Mineral grinding method

Abstract

The present invention relates to an adjustment to conventional mineral grinding or milling process to produce fine or ultrafine, activated minerals, particularly cementitious materials, clinker or pozzolans, with an increased mill's throughput or to enable ultrafine grinding with minimum changes or mechanical modifications to the closed loop milling circuit.

Description

[0001] MINERAL GRINDING METHOD

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of grinding mineral particles, more specifically cement particles. In particular, the invention describes an adjustment to the conventional mineral grinding process to produce fine or ultrafine, activated minerals, particularly cementitious materials, clinker or pozzolans, with an increased mill’s throughput or to enable ultrafine grinding with minimum changes or mechanical modifications to the closed loop milling circuit.

[0004] BACKGROUND OF THE INVENTION

[0005] Cement, one of the most essential materials in construction, owes much of its properties, such as strength and reactivity, to the process of grinding. Cement grinding is a crucial stage in the production of cement, transforming clinker, a hard, nodulous material, into the fine powder that forms the basis of concrete structures around the world.

[0006] The conventional cement grinding process involves several key stages, each contributing to the refinement of the final product. At the centre of the cement grinding process is the single or multi-chamber ball mill, a horizontal, cylindrical equipment rotating about its axis. The grinding media within the mill crushes the clinker nodules into fine particles, and mixes them with other materials, such as gypsum and cement additives, forming cement. Such materials help regulate the setting time and enhance the final product’s performance.

[0007] Another piece of equipment that is paramount in the cement grinding process is the classifier, also known as separator, which divides the ground material into coarse and fine particles, ensuring the desired quality of the final product.

[0008] The working principle of a classifier is simple. The material enters the classifier from the top through a feed chute, and it’s dispersed in the circulating air by the distribution plate. The particles are driven by three forces: 1) the centrifugal force due to the dispersing plate, which pushes the material towards the guide vanes; 2) the drag force due to the air flow, which pulls the material into the rotating cage, and 3) the gravity due to the mass of the particle. The coarse material leaves the classifier at the bottom, due to the action of gravity, and is fed into the mill for further processing. The fine material exits with the air flow by the upper, middle, or lower part of the classifier, depending on its configuration. The powdered material is transported to storage silos (Figure 1).

[0009] In the cement manufacturing process, several minerals may be processed other than clinker or cement particles using this conventional milling process described above, such as ore, limestone, silicate, hard waste (bottom ash, fly ash or slag), natural pozzolans, or other supplementary cementitious materials.

[0010] A common technique to increase the particles’ reactivity, and therefore reduce the clinker factor of the final concrete product, is to grind the particles until ultrafine particles are obtained. Such ultrafine particles will have an increased reactivity due to their higher surface area. When such ultrafine mineral particles are cement particles, their increased reactivity offers advantages in areas such as high-performance concrete, or advanced construction materials.

[0011] With the increasing demand on highly performing cements, the cement plants have been pushing their processes to produce finer and finer cements. This has directly impacted the production capacity of the grinding process. Furthermore, with the increasing need for cements with low clinker factor, where clinker is substituted by supplementary cementitious materials, such as pozzolans, slags, silica, limestone, fly or bottom ashes, such materials must also be subjected to extra fine grinding, to increase their pozzolanic activity.

[0012] Although such ultrafine particles may be obtained using conventional ball mills, this can be challenging. The primary limitation stems from the inherent design and operating principles of ball mills, which are optimized for coarse to fine grinding rather than ultrafine grinding. Also, achieving ultrafine particles in a conventional ball mill comes with low throughput. The mill’s throughput is influenced by several factors, one of them being the type of material to be milled (size and hardness), but normally a typical ball mill’s throughput is located between 10 up to 150-180 ton / hour.

[0013] Another limitation is because such fine milling overloads the classifier, arising several issues that impact the operation of the mill. When the classifier is overloaded, its ability to efficiently sort the material diminishes, leading to a reduction in the overall throughput of the mill. Moreover, an overloaded classifier can cause fluctuations in the loading of the grinding mill, leading to unstable mill operation. This instability can manifest as vibrations, which may not only affect the milling process but could also potentially cause mechanical damage.

