Claims:We Claim:

1. A process for grinding soft iron ore suitable for desired pelletization for working in making iron ore pellets for various end uses comprising:

step of subjecting the soft iron ore involving friable and low bond work index below 7 kWh/st to selective grinding and generate the iron ore suitable for pelletization free of Blaine number > 2800 and higher fractions of < 10 µm size iron ore thereby make it suitable for pelletization.

2.A process as claimed in claim 1 comprising subjecting said friable and low bond work index below 7 kWh/st to selective grinding free of any blending of hard ore.

3. A process as claimed in anyone of claims 1 or 2 comprising stage wise grinding of said soft ore for controlled ultra-fines below 10µ content and thereby achievement of control over Blaine number to working ranges of below 2400 cm2/gm required for quality iron ore pelletization.

4. A process as claimed in anyone of claims 1 to 3 comprising involving sieve of selective perforation size for separation of ultra-fines present in start iron ore and between grinding stages.

5.A process as claimed in anyone of claims 1 to 4 comprising monitoring ultrafine generation below 10µm at every stage and after screening and completion of all stage grinding mix batch Blaine is obtained.

6. A process as claimed in anyone of claims 1 to 5 wherein number of stages depend upon fraction of hard phase present in iron ore as from source to source iron ore is different and duration and grinding at each stage is controlled for desired targeted Blain Number with low bond work index.

7. A process as claimed in anyone of claims 1 to 6 comprising involving ball mill in series stages for controlled grinding to achieve desired granulometry and following working in ball mill in stage series operation with different charge feed (Iron ore) to grinding media ratio & involving of screen of specified mesh size charge in between stages.

8. A process as claimed in anyone of claims 1 to 7 comprising

for desired Blaine number of 2200to 2400preferably 2248 cm2/gm for iron ore with low bond work index of 6 to 7 preferably about 6.5 KWH/st, said stage wise grinding comprises 3 series stages and application of 600-µm sieve between successive stages including:

1st stage grinding with 0.5 to 0.6 preferably about 0.5 ‘ore to grinding media ratio’ and 40-minutesto 60-minutespreferably about 50-minutes duration;

2nd stage grinding with 0.3 to 0.5 preferably about 0.4 ‘ore to grinding media ratio’ and 20-minutes to 30-minutes preferably about 25-minutes duration; and

3rd stage grinding with 0.08to 0.12 preferably 0.1 ‘ore to grinding media ratio’ and 25-minutesto 30-minutes preferably 30-minutes duration such as to achieve particle size combination (cumulative% passing size)

Size in µm 10 25 45 75 150 500
% passing 22 to 30 preferably 26.8 35to 40 preferably 38.9 45 to 60 preferably 47.5 55 to 70 preferably 57 70 to 90 preferably 74.31 90 to 100 preferably 93

9. A process as claimed in anyone of claims 1 to 7 comprising for desired Blaine number of 0.4 to 0.6 preferably 2700 cm2/gm for iron ore with low bond work index of 6kWh/st to 7kWh/st preferably about 6.5 kWh/st, grinding with 3 series stages and apply of 500-µm sieve between successive stages including:

1st stage grinding with 0.4to 0.6 preferably about 0.5 ore to grinding media ratio and 15-minutes to 20-minutes preferably about 15-minutes duration;
2nd stage grinding with 0.4 to 0.6 preferably about 0.5 ore to grinding media ratio and 15-minutes to 20-minutes preferably about 15-minutes duration;
3rd stage grinding with 0.08 to 0.12 preferably about 0.1 ore to grinding media ratio and 30-minutes to 45-minutes preferably about 30-minutes duration, such as to achieve particle size

combination (cumulative% passing size)
Size in µm 10 25 45 75 150 500
% passing 28 to34 preferably 33.5 45to 50 preferably 47.9 50to 60 preferably 58.2 60 to 70 preferably 68.5 80 to 95 preferably 87.6 100

