microwave energy as it passes through; the loss factor is a measure of the ability of the material to dissipate the energy. In other words, ‘loss’ factor represents the amount of input microwave energy that is lost in the material by being dissipated as heat. Therefore, materials with high ‘loss’ factor are easily heated by microwave energy. The extent to which a material absorbs microwave energy is primarily determined by its conductivity. Materials with low conductivities, such as insulators, are effectively transparent to incident waves and, thus, do not store any of the energy in the form of heat. Materials with high conductivities, such as metals, reflect the microwaves which provide no significant heating effects. Materials, such as semiconductors, can be effectively heated from room temperature through the interaction of the materials with microwaves.

Depending on the response of materials to microwave energy they are classified as hyperactive & active to microwaves, less active and inactive, as shown in the Table.1 (Ref: Wong, D.,1975. “Microwave dielectric constants of metal oxides at high temperatures”: MSc thesis. Univ. of Alberta, Canada and Tinga, W.R., 1989. Microwave Dielectric Constants of Metal Oxides, part 1 and part 2. Electromagnetic Energy Reviews 2 (1), 349-351).
Table.1 Classification of materials as per their behaviour to microwaves

a. Hyperactive Materials Heating rate, oC /sec Max. Temp. (oC)
UO2 200 1100
MoS2 150 900
Fe3O4 20 1000
FeS2 20 500
CuCl 20 450
b. Active materials Heating rate, oC /min Max. Temp. (oC)
Ni2O3 400 1300
Co2O3 300 900
CuO 200 800
Fe2O3 170 1000
FeS 135 800
CuS 120 600
c. Less active materials Heating rate, oC /min Max. Temp. (oC)
Al2O3 80 1900
PbO 70 900
MgO 33 1300
ZnO 25 1100
MoO3 15 750
d. Inactive materials Heating rate, oC /min Max. Temp. (oC)
CaO 5 200
CaCO3 5 130
SiO2 2 70
Moreover, since the steel making converter slag comprises different silicates and oxides and some amount of metallic iron, their exposure to microwave energy causes different heating in different mineral phases. Silicates and CaO rich phases are inactive to microwave energy and hence they are not heated.

Fe2O3 and Fe3O4 are more active to microwave energy and hence they are heated to higher temperatures as compared to inactive mineral resulting in differential heating within the slag particles. This differential heating cause thermal stresses inside the slag particles and finally leads to micro cracks and also develop network of cracks at the interface between active and inactive minerals. These micro cracks help in decreasing the energy required for grinding.
In an embodiment, for microwave pre-treatment, four homogeneous steel making converter slag samples of size less than 3.5 mm are prepared. In another embodiment of the invention, steel making converter slag is grounded to a size of less than 106 microns using techniques comprising ball milling.
To evaluate the effect of microwave pre-treatment, each sample was treated in a constant power (maximum 900W) and 2.45GHz kitchen-type microwave oven for different pre-treatment durations of 0 minutes, 10 minutes, 20 minutes and 30 minutes. Samples were placed in the oven in a microwave transparent glass wear. The vessel was always located in the same central position in order to minimize the effect of the field pattern variation in the oven.
Further, micro-structural study of pre-treated steel making converter slag samples show clear evidence that due to the pre-treatment with microwave energy, new phases are precipitated within the grains of slag sample and micro cracks are generated within the particles, as shown in the Fig.1(a) and Fig.1(b). In order to measure the grinding energy requirement of pre-treated steel making converter slag, Bond work index test was carried out in ball mill which is of a standard dimension with fixed operating parameters. The bond work index and grindability (Gbp) which are output of the test, will give the scale of hardness. Higher the value of BWI, lower the value of grindability of the material means material will show higher resistance towards grinding. Though bond mill work index test is a batch grinding state, we can assume it as a steady state conditions because ball mill work index is reported only after