[0014] Although optimization strategies aimed at improving grinding and separation efficiencies can help solving these problems, achieving ultrafine particles may still involve significant decrease in production throughput due to the overall inability of the process. One such strategy to overcome this is to run the grinding process at lower capacity to avoid overwhelming the classifier and allow a better classification. The conventional proposed solution in these cases implies upgrading the classifier or installing additional classifiers which requires a costly capex and additional footprint.

[0015] One of the existing solutions passes by an upgrade of the classifier. According to “Energy Efficiency Improvement and Cost Saving Opportunities for Cement Making” (Ernst Worrell and Christina Galitsky, 2008: “Standard classifiers may have a low separation efficiency, which leads to the recycling of fine particles, resulting in extra power use in the grinding mill.”

[0016] Similar teaching to Worrell et al. is found in Innovations in Portland Cement Manufacturing. 2nd Edition (January 2011), from the Portland Cement Association. It is said in chapter 4.1 (Finish milling and grinding): “Therefore, higher circulating load provides lower mill exit fineness (leading to higher grinding efficiency) but higher classifier loading (leading to decrease in classifier efficiency, hence an increase in the returns of fines) and increase in the mill hold-up (leading to a decrease in the grinding efficiency, once above the optimum). Thus, there are opposing influences that result in an optimum circulating load.” (page 633).

[0017] Kazak et al. have suggested an improved design of the ball mill to improve the efficacy of dry materials’ grinding, namely the installation of longitudinal diametrical partitions with windows at right angles and prisms.

[0018] Sharapov et al. disclose an improvement in the design of air classifiers to improve the grinding process.

[0019] Other proposed solutions involve new or modified grinding media. For example, Worrell et al. also teach to use improved wear resistant materials in the grinding media to decrease energy consumption.

[0020] The design patent USD743457 discloses a modified grinding body that has a spherical tetrahedral form to improve the grinding of the material.

[0021] Not only modifications to the equipment or grinding media are disclosed in the prior art, but also modifications to the grinding process itself. For example, patent J P2004188368 discloses a two-stage grinding apparatus using a vertical grinder and a ball mill. In this patent, the raw material ground by a vertical grinder is separated into a coarse powder and a fine powder by a classifier, returning the coarse powder to the vertical grinder to grind it again, and feeding the fine powder into a ball mill to mill it. The inventors therefore disclose the need to use 2 set of equipment, one specifically for the coarse particles and the other for fine particles, to improve the milling process. A similar approach is disclosed by JP4811713.

[0022] EP3665133 discloses a method wherein cement starting material is ground in a ball mill and is subsequently separated into fine and coarse material in a classifier, the coarse material being fed to the ball mill again. Subsequently, the fine material of the classifier is quantitatively divided into a first partial amount and a second partial amount. The first partial amount forms a first cement, while the second partial amount of the fine material is fed to a dry-operated stirred ball mill for further grinding and the resulting second cement is selectively mixed with the first cement or used as a separate product. The method focuses only on re-grinding the already fine particles, which is a different approach to the present invention.

[0023] LIS20180208506 presents a method that aims at optimizing the energy consumption of the cement grinding process, achieving desired particle sizes efficiently, and enhancing the properties of the cement product. The patent comprises a method of manufacturing a cement from a cement clinker comprising at least two kinds of clinker phases with differing grindability, involving feeding the cement clinker to a first milling stage, grinding the cement clinker in the first milling stage with specific grinding power and time settings, transferring the output to a first separator, dividing the output into two fractions based on particle size, transferring the larger particle size fraction to a second milling stage, grinding it to a final maximum particle size smaller than the predetermined maximum particle size, and combining the fractions to achieve the desired particle size distribution. It is a complex method, involving two mills and two separators.

