Description:FIELD OF THE INVENTION

The present invention relates to an induration method and, more specifically, to pellet induration method directed to solve a major industrial problem of pellet spalling faced during pellet induration without any change in the raw material or reducing the speed or productivity of the induration furnace. Importantly, the present advancement provides for introduction of simple and cost-effective method of control of spalling of pellets following standard induration cycle which can be readily adopted for ensuring better quality pellet production.

BACKGROUND OF THE INVENTION

Iron ore pellets are preferred among iron bearing raw materials in iron making due to their raw material efficiency, positive impact on blast furnace performance, reduced environment footprint, enhanced pellet quality and ease of transportation. Globally, there is a trend to increase % pellet in the blast furnace feed burden.
Generally, iron ore pellets are susceptible to crack generation during the induration process, if raw material and processing conditions are not optimized. The major cause of crack generation is the inability of the gases (including steam) to escape from the pellet. Following parameters are known to affect the cracking behaviour:
1. Mass or volume of the gases/ steam: Green pellets containing higher fraction of ‘loss on ignition’ (LOI) and moisture generates high quantity of gaseous phase during the induration process
2. Rate of release of the gases/ steam: A higher heating rates cause sudden pressure drop across the outer shell, causing a crack generation
3. Hindrance for the escaping gases/ steam: Excessive compaction or presence of micro fines causes the reduction in the porosity, making it difficult to escape gases/steam and results in the build-up of pressure causing the crack
Pellet spalling is a problem observed in the pellets containing iron ores with high LOI (loss on ignition) which is due to the mineralogical constituents (i.e. Goethite/Kaolinite/Gibbsite). In such iron ores, these LOI starts liberating at temperatures >250°C and may continue up to 950°C depending on the type of LOI constituent, pellet bed depth, machine speed, length of the pellet machine, speed and the heating rate. Due to high heating rates in these temperature ranges, the gaseous products (i.e. steam) causes pressure on the external shell of the green pellets. When such pressure increases the strength of the outer shell, the gas (steam) may escape causing a crack or burst in the pellets which is referred as spalling. These broken fragment increases packing of bed, resulting into increase in pressure drop.
In standard industrial practice, during abnormal bed permeability conditions, reason of increase in pressure drop could be due to reasons other than situation arrived due to spalling. Reasons like wider distribution of pellet sizes, dissimilar feed to machine, plastic nature of green pellet, other lesser green pellet quality like green strength, drop number and temporary suction related machine design parameters. Operator during abnormal plant operation condition arising due to bed permeability needs to consider all above factor and difference out signs of pellet spalling. Thereafter, identification of exact spalling temperature within induration zone is critical to mitigate. Needless to say above tedious and highly skilled activity has other side of downtime and related production and quality loss of product pellet.
Based on mentioned technical constraints various published literature has elaborated spalling reasons for specific feed and plant conditions.
Guojing Wong, et al., Powder Technology Volume 342, 15 January 2019, Pages 873-879 discloses that as the ultrafine iron ore concentrate was used to prepare green pellets, the main problem was the serious cracking during the drying process. The fine particles contacted so closely that the porosity of green pellet was only 16.68%, which finally hindered vapor diffusion, and the cracking temperature was only 370?°C. Organic binder replacing parts of bentonite improved the cracking temperature to 500?°C, with comparable mechanical strength. Due to the action of organic binder, fine-grained concentrates were aggregated into large particles, with the porosity of green pellet increased from 16.68% to 23.15%.
DE1909037C3 discloses a process for the production of solid pellets intended for use in blast furnaces from iron-rich, pelletized iron ores or iron ore concentrates by firing at temperatures of 900-1200 °C. The method includes pellets to be burned in a neutral to slightly reducing atmosphere.
The green pellets are burned in the usual devices, such as rotary kilns, rotary hearth kilns, shaft kilns, pellet burning machines with straight or ring-shaped grates, etc. The pellets are heated to the firing temperatures within 10 to 90 minutes and left at the maximum firing temperature for 5 to 40 minutes. The fired pellets are cooled in such a way that the molar ratio of iron to oxygen obtained in the burning process is practically maintained.
Mixtures of ores of various chemical and / or physical properties, in which the fine grain fraction is below 0.045 mm is only 20 to 80% of the total amount of ore to be pelletized and at a minimum of 1250 ° C hardening.
The present invention relates to a method to solve a major industrial problem faced during the pellet induration, without increasing the induration cycle time or reducing the productivity. The present invention proposes to reduce spalling in the iron ore containing high amount of LOI, which was otherwise considered unsuitable for use in pellet feed mix due to the problem of spalling, reducing overall quality and productivity.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to advancements in induration cycle for pellet generation where the spalling can be avoided without change in the raw material or reducing the speed/ productivity of the induration furnace.
Another object of the present invention is directed to advancements in an induration method to solve a major industrial problem faced during the pellet induration without increasing the induration cycle time or reducing the productivity.
Another object of the present invention is directed to advancements in an induration method to assess the spalling temperature range and using that to solve the problem of pellet spalling/cracking.
Further object of the present invention is directed to advancements in an induration method by introducing a temperature holding step to the standard induration cycle that helps to reduce the spalling in iron ores.
Yet another object of the present invention is directed to advancements in an induration method which helps to reduce the spalling in the iron ore containing high amount of LOI which was otherwise considered unsuitable for use in pellet feed mix due to the problem of spalling, reducing overall quality and productivity.
Further object of the present invention is directed to advancements in an induration method to reduce spalling in the case of iron ores containing a wide range of mineralogical constituents such as Goethite, Kaolinite or Gibbsite.
Another object of the present invention is directed to advancements in an induration cycle by which low quality iron ores can be produced without affecting pellet productivity or quality.