10. A process as claimed in anyone of claims 1 to 9 wherein the soft iron ore involved comprises:
Size distribution is –
Size range in mm +10 -10 to +8 -8 to +5 -5 To +3 -3 To +1 -1 To +0.5 -0.5 To +0.15 -0.15
Weight% 4-5 4.5-5.5 11-14 10-12 23-26 10-12 10-12 15-23

Chemistry in wt%
Element/ Oxide CaO MgO SiO2 Al2O3 T.Fe MnO LOI*
Weight% range 0.15 to 0.22 preferably 0.2 0.08 to 0.12 preferably 0.1 3-4 preferably 3.8 2.3-2.8 preferably 2.6 61.5-63.0 preferably 62 0.1-0.7 preferably 0.5 2.5-4 preferably 3.8

*Loss on ignition (LOI) accounts for bonded hydroxide part with goethite and gibbsite

Mineralogy of iron ore:

Typical mineralogical phases of soft iron ore by XRD analysis are
Sr.No. Phase name Oxide Range
1 Hematite Fe2O3 85-87
2 Goethite Fe2O3.H2O 10-15
3 Quartz SiO2 3-4.5
4 Gibbsite Al(OH)3 0.5-1

11. A process as claimed in anyone of claims 1 to 10 comprising setting up of ball mill in stage series operation with different charge feed (Iron ore) to grinding media ratio and apply of screen of specified mesh size charge in between stages wherein Grinding media was stabilized as follows:
Size in mm 45-50 65-70
Weight% 42 58

Dated this the 27th day of February, 2020
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION

The present invention relates to a method for controlled grinding of iron ore for producing fines of desired particle size analysis(PSA) and Blaine number required for use in various industries, for example agglomeration, more specifically– iron ore pelletization. More particularly, the present invention is directed to provide a grinding circuit with selective use of a series of ball mills and sieving in between the ball mills of ground material so that removal of softer fines takes place at each stage. This will avoid further ultrafine generation (minimum of primarily below 10 µm) due to over grinding of already ground fines. For the target ore (soft iron ore having origin in eastern India) grinding Blaine number target was 2200-2400 cm2/gm (due high amount of ultrafine of specifically below 10 µm) for use in pellet process without any blending with any other ore. The method applied for grinding iron ore is batch ball mill grinding. This soft ore break down during grinding such that there is little control over ultrafine generation below 10µm size. Other option to utilize such iron ore is to blend in proper proportion with hard ore. Present method provides controlled dry grinding where ultra-fines is removed by application of sieves before and in between ball mill operation and oversize is crushed with higher load of grinding media. Overall control over fine generation lead to better control over granulometry thereby ease in further agglomeration and elimination of need of blending soft and hard ore achieving operation optimization.

BACKGROUND OF THE INVENTION
Grinding of iron ore in iron making section takes place primarily by means of ball mill where coarser iron ore particles are charged along with grinding media (usually high chrome spherical balls) and ball mill is internally equipped with lifters, after specified retention material grounded is removed from ball mill and classified to desired size ranges.

Grinding of ore is an initial step of pelletization process wherein iron ore, fluxes & coke are ground to specified ultrafine size ranges and blended in certain proportions. Green balls of are made using disc pelletizers and harden it by flame roasting.

Grinding of iron ore in pellet process takes place primarily by means of ball mill where coarser iron ore particles are fed along with grinding media (usually high chrome steel spherical balls) . This ball mill is rotated at fixed revolutions speed i.e. revolutions per minute (rpm). Due to the interaction between the hard grinding media and iron ore, breakage of iron ore takes place leading to the reduction in size of the iron ore. After specified retention time and number of revolutions of the ball mill, ground material (reduced in size) is removed from ball mill and classified to desired size ranges using classifier. Samples are further tested in lab for determining size distribution and Blaine number.