[0024] LIST OF DEFINITIONS

[0025] Activated minerals or particles. Minerals or particles to which were applied grinding technology that can couple ultrafine grinding and crystal deformation through shearing (hence also called “Mechanically activated minerals or particles”). Consequently, the material undergoes crystalline-to-amorphous transformation, thereby increasing its chemical reactivity in addition to the “typical” physical reactivity resulting from higher surface area.

[0026] Admixtures. A combination of chemical substances that are added to concrete or mortar before or during mixing to modify its properties and enhance performance.

[0027] Ball-mill. Type of grinder used to grind, blend, and sometimes mix materials for use in various industrial processes. It operates on the principle of impact and attrition, where the size reduction of the materials is achieved by impact as the balls drop from near the top of the shell. It may be “Single” with 1 chamber, or “Multi-chamber” with more than 1 chambers.

[0028] Binder. Material with cementing properties that sets and hardens due to hydration even under water. Hydraulic binders produce calcium silicate hydrates also known as CSH.

[0029] Blaine method. Standardized procedure used to determine the specific surface area of powders, most commonly in the context of cement and other fine materials. The method provides an indication of the fineness or particle size distribution of a powder, which is important for assessing its performance in various applications. It involves compacting the powder into a bed and measuring the resistance of air flow through it, with the results indicating the powder's surface area in square meters per kilogram.

[0030] Bottom ash. By-product of the combustion of coal in a coal-fired power plant or industrial boiler. Cement. Binder that sets and hardens and bring materials together. The most common cement is the ordinary Portland cement (OPC) and a series of Portland cements blended with other cementitious materials.

[0031] Cement additives. Substances incorporated into cement or concrete mixtures to modify and enhance specific properties of the final product. These additives can be used to improve performance characteristics, adjust setting times, and optimize the workability and durability of cement.

[0032] Cementitious materials. Cement-based materials that have the ability to act as a binder and set, harden, and adhere to other materials, forming a solid mass. These materials are crucial in construction as they are the primary component in concrete, mortar, and other building products.

[0033] Circulation load. The amount of material that is returned to the grinding circuit from the classifier or separator, including coarse particles that need further grinding. It is usually expressed as a ratio or percentage of the total amount of material being processed.

[0034] Clay. A natural, fine-grained soil or sediment composed primarily of very small particles of minerals and organic matter.

[0035] Clinker. A nodular material produced in the manufacturing process of cement, specifically Portland cement.

[0036] Clinker factor. The proportion of clinker used in the production of cement relative to the total mass of the final cement product.

[0037] Closed loop milling circuit. Type of milling process where the material is continuously cycled through the milling equipment until it reaches the desired particle size or other specified properties. This process involves the recirculation of oversized particles back into the milling system for further grinding, ensuring that the final product meets the required specifications.

[0038] Coarse minerals or particles. Solid particles that are relatively large in size compared to other particles in a given material or mixture. In the context of the present invention, coarse particles have a d(90) higher than 38 pm.

[0039] Concrete. Concrete is primarily a combination of hydraulic binder, sand and / or aggregates, water. Admixture can also be added to provide specific properties such as flow, lower water content, acceleration, etc. d(10): The particle size (diameter) at which 10% of the total sample’s mass or volume is comprised of particles smaller than this size. Conversely, 90% of the particles are larger than this size. It is determined by sieve analysis; the sieve size [pm] at which 10% of the particles pass through, meaning that 10% of the particles will have a smaller size than d(10) and 90% have a bigger size than d(10). d(50): Median particle size. Specifically, it is the particle size at which 50% of the sample's mass is finer and 50% is coarser. It is determined by sieve analysis; the sieve size [pm] at which 50% of the particles pass through, meaning that 50% of the particles will have a smaller size than d(50) and 50% have a bigger size than d(50). d(90): The particle size at which 90% of the total sample mass or volume is smaller. In other words, it is the size below which 90% of the particles fall. It is determined by sieve analysis; the sieve size [pm] at which 90% of the particles pass through, meaning that 90% of the particles will have a smaller size than d(90) and 10% have a bigger size than d(90).