SUMMARY OF THE INVENTION

Thus according to the basic aspect of the present invention there are series of steps provided for a process for controlling the spalling of pellets during pellet induration process based on the constitution of the pellets comprising:
i) providing green pellet for induration;
ii) identifying the temperature ranges of spalling following induration cycles depending on the composition and mineralogy of the green pellets;
iii) carrying out the induration of the pellets following such induration cycles wherein based on the identified spalling temperature range holding the temperature of induration of the pellets for maintaining a temperature buffer and only after crossing the said spalling temperature range under such holding condition, allowing the completion of induration process including any required ramping up to maintain constant induration cycles.
According to another aspect there is provided for a process as above wherein said step of identifying the spalling temperature ranges corresponding any immediate increase in pressure drop value by at least about 60% to 140% to -4 to -6 kPa which is beyond the typical operating range of pressure drop in the range of -2.5 to -3.5 kPa.
According to yet another aspect there is provided for a process as above wherein said spalling temperature range is identified based on effect of fluxing and iron ore mineralogy on observed spalling temperature range including at least anyone of lab scale and industrial scale operations comprising:
% Goethite (FeOOH) % Kaolinite
(Al2Si4O5) % Gibbsite Al(OH)3 % LOI from iron ore % CaCO3 basicity (B2) % MgO Spalling temperature range °C
A 0-60 8-10 6-8 2-10 0-2 0.1-0.4 0.5 350-500
B 0-60 8-25 0-25 2-10 0-10 0.4-2 1.5 350-500 & 800-1100

and said step of controlling holding time for temperature of induration include introducing 10-60 seconds holding by switching off the burners/heat source in the required induration zone or maintaining temperature buffer in industrial induration furnace and subsequently after crossing the spalling temperature range, ramping up heating to maintain constant cycle time such as to achieve advancement (i.e. reduction) in the spalling in the range of 7.3% to 40 % alongwith CCS (cold crushing index) from 172 to 250 kgf/pellet for variation in raw material and fluxing considerations.