recirculation load getting constant. This means that although ball mill is not fully continuous, work index will be calculated only after it reaches to equilibrium. Work index is reported as the measure of hardness as we discussed earlier. For this experiment to carry out, the required feed size should be having a size distribution in such a way that entire cumulative material should pass through particle sizes of 3.5mm.
Bond Work Index (BWI) Procedure
1) A standard laboratory bond mill has been used to carry out the BWI experiment with 12 in X 12 in (305 mm X305 mm) with rounded comers.
2) The entire feed size distribution is determined by using the sieve shaker (Uses Taylor’s root 2 series).
3) 700 cm3 of microwave pre-treated material has been taken by tapping down several times in a cylindrical vessel.
4) The total ball load is weighing around 20.125 kgs with different sizes ranging from 12.7 mm to 38 mm in diameter.
5) The ball mill run with the 70 rpm i.e. 85% of its critical speed.
6) The procedure involves grinding the charge for a short sieving at a desired screen size (106 micron in this case) to remove the undersize, and replacing the weight of undersize with an equivalent weight of original feed.
7) This newly added feed with the aid of the previous oversize samples were again subjected to the grinding in bond mill with different sets of rotations depending upon the respective calculations based on -106 microns produced during grinding at previous stages.
8) This process of regrinding the material at different mill revolutions is continued until a constant mass ratio of oversize to undersize equal to 2.5 is

achieved and the net grams of undersize produced per mill revolution (denoted by GbP) becomes constant.
9) Screen analysis is performed on the undersize product. Then the Bond Work Index is calculated, in kilowatt hours per ton from the following formula.
Where, Pi= Size of the closing sieve
Gbp= Grindiability, net undersize weight per revolutions
P80= 80% cumulative passing of product after grinding
F80= 80% cumulative passing of feed
Results of the bonds work index test to estimate the grinding energy requirement of microwave pre-treated steel making converter slag are shown in the Table.2.
Table.2 Results of Bonds work index test

Results 0 mins 10 mins 20 mins 30 mins
F80(in microns) 1000 1150 1120 1100
P80 (in microns) 78 78 78 74
Gbp (gram/revolution) 0.87 0.98 1.13 1.14
BWI in kilowatt hours per ton 23.5 20.7 18.4 17.6
Further, Fig.2 illustrates the relationship between the effects of microwave pre-treatment time on the grindability of steel making converter slag whiles Fig. 3 shows the relationship between the effects of microwave pre-treatment time on the work index of steel making converter slag. From these results, the following conclusions are drawn:

1. Pre-treatment of steel making converter slag with microwave energy induces some structural changes in the particles of slag. Precipitation of new phases and generation of micro cracks takes place within the slag particles due to the pre-treatment with micro wave energy.
2. The effect of pre-treatment of steel making converter slag with microwave energy increases with increasing the duration of pre-treatment.
In order to further establish the above conclusions, the following exemplary embodiments with experimental results are produced:
Example-1: Bond mill work test of Steelmaking Converter Slag after microwave treatment for 10 min
Steelmaking converter slag with chemical composition as described in Table. 3 below and having particle size distribution of 3.15 mm – 106 microns is taken as raw material.
Table.3 Composition of the Steelmaking Converter slag used in the process
MgO
0.8-2.13 S
0.03-0.21
MnO TiO2
0.22-1.07 0.8-1.4
Fe(T) CaO SiO2 Al2O3 P205
49- 12-Wt% 12-21 56 20
First, 16 kg Steelmaking converter slag is taken and made into four homogenous samples using coning and quartering technique. From these, one of the samples was used for the bond mill test with microwave treatment of 10 min. For this, about 300 g of steel making converter slag is filled in each of the three semi-cylindrical ceramic boats and placed on the microwave transparent glass wear. The samples were treated in batch process and in a 2.45GHz kitchen-type microwave oven at a constant power (maximum 900W) for 10 min. The vessel is always located in the same central position to minimize the effect of the field pattern variation in the oven. After the