[0040] Fine minerals or particles. Finely ground minerals or particles. In the context of the invention, fine particles have the following particle size distribution: d(10):2-3 pm, d(50): IQ- 12 pm, d(90): 21-38 pm.

[0041] Fly ash. By-product of burning pulverized coal in electric power generating plants.

[0042] Grinding aids. Chemical additives that are introduced into the grinding mill to improve the grinding process. They work by reducing the energy required for grinding and improving the flow characteristics of the material.

[0043] Grinding media. Spherical or cylindrical materials used within a grinding mill to facilitate the process of grinding or milling. These materials are essential for breaking down and reducing the size of the particles of the material being processed.

[0044] Grinding process. Method used in various industries to reduce the size of particles by mechanical means. It involves breaking down materials, typically solid substances, into smaller fragments through the application of force, often by using grinding machinery.

[0045] Gypsum. A soft, sulfate mineral composed primarily of calcium sulfate dihydrate (CaSO4'2H2O).

[0046] High-performance concrete. Concrete that is engineered to provide superior performance characteristics compared to conventional concrete. These characteristics typically include increased strength (higher than 40 MPa), durability, workability, and resistance to environmental factors.

[0047] Horizontal roller mills. Grinding machines that operate with horizontal cylindrical rolls rotating on a horizontal axis to crush or grind materials. The material is fed between the rolls, where it is subjected to compressive forces, leading to size reduction.

[0048] Limestone. A sedimentary rock primarily composed of calcium carbonate (CaCO3), usually in the form of calcite or aragonite.

[0049] Mineral particles. Small, discrete pieces of minerals that can vary in size from microscopic to macroscopic. These particles are the basic components of rocks, soils, and sediments and are characterized by their chemical composition and physical properties.

[0050] Mode diameter. The diameter that occur with the highest frequency in a sample. It indicates the most common diameter(s) within a given sample of particles. Mortar. A mixture of binding agents (such as cement, lime, or gypsum), sand, and water, used to bond and seal masonry units and to fill joints and gaps in construction.

[0051] Ore. A naturally occurring mineral or rock which a key raw material in the production of iron and steel, which are crucial for various industrial applications and construction.

[0052] Particle Size Distribution (PSD). The distribution of particle sizes within a material, typically represented as a percentage or fraction of particles within specific size ranges or classes.

[0053] Pozzolans. Aluminosilicate material of volcanic origin that reacts with calcium hydroxide to produce calcium silicate hydrates or CSH as known in Portland cement hydration. When natural in origin, also called “Natural Pozzolans”.

[0054] Pozzolanic activity. Refers to the ability of certain materials to react and form compounds that contribute to the strength and durability of concrete.

[0055] Pumice. A volcanic glass that is highly porous and lightweight, formed from rapidly cooled lava that contains gas bubbles. It is used in various industrial and construction applications due to its unique physical properties.

[0056] Reactivity. The tendency or ability of a particle to engage in chemical reactions. This property is influenced by factors such as the particle's size, surface area, chemical composition, and the presence of active sites on its surface.

[0057] Return grits. Coarse, oversize particles or fragments that are returned to a grinding or milling process after an initial pass.

[0058] Roller press. A machine designed for compressing and grinding bulk materials between two counter-rotating rollers. It is used to achieve size reduction and improve material handling properties.

[0059] Setting time. The time it takes for a material, typically cement or concrete, to undergo the transition from a liquid or semi-liquid state to a solid state. This process involves the hardening and curing of the material, which is crucial for its final strength and stability.

[0060] Silicate minerals. Group of minerals that contain silicon and oxygen as their primary chemical components.

[0061] Slag. A by-product generated during the smelting and refining processes of metals, such as iron, copper, and lead. It consists of a mixture of metal oxides, silicates, and other impurities that are separated from the metal during the smelting process.

[0062] Strength (Compressive strength). The ability of a material, such as concrete, to withstand axial loads that tend to compress or shorten it. The measurements are normally done at 1 (early-age), 7 and 28 (later-age) days.