According to yet further aspect of the invention there is provided for a process as above wherein green pellet composition comprising:
Chemical composition ranges of green pellets
Pellet Type T Fe SiO2 Al2O3 CaO Loss on ignition
A 63-65 3-4 2.5-3.0 0.1-0.5 2-5
B 60-63 4-6 2.5-3.5 0.5-5 3-10

is used, the identified spalling temperature ranges between 250?C-500?C to 650?C to 900?C and the holding time during said spalling temperature range is varied as 10 to 60 seconds.

Yet another aspect of the present invention provides for a process as above which is carried out following standard induration cycle comprising of drying 5-20 minutes, preheating 1-20 minutes, firing 5-25 minutes, cooling 5-25 minutes.

According to another aspect of the present invention there is provided for a process as above wherein said step of holding is based on spalling activity based on state of liberation of LOI which starts liberating at temperatures >250°C and may continue up to 950°C depending on the type of LOI constituent, pellet bed depth, machine speed, length of the pellet machine, speed and the heating rate.

According to yet another aspect of the present invention there is provided for a process as above comprising selectively carrying out anyone or more of:
a) detailed characterization of the raw materials following:
i) X-ray Fluorescence (XRF) analysis including non-destructive analytical technique to determine the elemental composition of a material;
ii) % Loss on Ignition (LOI) is percentage weight loss measured by calcination at around 900?C.; and
iii) XRD phase analysis

b) identifying LOI liberation protocol;
c) following industrials scale induration process to identify the location of the spalling in the specific heating zone; and
d) generating spalling range corresponding to the industrial scale.

According to another aspect of the present invention there is provided for a process as above wherein said step of identifying temperature ranges critical for spalling include:

i) providing apparatus for carrying out induration of pellets;
ii) performing heating cycle in lab scale furnace to match induration as per plant cycle;
iii) identifying temperature ranges critical for spalling which is used as reference temperature enabling modifying heating cycle to eliminate spalling.

DETAILED DESCRIPTION OF THE INVENTION
In order to advance the above art of induration of pellets, the present invention was carried out following the studies, its findings and related developments on the lines discussed hereunder:
The present invention relates to an induration method where pellet spalling can be avoided without any change in the raw material or reducing the speed/ productivity of the induration furnace.
Detailed characterization of the raw materials present in iron ore was carried out as shown in Table 1. Loss on ignition (LOI) of various mineral phases present in iron ore is important to understand their volatile content and thermal behaviour.
Goethite (FeO (OH)): Goethite is an iron oxide mineral that contains chemically bound water within its structure. As a result, goethite can exhibit a significant LOI during heating. The heating process drives off the chemically bound water, leading to a weight loss.
Clay minerals (e.g., kaolinite, gibbsite): Clay minerals are common impurities in iron ore. They often contain chemically bound water in their structures and can exhibit a notable LOI during heating. The loss of this bound water leads to weight loss during the heating process.
As observed from Table 2, Goethite reported mass loss of 10% at a temperature range of 250-300°C, Kaolinite reported mass loss of 12-14% at a temperature range of 450-650°C and Gibbsite reported mass loss of 34-35 % at a temperature range of 300-400°C.
It was observed that most of the spalling takes place in the temperature range of 300-500°C which was identified by immediate increase in the pressure drop value by at least 10% due to the reduction in the void area of the pallet car cause by the debris generated by the spalling.
A laboratory scale setup was used to simulate the spalling and the setup could successfully simulate the spalling observed in the industrial scale. A series of experiments was designed to identify exact temperature range of spalling while using industrial scale induration cycle under various combination of raw materials. Further, experiments were done to reduce the heating rates in the specific ranges without increasing the overall induration time, thus induration furnace machine speed need not be reduced.
Pellets were subjected to the standard induration cycle as per the industrial practice. Using the spalling temperature range obtained from the experiments reported in Table 3, inventive step of introducing 0-1minute hold was performed. This is equivalent of switching off the selective burners/heat source in the specific zone or maintaining temperature buffer in an industrial induration furnace. Subsequently, after crossing the spalling temperature range, heating was ramped up so that the total cycle time remains constant. The effect of introduction of 10-60 second hold time on various pellet quality parameters including spalling was evaluated as shown in Table 4.
It was observed from Table 4 that along with the improvement in the Spalling in the range of 7.3% to 40 %, CCS (cold crushing index) has also improved from 172 to 250 kgf/pellet for variation in raw material and fluxing perspective.
The advancement was further followed up based on the following non-limiting exemplary illustrations as discussed here under:

Examples
Example 1: Characterization of the raw materials

The detailed characterization of the raw materials present in the iron ore is shown is carried out following:
i) X-ray Fluorescence (XRF) analysis including non-destructive analytical technique to determine the elemental composition of a material;
ii) % Loss on Ignition (LOI) is percentage weight loss measured by fusion at 900?C; and
iii) XRD phase analysis
More specifically, the process of characterization of the raw materials is done following:
X-ray Fluorescence (XRF) analysis is a non-destructive analytical technique used to determine the elemental composition of a material. XRF analysis provides information about the types and concentrations of elements present in a sample.
Loss on ignition is done to determine the amount of volatile or combustible substances present in a material. The test involves heating a sample to a high temperature to drive off volatile components, such as water, carbon dioxide, and other organic materials, and measuring the weight loss as a percentage of the initial sample weight. % Loss on Ignition (LOI) is percentage weight loss measured by fusion at 900?C.
X-ray Diffraction (XRD) phase analysis is an analytical technique used to determine the crystalline phases present in a sample. It is used to identify and quantify the types of crystalline structures that make up iron ore fines.
The detailed characterization of the raw materials present in the iron ore is shown in Table 1.

Table 1: Raw material major composition and phase analysis of iron ore
XRF analysis Weight loss method XRD phase analysis using Reitveld method
Iron ore T Fe SiO2 Al2O3 % LOI % Goethite % Kaolinite % Gibbsite
A 60-64 3-4 3-4 3-4 5-25 0-10 0-8
B 60-62 4-6 2.5-3.5 4-5.5 25-40 5-15 4-8

Example 2: LOI liberation temperature of different mineralogical phases

Loss on ignition (LOI) of various mineral phases present in iron ore is important to understand their volatile content and thermal behaviour.

Goethite [FeO (OH)]: Goethite is an iron oxide mineral that contains chemically bound water within its structure. As a result, goethite can exhibit a significant LOI during heating. The heating process drives off the chemically bound water, leading to a weight loss.

Clay minerals (e.g., kaolinite, gibbsite): Clay minerals are common impurities in iron ore. They often contain chemically bound water in their structures and can exhibit a notable LOI during heating. The loss of this bound water leads to weight loss during the heating process.

Table 2 provides a list of mineralogical phases associated with iron ore which are responsible for significant release of mass during Loss on Ignition (LOI) along with the LOI liberation temperature for each phase. Determination of Loss of Ignition was carried out as per standard ISO 11536-2015.

Table 2: List of mineralogical phases associated with iron ore responsible for significant release of mass during Loss on Ignition (LOI) test as per standard ISO 11536- 2015.

Phase Name Chemistry Mass loss % LOI liberation temperature range °C
Goethite FeO(OH) 10 250-300
Kaolinite Al2Si2O5(OH)4 12-14 450-650
Gibbsite Al(OH)3 34-35 300-400

As observed from Table 2, Goethite reported mass loss of 10% at a temperature range of 250-300°C, Kaolinite reported mass loss of 12-14% at a temperature range of 450-650°C and Gibbsite reported mass loss of 34-35 % at a temperature range of 300-400°C.

Example 3: Study of the industrial data of spalling

A study was performed of the industrial scale induration process to identify the location of spalling in the specific heating zone. It was observed that most of the spalling takes place in the temperature range of 300-500°C which is identified by immediate increase in the pressure drop value by at least 10% due to the reduction in the void area of the pallet car cause by the debris generated by the spalling.

Example 4: Lab scale simulation of the spalling corresponding to the industrial scale

A laboratory scale setup was used to simulate the spalling and the setup could successfully simulate the spalling observed in the industrial scale. Once, established the setup was used to study the phenomena in greater depth and identification of possible solutions.