Various characteristics of iron ore such as feed size, morphology and phases of iron plays major role for determination of ore hardness, output Blaine size andparticle size distribution. Bond work index (BWI) is a measure of hardness of the ore and it indicates the energy required to grind one ton of iron ore to a specific size (80% below 75-µm size). BWI typically varies from 6 to 15 kWh/st for various iron ores evaluated from sources across the globe. Although materials with higher Blaine number (BWI>10) requires more energy to grind, it is easier to control PSA and Blaine number. While softer iron ores (BWI<7) poses another problem of very fine grinding and leads to high fine fractions after grind (Blaine number > 2800 & higher fraction of <10-µm size fraction) which results in high compactness (leading to less porosity) and deteriorates pellet quality and further performance in iron making. A standard solution in existing practice is to use a blend of softer & harder ores to get desired particle size distribution after balling.

Blaine number is surface area based parameter determined using air permeability technique. It is measurement of fineness of powder. SI unit of Blaine number is m2kg-1.Blaine number affects particle packing for single pellet and reduces porosity. Lower porosity leads to reduced penetration of hot gas during pelletization stage and leads to deterioration in pellet quality parameters such as strength (measured as cold crushing strength in Kg/pellet), % Porosity, abrasion index and thereby increased dust generation while handling and processing. Also production rate is reduced due to increase in pressure drop between entry and exit of hot gases.
Further downstream iron making process of ‘Sponge Iron Plant’ reduction method is via. gas solid interaction. Thus less passage of reducing gases will have adverse impact on metallic Fe yield.

CA 2476579 disclosed use of single rod mill to achieve iron ore concentrate grinding size to Blaine fineness of 1600 – 2200. This is achieved by means of classifying machine discharge to size distribution in dry condition, separating & recycling oversize.

The above referred prior art does not disclose operation of ball mill for control over quality parameters like granulometry, Blaine number, control over coarser fractions. This document discloses popular means to get range of Blaine number.

CN107552203A disclosed a method for improving the ore grinding efficiency of a ball grinding mill. The method includes two stages of ore grinding processes. The original mode that discharged ores of the second-stage ball grinding mill flow to a middling ore pump pool is transformed into the mode that the discharged ores of the second-stage ball grinding mill flow to a roughing magnetic separator firstly, are subjected to recleaning for tailing discarding, then flow to the middling ore pump pool, and are sent by a sand pump to a high-frequency fine screen to be screened and graded, and thus anew closed loop circulation is formed. According to the method for improving the ore grinding efficiency of the ball grinding mill, through optimizing, throwing as early as possible is achieved in the second-stage ore grinding circulation, the grading efficiency of the high frequency fine screen is improved, the circulation load of second-stage grinding and separation is lowered significantly, the processing capability of a grinding and separation system can be improved, the grinding and separation cost expenditure is reduced, and economic benefits are improved greatly.

The above patent publication mentions about magnetic separator - It is developed for iron ore bearing magnetic property, whereas ore which is used for the purpose of present invention are 100% non magnetic phase (0 % magnetite). This Publication mentions of using ball mill with two sections and control over grinding of magnetic part whereas in the present invention operation of ball mills are in series and with intermittent screening to take care of ultrafine generation. Thus this prior art would fail to achieve desired Blaine number suitable for pelletization in case of soft iron ore which is non strongly magnetic. Our current invention is aimed to give desired results irrespective of magnetic properties of any soft iron ore consisting of low BWI (below 7 kWh/st), having porous hematite, goethite (10-15%).

Present invention related to grinding of iron ore with friable nature, low bond work index (Bond work index BWI- <7 kWh/st), porous hematite, goethite and methodology for getting optimum Blaine number. The area of application of this invention is grinding of iron ore for making iron ore pellets for input to sponge iron plant (SIP) or blast furnace (BF). This invention is related to the grinding of iron ore above characteristics in controlled manner to achieve desired particle size distribution which is required for pelletization process.

Present invention thus is targeted to grinding of iron ore exclusively with friable nature & low bond work index (Bond work index BWI– below 7 kWh/st) and to optimize the grinding process to arrive at required particle size distribution. An ore used in this study consists of low BWI (below 7 kWh/st)and have porous hematite, goethite (10-15%). To evaluate grinding of above stated nature iron ore lab scale ball mill is considered.