[0063] Supplementary cementitious materials (SCMs). Materials used in concrete production to enhance or modify the properties of the concrete mix, often by partially clinker or cement. SCMs are added to improve the sustainability, durability, and performance of concrete while potentially reducing the overall cost.

[0064] Surface area. Total area of the outer surface of a particle. This measure is critical in many scientific and industrial applications because it impacts various properties of materials, such as reactivity, strength, and interaction with other substances.

[0065] Throughput. Measure of the amount of material or items passing through a system or process within a given time period (for example, units can be ton / hour). It is a key performance indicator in various industries, reflecting the efficiency and capacity of operations.

[0066] Total Setpoint. A predefined target value or control parameter that guides the operation of a system or process. In a milling or grinding process, it typically refers to the total desired output or performance level for the operation, such as the target particle size, throughput, or quality metrics.

[0067] Tube mill. A cylindrical grinding mill with a horizontal orientation, used for the fine grinding of materials such as cement, minerals, and ores. It operates by rotating a tube filled with grinding media, which impacts and grinds the material inside.

[0068] Ultrafine minerals or particles. Ultra-finely ground minerals or particles. These minerals or particles possess unique properties due to their fine particle size and high surface area, which can significantly influence their physical and chemical behaviour. In the context of the invention, ultrafine particles have a d(90) between 5 and 20 pm.

[0069] Vertical mill. A type of grinding machine where material is fed into a rotating grinding table, and grinding rollers press down on the material to grind it. The grinding process occurs in a vertical plane.

[0070] DESCRIPTION OF THE INVENTION

[0071] The present invention can increase the mill’s throughput by 10-15 % in a conventional ball mill, not only when the objective is ultrafine grinding of particles, but also when conventional clinker fineness is targeted, and solve several problems, namely:

[0072] 1 . To provide a versatile solution that can be easily retrofitted to existing mills.

[0073] 2. To produce ultrafine mineral ground particles or to substantially increase the mill’s throughput targeting conventional clinker fineness, using conventional milling equipment.

[0074] 3. To provide a wider range of minerals (pozzolans, clinker, slags, silica, limestone, fly or bottom ashes) that can be milled in the same equipment.

[0075] 4. To mechanically activate minerals or industrial wastes in order to provide them with pozzolanic activity that cannot be achieved with a conventional milling process, with acceptable production throughput. The present inventors have also found out that, returning a portion of the milled material back to the mill, without passing by the classifier, in fact increases the grinding ability, without the need to use special classifiers.

[0076] The proposed invention is thus a breakthrough technique, yet providing incremental benefits in respect to the prior art.

[0077] The inventors have found an ingenious approach that allows ultrafine mineral particles to be produced in any mill used to grind mineral particles, without overloading the classifier (and consequently not leading to a reduction in the overall throughput of the mill). They also found out that this method is able to increase the mill’s throughput in conventional milling. What the inventors have found is that, by modifying the conventional configuration of the grinding process, a better PSD and finer product are obtained. This process modification brings several advantages:

[0078] 1 . Maximizing production throughput: the proposed solution allows, with the same process conditions and capabilities, to reach higher throughput with the same or higher quality and lower energy consumption.

[0079] 2. Maximizing attainable fineness: the proposed solution reaches higher fineness, and it’s suitable for ultrafine grinding of mineral particles - D(90) up to 20 pm - without decreasing the overall capacity of the grinding process.

[0080] 3. Maximizing attainable steady feed: the proposed solution does not overload the classifier nor the mill.

[0081] 4. Minimizing energy: the proposed solution allows the production of ultrafine mineral particles, maintaining or even decreasing the overall energy consumption of the milling process.

[0082] These problems are normally individually tackled using various solutions (changing elements of the mill, using different additives or grinding media, etc...) as the prior art has shown, whereas this invention presents one technical solution that can solve all identified problems. Furthermore, the present invention is not limited to cement particles, but can be used in the grinding process of any mineral particles (ore, limestone, silicate, hard waste - bottom ash, fly ash or slag -, natural pozzolans, or other supplementary cementitious materials), using any suitable mill. Indeed, although the ball mill is the most conventional mill in a cement plant, there are other mills suitable for cement milling, for example vertical mill, roller presses, tube mills or horizontal roller mills.