complete sample is treated and allowed to cool down the sample is then subjected to the bond work index test. In an embodiment, a standard laboratory bond mill has been used to carry out the BWI experiment with 12 in X 12 in (305 mm X305 mm) with rounded comers. The entire feed size distribution is determined by using the sieve shaker (Uses Taylor’s root 2 series). 700 cm3 (1229.9 g) of microwave pre-treated material has been taken by tapping down several times in a cylindrical vessel. The total ball load is weighing around 20.125 kg with different sizes ranging from 12.7mm to 38mm in diameter. The ball mill runs with 70 rpm i.e. 85% of its critical speed. The procedure involves grinding the charge for a short sieving at a desired screen size (106 micron in this case) to remove the undersize, and replacing the weight of undersize with an equivalent weight of original feed. This newly added feed with the aid of the previous oversize samples were again subjected to the grinding in bond mill with different sets of rotations depending upon the respective calculations based on -106 microns produced during grinding at previous stages. This process of regrinding the material at different mill revolutions is continued until a constant mass ratio of oversize to undersize equal to 2.5 is achieved and the net grams of undersize produced per mill revolution (denoted by GbP) becomes constant. The whole process is described in Table 4. Screen analysis is performed on the undersize product. Then the Bond Work Index is calculated, in kilowatt hours per ton using the formula shown in Bond Work Index Procedure. The result of the bond work index test is 20.7 kW-h/ton. Thus the prior treatment for 10 min reduces the specific energy consumption approximately by 12% when compared to bond work index of untreated steel converter slag i.e., 23.5 kW-h/ton.
Table. 4 Bond work index sheet

Grindi
ng Stage Revolut
ions
Used
for
Grindin
g Mass (+100µm) Produced Mass (-
100µm)
Produced Refill Required Total Refill Used Mass-
100µm
produced Gbp
=F/Revol
ution RPM =H/G
1 1229.9 1229.9 100
2 100 894.8 335.1 335.1 1229.9 177.3493 1.773493 173.9048

3 174 965.1 264.8 264.8 335.1 221.8191 1.274822 249.0041
4 250 911.6 318.3 318.3 264.8 284.336 1.137344 273.0695
5 273 922.1 307.8 307.8 318.3 266.9739 0.977926 318.9613
6 319 880 349.9 349.9 307.8 310.4206 0.973105 314.9923
Example-2: Bond mill work test of Steelmaking Converter Slag after microwave treatment for 20 min
Experimental procedure and conditions are similar to as mentioned in Example 1. However pre-treatment time of Steelmaking Converter Slag is maintained for the duration of 20 minutes. The bond work index obtained after 20 minutes pre-treatment is 18.4 kW-h/ton.
Example-3: Bond mill work test of Steelmaking Converter Slag after microwave treatment for 20 min
Experimental procedure and conditions are similar to as mentioned in Example 1. However pre-treatment time of Steelmaking Converter Slag is maintained for the duration of 20 minutes. The bond work index obtained after 20 minutes pre-treatment is 17.6 kW-h/ton.
Advantages:
The disclosed method decreases the high grinding energy required for steel making converter slag by pre-treating the slag with micro wave energy before grinding in order to facilitate selective heating or reaction of the microwave-active and micro-inactive phases present in the slag without using any grinding aids which result in additional costs and changes in the chemical properties of the ground product.
The foregoing is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. While the

preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims.

We claim:
1. A method for energy efficient grinding of steel making converter slag,
the method comprising:
pre-treating steel making converter slag with the microwave energy for a pre-determined duration; and
grinding the pre-treated steel making converter slag.
2. The method as claimed in claim 1, wherein the pre-determined duration is preferably in the range of 10 to 30 mins.
3. The method as claimed in claim 1, wherein the steel making converter slag is grounded to a size of less than 106 microns using ball milling technique.
4. The method as claimed in claim 1, wherein the pre-treatment of the steel making converter slag is performed at a microwave frequency of 2.45 GHz.
5. The method as claimed in claim 1, wherein the pre-treatment of the steel making converter slag is performed by maintaining microwave power between 900 watts to 1100 watts.
6. The method as claimed in claims 1-4, wherein when the pre-treatment duration is maintained for 10 mins bond work index (BWI) of steel making converter slag decreases to 20.7 kW-h/ton.
7. The method as claimed in claims 1-4, wherein when the pre-treatment duration is maintained for 20 mins the BWI of steel making converter slag decreases to 18.4 kW-h/ton.

8. The method as claimed in claims 1-4, wherein when the pre-treatment duration is maintained for 30 mins the BWI of steel making converter slag decreases to 17.6 kW-h/ton.
9. The method as claimed in claim 1, wherein the composition of the steel making converter slag comprises:

MgO
0.8-2.13 S
0.03-0.21
MnO TiO2
0.22-1.07 0.8-1.4
Wt% 12-21
Fe(T) CaO
49-56

SiO2 Al2O3 P205
2.7-4.3
12-20 0.24-1.54