Specific Process modification is for iron ore source from eastern location in India. Broadly being friable nature ore which leads to high ultrafine generation and difficult to control fineness (Measured as Blaine number working range for other source iron is 1500-1800 cm2/gm, For soft iron ore grinding Blaine number target was 2200-2400 cm2/gm).

OBJECTS OF THE INVENTION

The basic objective of the present invention is to develop a process for dry grinding of soft iron ore (Bond work index BWI–below 7 kWh/st) specifically the one from eastern mines region, without blending with hard ore, using lab scale ball mill to get desired PSA and Blaine number suitable for pelletization.

A further object of the present invention is directed to provide a process for grinding soft iron ore to achieve desired Blaine number wherein said ore comprising consisting of low BWI (<7 kWh/st), having porous hematite, goethite (10-15%) but no magnetite.

A still further object of the present invention is directed to a process for grinding of soft iron ore to obtain desired particle size distribution and Blaine number involving controlled dry grinding where ultra-fines is removed by application of sieves before and in between ball mill operations and oversize is crushed with higher load of grinding media.

A still further object of the present invention is directed to a process for grinding of soft iron ore to obtain desired particle size distribution and Blaine number involving series set of ball mill and sieving within screen of ground material so that removal of softer fines takes place.

Another object of the present invention is directed to a process for grinding of soft iron ore to obtain desired particle size distribution and Blaine number which will avoid further ultrafine generation (minimum of primarily below 10 µm).

Yet another object of the present invention is directed to a process for grinding of soft iron ore to obtain desired particle size distribution and Blaine number for the target ore (soft iron ore) grinding Blaine number target being 2200-2400 cm2/gm (due high amount of ultrafine of specifically below 10 µm).

A still further object of the present invention is directed to a process for grinding of soft iron ore to obtain desired particle size distribution and Blaine number which is applied for grinding iron ore of less bond work index in batch ball mill grinding.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1:shows Schematic layout of ball mill.
Figure 1(a): shows the typical Flow diagram for existing continuous type process step of ball mill at plant.
Figure 1(b): shows the typical batch type ball mill operation flow diagram.
Figure 2: shows a typical microstructure of the soft ore used in this study that mainly consists of three features: A: Iron ore particles- Mostly hematite and harder phase: Gangue – Consists of various mineralogical phases including silicates, aluminates and other impurities and C: Porosity in iron ore particles consisting of micro pores.
Figure 3: shows existing System and sequence of operation for grinding soft Iron ore mixed with hard ore at plant level.
Figure 4: shows schematically the Flow diagram for Batch type ball mill operation according to present invention for controlling ultrafine generation while grinding soft iron ore having low Bond work index (BWI) below 7 kWh/st to achieve desired PSA and Blaine number suitable for agglomeration/pelletization.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a process for producing soft iron ore suitable for desired pelletization for working in making iron ore pellets for various end uses comprising:

step of subjecting the soft iron ore involving friable and low bond work index below 7 kWh/st to selective grinding and generate the iron ore suitable for pelletization free of Blaine number > 2800 and higher fractions of < 10 µm size iron ore thereby make it suitable for pelletization.

A further aspect of the present invention is directed to said process comprising subjecting said friable and low bond work index below 7 kWh/st to selective grinding free of any blending of hard ore.

A still further aspect of the present invention is directed to said process comprising stage wise grinding of said soft ore for controlled ultra-fines below 10µ content and thereby achievement of control over Blaine number to working ranges of below 2400 cm2/gm required for quality iron ore pelletization.

Another aspect of the present invention is directed to said process comprising involving sieve of selective perforation size for separation of ultra-fines present in start iron ore and between grinding stages.

Yet another aspect of the present invention is directed to said process comprising monitoring ultrafine generation below 10µm at every stage and after screening and completion of all stage grinding mix batch Blaine is obtained.