[0083] The invention thus concerns a method to increase the throughput of the grinding or milling process of mineral particles using a single and / or multi-chambers ball mill, the method comprising the steps of: a) Feeding minerals (Ftotal) into a mill; b) Grinding the minerals in the mill; c) First discharging fine particles (FF) directly to the final product silo using the air stream from the mill; d) Second discharging the remaining mineral particles (FE) from the mill, wherein FE equals Ftotal minus FF (FE=Ftotal-FF); e) Splitting online the discharged ground mineral particles (FE) after step d) in at least two fractions (FM and FS) using a splitting device (SD); f) Feeding fraction FM back to the mill for further re-grinding; and g) Feeding fraction FS into a separation device (that will sort out the particles according to their size), wherein the sum of all particles of said at least two fractions (FM and FS) equals FE; h) Sorting, in the separation device, the mineral particles from fraction FS into fine (FSf) and coarse (FSC) particles; i) Discharging the fine mineral particles (FSf) from the separation device into storage; j) Feeding the coarse mineral particles (FSC) from the separation device back into the mill for further re-grinding, whereas the splitting of the fractions FM and FS is operated using a splitting device SD that enables to set the ratio between FM and FS.

[0084] In step a) of the method of the invention, minerals are fed from a feed silo (a) into a ball mill (c) through a mill’s inlet (b). These minerals can be clinker, but also can be limestone, silicate, hard waste - bottom ash, fly ash or slag, natural pozzolans, or other supplementary cementitious materials.

[0085] The purpose of the mill is to grind mineral particles, for example clinker and any additions (for example, pumice, gypsum, limestone, pozzolans, etc.), into a fine powder, increasing the surface area and consequently the reactivity of the cement with water, for concrete or mortars production. The grinding of the minerals in a cement mill (c) is carried out in step b) of the method of the invention.

[0086] The main step in the cement production process consists of heating raw meal (a mixture of limestone, clay, iron ore, among others) up to 1500°C, to produce the main component of cement: clinker, a dark grey nodular material. The clinker contains minerals that are responsible for the unique properties of cement. After being produced, the clinker is cooled and stored until it is turned into cement in a cement mill.

[0087] The cement mill is where the clinker particles and any additional substances are finely ground into a powder. There are four primary types of cement mills: the ball mill, vertical roller mill, roller press, and horizontal roller mill. The present invention is suitable for any such type of mills. In a ball mill, particle size reduction occurs through impact and attrition. In this type of mill, spherical balls inside the rotating shell drop onto the particles as the mill rotates. Initially, larger balls (60-80 mm in diameter) in the first chamber crush the clinker nodules, while in the second chamber, smaller balls (15-40 mm in diameter) produce fine or ultrafine particles. Typically, ball mills have two chambers, but some may have up to four. When a mill has more than 2 chambers, it is often called multi-chamber ball mill.

[0088] In a conventional milling process using a ball mill (Figure 2, example of a double chamber ball mill), the fresh feed particles (or mill feed particles, material entering the mill for the first time) leave the fresh feed silos (1), are transported through weight feeder / conveyor belt(s) (2) to the mill fresh feed chute (3) and enter the mill, where are grounded in the first chamber of the mill (4). The final product size depends on the type of mineral being process and final application. For example, cement particles are normally ground in the first chamber until no more than 1 wt.% of the particles are retained on 1 mm sieve and not more than 50 wt.% of the particles are retained on a 90 pm sieve. The large grain size (> 6 mm) particles are screened by an intermediate diaphragm (5), which has a series of apertures located on the walls that stop particles bigger than 6 mm from exiting the first chamber (4). The larger material remains in the first chamber (4) to be further ground. When the particles are fine enough (< 6 mm), they go through the apertures of the intermediate diaphragm (5) and are discharged into the second chamber (6). When the material reaches the end of the second chamber, it is lifted by a diaphragm called discharge diaphragm (7) to be transported to the drop-out-box (8), where a small portion of the fine particles is lifted through the mill filter line (9) to the mill filter (10) by the air draft created by the mill fan (11). The collected filtered material on (10) is returned by a screw conveyor (11) and subsequently through a discharge duct (13) to the mill outlet air slide (14) (the air slide is a pneumatic conveying system designed to transport powdered materials) where it joins the discharged material from the mill and both are transported to the bucket elevator (15) which lifts the material to the air classifier or separator (16).