Example 5: Development of unique processing window of induration to avoid spalling (without reducing the productivity) with a given set of raw material:
The present example discloses the use of an apparatus for carrying out induration of pellets along with a method of performing heating cycle in muffle furnace to match induration as per plant cycle and a method to find out temperature ranges critical for spalling by above mentioned method and further enabling to modify heating cycle to eliminate spalling.
A series of experiments were designed to identify exact temperature range of spalling while using industrial scale induration cycle under various combination of raw materials. Further, experiments were done to reduce the heating rates in the specific ranges without increasing the overall induration time, thus induration furnace machine speed need not be reduced.

Example 5a: Chemical Composition of green pellet
The chemical composition of green pellet used for the process of induration is demonstrated in Table 3.

Table 3: Chemical Composition of green pellets
Chemical composition ranges of green pellets
Pellet Type T Fe SiO2 Al2O3 CaO Loss on ignition
A 63-65 3-4 2.5-3.0 0.1-0.5 2-5
B 60-63 4-6 2.5-3.5 0.5-5 3-10

Example 5b: Determination of the spalling temperature range

Induration cycle from the industrial scale was simulated in the lab scale furnace and pellets were indurated using the similar thermal cycle. The process for such industrial simulation was carried out involving system and method as already published and disclosed under prior dated granted Indian Patent application number 201921026025 for operation of system for induration of green pellet. The temperature at which spalling occurs was recorded for each type of feed mix. For various combinations of raw materials, following temperature range have been identified using lab scale experiments as shown in Table 4.

Table 4: Effect of fluxing on observed spalling temperature ranges
% Goethite (FeOOH) % Kaolinite
(Al2Si4O5) % Gibbsite Al(OH)3 % LOI from iron ore % CaCO3 basicity (B2) % MgO Spalling temperature range °C
A 0-60 8-10 6-8 2-10 0-2 0.1-0.4 0.5 350-500
B 0-60 8-25 0-25 2-10 0-10 0.4-2 1.5 350-500 & 800-1100

Example 5c: Induration method to avoid pellet spalling
The induration process for pellets involves 4 main stages: (1) drying, (2) preheating, (3) heating and (4) cooling.
Pellets were subjected to the standard induration cycle as per the industrial practice (Meyer, K. 1980). Now, using the spalling temperature range obtained from the experiments reported in Table 4, inventive step of introducing 10-60 seconds of hold was performed. This is equivalent of switching off the selective burners/heat source in the specific zone or maintaining temperature buffer in an industrial induration furnace. Subsequently, after crossing the spalling temperature range, heating was ramped up so that the total cycle time remains constant. The effect of introduction of 10 to 60 seconds hold time on various pellet quality parameters including spalling was evaluated as shown in Table 5.

Table 5: Effect of novelty induration method on pellet quality with different sample blend conditions (based on mineralogical variations and fluxing)
Spalling % Cold Crushing Strength (CCS)

kgf/pellet
Base case With Novelty Base case With Novelty
Blend 1 17.3 1.4 172 202
Blend 2 40 7.3 180 250
Blend 3 7.3 1 199 205
Blend 4 8.5 1.3 192 211

It can be observed from Table 5 that along with the improvement in the spalling in the range of 7.3% to 40 %, CCS (cold crushing index) has also improved from 172 to 250 kgf/pellet irrespective variation in raw material and fluxing perspective.

It is thus possible by way of the present invention to provide for an induration method and, more specifically, to pellet induration method directed to solve a major industrial problem of pellet spalling faced during pellet induration without any change in the raw material or reducing the speed or productivity of the induration furnace. Advantageously, the present advancement as traversed hereinbefore would go to demonstrate the special and unexpected achievements by way of introduction of simple and cost-effective method of control of spalling of pellets following standard induration cycle which can be readily adopted for ensuring better quality pellet production.