A further aspect of the present invention is directed to said process wherein number of stages depend upon fraction of hard phase present in iron ore as from source to source iron ore is different and duration and grinding at each stage is controlled for desired targeted Blain Number with low bond work index.

A still further aspect of the present invention is directed to said process comprising involving ball mill in series stages for controlled grinding to achieve desired granulometry and following working in ball mill in stage series operation with different charge feed (Iron ore) to grinding media ratio and involving of screen of specified mesh size charge in between stages.

A still further aspect of the present invention is directed to said process comprising

for desired Blaine number of 2200 to 2400 preferably 2248 cm2/gm for iron ore with low bond work index of 6kWh/st to 7kWh/st preferably about 6.5 kWh/st, said stage wise grinding comprises 3 successive stages and application of 600-µm sieve between successive stages including:

1st stage grinding with 0.5 to 0.6 preferably about 0.5 ore to grinding media ratio and 40-minutes to 60-minutes preferably about 50-minutes duration;

2nd stage grinding with 0.3 to 0.5 preferably about 0.4 ore to grinding media ratio and 20-minutes to 30-minutespreferably about 25-minutes duration; and

3rd stage grinding with 0.08 to 0.12 preferably 0.1 ore to grinding media ratio and 25-minutes to 35-minutes preferably 30-minutes duration such as to achieve particle size combination (cumulative% passing size)

Size in µm 10 25 45 75 150 500
% passing 22 to 30 preferably 26.8 35 to 40 preferably 38.9 45 to 60 preferably 47.5 55 to 70 preferably 57 70 to 90 preferably 74.31 90 to 100 preferably 93

A still further aspect of the present invention is directed to said process comprising for desired Blaine number of 2500 cm2/gm to 2700 cm2/gm preferably 2700 cm2/gm for iron ore with low bond work index of 6kWh/st to 7kWh/st preferably about 6.5 kWh/st, grinding with 3 successive stages and apply of 500-µm sieve between successive stages including:

1st stage grinding with 0.4 to 0.6 preferably about 0.5 ore to grinding media ratio and 15-minutes to 16-minutes preferably about 15-minutes duration;
2nd stage grinding with 0.4 to 0.6 preferably about 0.5 ore to grinding media ratio and 15-minutes to 16-minutes preferably about 15-minutes duration;
3rd stage grinding with 0.08 to 0.12 preferably about 0.1 ore to grinding media ratio and 30-minutes to 45-minutes preferably about 30-minutes duration, such as to achieve particle size combination (cumulative% passing size)
Size in µm 10 25 45 75 150 500
% passing 28 to 34 preferably 33.5 45 to 50 preferably 47.9 50 to 60 preferably 58.2 60 to 70 preferably 68.5 80 to 95 preferably 87.6 0

A still further aspect of the present invention is directed to said process wherein the soft iron ore involved comprises:

Size distribution is –
Size range in mm +10 -10 to +8 -8 to +5 -5 To +3 -3 To +1 -1 To +0.5 -0.5 To +0.15 -0.15
Weight % 4-5 4.5-5.5 11-14 10-12 23-26 10-12 10-12 15-23

Chemistry in weight%-
Element/ Oxide CaO MgO SiO2 Al2O3 T.Fe MnO LOI*
Weight % range 0.15 to 0.22 preferably 0.2 0.08 to 0.12preferably 0.1 3-4 preferably 3.8 2.3-2.8 preferably 2.6 61.5-63.0 preferably 62 0.1-0.7 preferably 0.5 3-4 preferably 3.8

*Loss on ignition (LOI) accounts for bonded hydroxide part with goethite and gibbsite

Mineralogy of iron ore:

Typical mineralogical phases of soft Iron ore by XRD analysis are
Sr.No. Phase name Oxide Range
1 Hematite Fe2O3 85-87
2 Goethite Fe2O3.H2O 10-15
3 Quartz SiO2 3-4.5
4 Gibbsite Al(OH)3 0.5-1