[0089] In the classifier (16), particles are sorted into fine cement particles and coarse cement particles. The definition of fine or coarse particles in this sorting process may change, as it is related to the type of cement being produced or classifier being used. Depending on the grinding technology, grinding parameters, and admixtures the particle size reduction achieved can vary, and in the specific case of clinker the follow categories are defined: Typical fineness: d(90): 39-65 pm, with a mode diameter around 22 pm; Fine grinding: d(10):2-3 pm, d(50):10-12 pm, d(90): 21-38 pm;

[0090] Ultrafine grinding: d(90): 5-20 pm

[0091] The fine cement particles leave the classifier (16) via finished product line (18) and are collected in a dust collecting filter (19) that is supported by a fan (20). The particles collected in the dust collecting filter (19) are transported by a screw conveying system (21) to the finish product silo (not shown).

[0092] On the other hand, the coarse particles are returned from the classifier (16) via a return line (17) (a conveying system carrying the coarse particles from the separator, pneumatically or by belt conveying) to the mill fresh feed chute (3) - hence the coarse particles can also be called “return grits” - for further milling, together with fresh feed in the first chamber (4).

[0093] Other than the main equipment mentioned above, in a conventional milling process there is also a low speed coupling that transmits power, accommodates missed alignment and compensates for axial movement (22), a gear box, which is a mechanical device used to increase the output torque and the speed of the mill (23) and a motor, which converts the electrical energy into mechanical energy, wherein the power installed in the motor is the one that determines the mill capacity (24).

[0094] Normally, fine particles and return grits experience very little processing in the first chamber, due to the reduced contact between material and grinding balls, which are designed to crush large particles. Therefore, smaller particles move faster into the second chamber (6).

[0095] This closed circuit operation is meant to increase the mill’s efficiency, with the return grits being returned several times to the mill. In normal conditions, meaning the traditional process of cement milling, the fresh feed represents only 1 / 3 in weight of the total material entering the first chamber (4) of the mill whereas the other 2 / 3 in weight is represented by the return grits coming from the classifier (16). The amount of material that is returned to the mill for further grinding after it has already been classified (separated) by the separator is called the “circulation load”. The targeted or desired amount of raw material (feed) that is introduced into the milling system to achieve the optimal grinding performance and desired product quality is commonly designated as “Total Setpoint”.

[0096] What the inventors have found is that the mill’s throughput increases as the separator feed mass is lowered and that, when a classifier receives a finer (re-grinded) material, it becomes more efficient, increasing its throughput. The PSD is more uniform, with a higher number of finer particles. Nevertheless, it was also found that this advantage is not significantly improved by recycling back to the mill only the coarse particles sorted by the classifier as commonly done by the cement industry. This advantage is already very significant when a portion of the particles that exit the mill, independently of their particle size, are recycled back to the mill. According to the invention, a circulation load between 50% and 600% should be verified.

[0097] This goes against the common knowledge in grinding technology. In fact, in Innovations in Portland Cement Manufacturing. 2nd Edition (January 2011), from the Portland Cement Association, it is said in chapter 4.1 (Finish milling and grinding): “Therefore, higher circulating load provides lower mill exit fineness (leading to higher grinding efficiency) but higher classifier loading (leading to decrease in classifier efficiency, hence an increase in the returns of fines) and increase in the mill hold-up (leading to a decrease in the grinding efficiency, once above the optimum). Thus, there are opposing influences that result in an optimum circulating load.” (page 633). This goes against what the inventors have discovered and the teaching of the present invention.