, Claims:We Claim:
1. A process for controlling the spalling of pellets during pellet induration process based on the constitution of the pellets comprising:
i) providing green pellet for induration;
ii) identifying the temperature ranges of spalling following induration cycles depending on the composition and mineralogy of the green pellets;
iii) carrying out the induration of the pellets following such induration cycles wherein based on the identified spalling temperature range, holding the temperature of induration of the pellets for maintaining a temperature buffer and only after crossing the said spalling temperature range under such holding condition, allowing the completion of induration process including any required ramping up to maintain constant induration cycles.
2.The process as claimed in claim 1 wherein said step of identifying the spalling temperature ranges include monitoring corresponding any immediate increase in pressure drop value by at least about 60% to 140% to -4 to -6 kPa which is beyond the typical operating range of pressure drop in the range of -2.5 to -3.5Pa.

3. The process as claimed in anyone of claims 1 or 2 wherein said spalling temperature range is identified based on effect of fluxing and iron ore mineralogy on observed spalling temperature range including at least anyone of lab scale and industrial scale operations comprising:

% Goethite (FeOOH) % Kaolinite
(Al2Si4O5) % Gibbsite Al(OH)3 % LOI from iron ore % CaCO3 basicity (B2) % MgO Spalling temperature range °C
A 0-60 8-10 6-8 2-10 0-2 0.1-0.4 0.5 350-500
B 0-60 8-25 0-25 2-10 0-10 0.4-2 1.5 350-500 & 800-1100

and said step of controlling holding time for temperature of induration include introducing 10-60 seconds holding by regulating the heat input following any regular operative control options selected from burner gas flow, blending of gases for temperature control in a specific location, switching off the burners/heat source in the required induration zone or maintaining temperature buffer in industrial induration furnace and subsequently after crossing the spalling temperature range ramping up heating to maintain constant cycle time such as to achieve advancement in the spalling in the range of 7.3% to 40 % alongwith CCS (cold crushing index) from 172 to 250 kgf/pellet for variations in raw material and fluxing considerations.

4. The process as claimed in anyone of claims 1 to 3 wherein pellet composition comprising:

Chemical composition ranges of green pellets (%)
Pellet Type T Fe SiO2 Al2O3 CaO Loss on ignition
A 63-65 3-4 2.5-3.0 0.1-0.5 2-5
B 60-63 4-6 2.5-3.5 0.5-5 3-10

is used , the identified spalling temperature ranges between 250?C-500?C to 650?C to 900?C and the holding time during said spalling temperature range is varied as 10 & 60 seconds.

5. The process as claimed in anyone of claims 1 to 4 which is carried out following standard induration cycle comprising of drying 5-20 minutes, preheating 1-20 minutes, firing 5-25 minutes, cooling 5-25 minutes.

6. The process as claimed in anyone of claims 1 to 5 wherein said step of holding is based on spalling activity based on state of liberation of LOI which starts liberating at temperatures >250°C and may continue up to 950°C depending on the type of LOI constituent, pellet bed depth, machine speed, length of the pellet machine, speed and the heating rate.

7. The process as claimed in anyone of claims 1 to 6 comprising selectively carrying out anyone or more of:
a) detailed characterization of the raw materials following :
i) X-ray Fluorescence (XRF) analysis including non-destructive analytical technique to determine the elemental composition of a material;
ii) % Loss on Ignition (LOI) is percentage weight loss measured by fusion at 900?C; and
iii) XRD phase analysis
b) identifying LOI liberation protocol;
c) following industrials scale induration process to identify the location of the spalling in the specific heating zone; and
d) generating spalling range corresponding to the industrial scale.

8. The process as claimed in anyone of claims 1 to 7 wherein said step of identifying temperature ranges critical for spalling include:

i) providing apparatus for carrying out induration of pellets;
ii) performing heating cycle in lab scale furnace to match induration as per plant cycle;
iii) identifying temperature ranges critical for spalling which is used as reference temperature enabling modifying heating cycle to eliminate spalling.

Dated this the 17th day of November, 2023. Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent & Advocate)
IN/PA-199