A still further aspect of the present invention is directed to saidprocess comprising setting up of ball mill in series operation with different charge feed (Iron ore) to grinding media ratio and application of screen of specified mesh size charge in between stages wherein Grinding media was stabilized as follows:
Size in mm 45-50 65-70
Weight% 42 58

The above and other objects and advantages are described hereunder in greater details with reference to following accompanying non limiting illustrative drawings.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is related to grinding process of iron ore exclusively with friable nature & low bond work index (Bond work index BWI– below 7 kWh/st) and to optimize the grinding process to arrive at required particle size distribution. The present invention is directed to provide a grinding circuit with selective use of series set of ball mill and sieving within screen of ground material so that removal of softer fines takes place. This will avoid further ultrafine generation (minimum of primarily below 0.010 mm).

The method is applied for grinding iron ore of less bond work index in batch ball mill grinding. An ore used in present study is from eastern regions of India, with low BWI (below 7 KWh/st), having porous hematite, goethite (10-15%). To evaluate grinding of above stated nature iron ore lab scale ball mill is considered. For the target ore (Soft iron ore) grinding Blaine number target was 2200-2400 cm2/gm (due to high amount of ultrafines of specifically below 10 µm).

Ball mills, rod mills are widely used for comminution- grinding of coarser size to fine size.

Specifications of ball mill used for the purpose of this invention is as follows: -
Machine Name - Ball mill- batch type (Lab scale)

Specifications – 1.5 HP motor, Single phase, Cylindrical drum has one opening (for loading and unloading), Size- 18-inch diameter 30-inch length, Speed of drum revolutions is fixed to 70 rpm.

Each ball mill is designed with a specific ore type & output size requirement. Hardness of ore (measured as bond work index) varies with the origin & mineralogy of the iron ore. Generally, softer ore are difficult to grind because of their tendency of high ultrafine generation and difficult to control fineness.

Accompanying Figure 1 shows the schematic view of a ball mill set-up. Ball mill has one opening for loading and unloading of grinding media and grinding charge.
Accompanying Fig. 1(a) shows the typical Flow diagram for existing process step of ball mill at plant.

Plant ball mill flow diagram as shown in Figure 1(a) is a continuous type operation. There is no such fix combination for optimum productivity and granulometry. Certain set of circuit combination comprising grinding charge (iron ore), grinding media, water flow, machine speed works well for one ore type may be not working for another source of iron ore.
In running plant operation only two variables can be independently controlled by operator are feed rate and water.
Principle variable affecting control of grinding mills are feed rate, circulating loads, size distribution of ore, hardness of ore (bond work index), rate of water addition, stoppage.
Grinding media can be controlled at start after major stoppage.
Similarly, a typical batch type ball mill operation flow diagram is illustrated in the accompanying Figure 1(b).

Input/Output, Process -

Input for ball mill is iron ore to be ground (particle size above 0.15 mm is 60-80%) & grinding media balls (high chrome round balls of three diameters 25-30 mm, 45-50mm, 65-70 mm).

Typical characterization of iron ore:

Size distribution is as given below:–
Size range in mm +10 -10 to +8 -8 to +5 -5 To +3 -3 To +1 -1 To +0.5 -0.5 To +0.15 -0.15
Wt% 5.33 5.38 12.43 11.26 28.7 10.64 12.27 13.99

Chemistry in wt%:
Element/ Oxide CaO MgO SiO2 Al2O3 T.Fe MnO LOI*
Wt% range 0.2 0.1 3-4 2.3-2.8 61.5-63.0 0.1-0.7 2.5-4

*Loss on ignition (LOI) accounts for bonded hydroxide part with goethite and gibbsite.

Mineralogy of iron ore:

Typical mineralogical phases of ‘Soft Iron’ ore by XRD analysis are found as below:
Sr.No. Phase name Oxide Range
1 Hematite Fe2O3 85-87
2 Goethite Fe2O3.H2O 10-15
3 Quartz SiO2 3-4.5
4 Gibbsite Al(OH)3 0.5-1

Microstructure: In accompanying Figure 2, a typical microstructure is shown of the soft ore used in the present study. This Ore mainly consists of three features:
A: Iron ore particles- Mostly hematite and harder phase,
B:Gangue – Consists of various mineralogical phases including silicates, aluminates and other impurities
C: Porosity in iron ore particles consisting of micro pores

Existing System and sequence of operation for grinding soft Iron ore mixed with hard ore at plant level is illustrated in accompanying Figure 3, that consist of:–
Sequence of operation at plant level– Iron ore feed to ball mill – ball mill operation - cyclone classifier – coarser return to ball mill (along with fresh iron ore feed) & fine fraction to further processing (dewatering).
Existing Procedure for achieving desired (PSA) &Blaine number from ball mill:

a) Soft ore (BWI < 7 kWh/st) is mixed with hard ore (BWI >10 kWh/st) to produce output material with required Blaine number & PSA. This is generally the most widely used method to optimize productivity and quality.

b) Adjustment of cut size at classifier and thereby control of passing size for further process.

c) Flow of water to iron ore feed – flow of water is proportional to retention time in ball mill. For reduction in Blaine number water flow is kept higher.

Output for ball mill is grounded iron ore (After specified duration particle size below 0.15 mm is 80-95%) and grinding media balls (No change).

Basic feature of present invention is a method used for controlled grinding of soft ore exclusively without blending with hard ore and to achieve required Blaine number & PSA. Method has been evolved through a series of experiments at lab scale batch type ball mill.

Accompanying Figure 4 shows schematically the Flow diagram for Batch type ball mill operation according to present invention for controlling ultrafine generation while grinding soft iron ore to achieve desired PSA and Blaine number.

For most of all ball mill, there is no such universal grinding circuit that will work for all type of ores. Thus a grinding circuit was developed for the purpose of present invention through experimental trials to achieve the target ore (Soft iron ore) grinding Blaine number target of 2200-2400 cm2/gm as follows, minimizing ultrafine generation.

The new batch type ball mill dry grinding circuit as illustrated in Figure 4 broadly include the stages from feeding input iron ore as per the characteristics given above to final product of desired Blaine number comprising of use of classification by means of sieve for starting material and in process material within stages. This new approach is opted to successive separation of fine grind generated at each step to avoid any over grinding of fine size fractions.

Example:

Initial experiments details are as below:

Processing variables for Ball mill operations are as follows:
a) Grinding media ball size;
b) Ore to grinding media ratio;
c) Grinding time.

Typical experiments with soft ore and standard processing variables (grinding media ratio0.25, grinding time60-minutes& grinding media ball size25mm to 70mm) results in over grinding of lower fraction (<150 µm size particles) and add up in ultrafine generation (< 10 µm) which increases the Blaine number to >2800 cm2/gm, which is undesirable for pellet making.

Hence, object of present invention was to determine the process route & parameters to achieve desired PSA &Blaine number.

Grinding media categorized into 3 parts. 1. Size 25-30, 2. 45-50, 3. 65-70 Range is considered to mitigate dimensional tolerance for industrial scale variation.
a) Preliminary test done with single stage grinding operation with iron ore to grinding media 0.2 for over 1hour duration.

Grinding media distribution is placed in below table.
Size in mm 25-30 45-50 65-70
Wt% 25 30 45

It was found that over 98% material passed through 500-µm size sieve. Blaine number was 3400 cm2/gm. This was due to higher ultrafine content particularly below 10-µm size which is over 35%.

It was also found that particular lower size fraction grinding media generate more ultra-fines due to higher contact incidences. So further to streamline lower fraction of grinding media is not considered.

Thus to achieve the desired process parameters following steps were followed:

Highlight of inventive step of the process according to present invention include:
1. Use of ball mill in series stages for controlled grinding to achieve desired granulometry.
This has been achieved with setting up of ball mill in stage series operation with different charge feed (Iron ore) to grinding media ratio & apply of screen of specified mesh size charge in between stages. Grinding media stabilized to below table.

Size in mm 45-50 65-70
Wt% 42 58

2. To achieve desired Blaine number of 2248 cm2/gm for iron ore with low bond work index of 6.5 KWH/st, grinding has been carried with 3 successive stages and application of 600-µm sieve (TYLER 28) between successive stages. 1st stage grinding in with 0.5 ore to grinding media ratio and 50-minutes duration. 2nd stage grinding is with 0.4 ore to grinding media ratio and 25-minutes duration. 3rd stage grinding is with 0.1 ore to grinding media ratio and 30-minutes duration.
Particle size achieved with this combination is below. (cumulative% passing size)
Size in µm 10 25 45 75 150 500
% passing 26.8 38.9 47.5 57 74.31 93

3. To achieve desired Blaine number of 2700 cm2/gm for iron ore with low bond work index of 6.5 kWh/st, grinding has been carried with 3 successive stages and application of 500-µm sieve (TYLER 28) between successive stages.

1st stage grinding in with 0.5 ore to grinding media ratio and 15-minutes duration.
2nd stage grinding is with 0.5 ore to grinding media ratio and 15-minutes duration.
3rd stage grinding is with 0.1 ore to grinding media ratio and 30-minutes duration.
Particle size achieved with this combination is below. (cumulative% passing size)

Size in µm 10 25 45 75 150 500
% passing 33.5 47.9 58.2 68.5 87.6 0

Two sieve sizes are used 500 µm and 600 µm.
Irrespective of focus on multiple fraction size ranges, only top sieve size considered. And ultrafine generation is monitored at every stage. After screening and completion of all stage grinding mix batch Blaine is considered for achievement evaluation. The result of experiments are shown in Table 1 below.
For control of Blaine number to pelletizing norms i.e. below 2400 cm2/gm, in experiment set 3, 600-µm sieve is used.

Table 1: for Key parameters in experiments and Blaine number
Set. 1 Set. 2 Set. 3
Grinding Media
Size 25-30 mm in kg 25
Size 45-50 mm in kg 30 30 30
Size 65-70 mm in Kg 45 25 25
Iron ore quantity in Kg 25 25 25
Grinding stage 1 1 1
Grinding time (minutes) 60 15 15
Grinding stage 2 2
Grinding time (minutes) 15 15
Grinding stage 3 3
Grinding time (minutes) 15 15
Grinding stage 4 4
Grinding time (minutes) 25 25
Sieve size used in µm 500 500 600
Blaine number cm2/gm 3400 2700 2248
Particle size %(below 10 µm) 34 32 28
Particle size% (above 500 µm) 10 0 7
In the above experiments, micro fines (below 10µm) is monitored as the controlling parameter. Number of stages depend upon fraction of hard phase present in iron ore as from source to source iron ore is different. Last stage grinding time kept more so as to complete crushing activity w.r.t. sieve size consideration. Since softer phases got sieved out in previous stages, ultrafine generation in last stage will be minimum.

For experiment set 2, 500 µm sieve is used for sieving between 1st and 2nd stage grinding and also between 2nd and 3rd stage grinding , whereby the resulting blain number achieved was 2700 cm2/gm.

So, for control of Blaine number to pelletizing norms i.e. below 2400 cm2/gm, in experiment set 3, 600-µm sieve is used.

It is thus possible by way of the present invention to provide a process for dry grinding of soft iron ore using a grinding circuit with selective use of series set of ball mill and sieving within screen of ground material so that removal of softer fines takes place. This will avoid further ultrafine generation (minimum of primarily below 10 µm). For the target ore (soft iron ore) grinding, Blaine number target achieved is 2200-2400 cm2/gm (due high amount of ultra-fines of specifically below 10 µm).The method is applied for grinding iron ore of less bond work index in batch ball mill grinding. Present method provides controlled dry grinding where ultra-fines is removed by application of sieves before and intermittent ball mill operation and oversize is crushed with higher load of grinding media. Overall control over fine generation lead to better control over granulometry thereby ease in further agglomeration and elimination of need of blending soft and hard ore achieving operation optimization.