[0098] For simplification purposes, and to better convey the invention, Figure 3 represents a simplified schematics of a conventional cement milling process, wherein (a) are the feed silos, (b) is mill’s inlet, (c) is the cement mill, (d) is the mill’s outlet, (e) is the stream of fine particles leaving the mill using the air stream from the mill (which can either go back to the mill or to the final product silo hence there is no final destination on stream (e)), (f) is a separation device, (g) is the classifier’s return outlet, which will join the mill’s fresh feed and together enter the mill through (b), (h) is the classifier’s finished product line and (i) is the dust collecting filter or final product silo.

[0099] Figure 4 shows a simplified schematics of a cement milling process according to the method of the invention, wherein (a) are the feed silos, (b) is mill’s inlet, (c) is the cement mill, (d) is the mill’s outlet, (e) is the stream of fine particles leaving the mill using the air stream from the mill (which can either go back to the mill or to the final product silo hence there is no final destination on stream (e)), (f) is a separation device, (g) is the classifier’s return outlet, which will join the mill’s fresh feed and together enter the mill through (b), (h) is the classifier’s finished product line, (i) is the dust collecting filter or final product silo, (SD) is a splitting device that splits stream (d) into 2 or more streams, (k) is the stream or streams of particles split by the splitting device (SD) that feed into the is a separation device (f), (j) is the stream or streams of particles split by the splitting device that return to the mill (c). Ftotal, FE, FS, FM, FF, FSf and FSc represent the total amount of particles respectively in the streams (b), (d), (k), (j), (e), (h) and (g).

[0100] In step c) of the method of the invention, the fine particles (FF) are directly discharged to the final product silo using the air stream from the mill (e).

[0101] In step d) of the method of the invention, the remaining mineral particles (FE) are discharged from the mill (c) through mill’s outlet (d), wherein FE=Ftotal-FF. This simple mass balance, wherein Ftotal=FE+FF ensures that all the particles that enter the mill (c) through stream / mill’s inlet (b) (Figure 4) exit the mill (c), either through stream / mill’s outlet (d) or through stream (e) (Figure 4).

[0102] In step e) of the method of the invention, a splitting device SD is used to split Online the discharged ground mineral particles (FE) after step d) in at least two fractions of particles (FM and FS). According to another embodiment of the invention, the splitter allows to split the discharged ground mineral particles in a third stream of particles that can be sent to another mill or to be post processed.

[0103] The splitting device SD can split stream (d) (Figure 4) in more than 2 streams, as long as the total amount of particles in the fraction that enter the mill equal the total amount of particles in the fractions that exit the mill. The particles in the other streams can be sent to another mill, to the product silo or can be stored for further processing.

[0104] Figure 5 shows a simplified schematics of a cement milling process according to the method of the invention, wherein the splitting device SD is splitting the stream (d) into 3 streams ((k), (j) and (I)). In this figure, (m) represents a second mill, a material silo or a dust collecting filter and FT represents the total amount of particles in stream (I). In this case, FE=FS+FM+FT.

[0105] The splitting device can be, for example, a rotary valve, a diverter valve, a splitter or distributor, like chutes or hoppers, or a slot in the conveyor belt. For example, when the splitting device is a slot in the conveyor belt, the flow of particles being diverted is regulated by changing the width of said slot or gap. Yet another example, when the splitting device is a rotary valve, its angle controls the flow of diverted particles.

[0106] According to the invention, the splitting device SD enables to set the ratio between FM and FS (FM / FS), with FM / FS comprised anywhere between 10 / 90 and 90 / 10. Without being bond to a specific example, the split FM / FS may be equal to 10 / 90, 20 / 80, 30 / 70, 40 / 60, 50 / 50, 60 / 40, 70 / 30, 80 / 20 or 90 / 10.

[0107] When the splitting device splits stream FE into 3 streams (FM, FS, and FT), the ratio FM / (FS+FT) is also comprised anywhere between 10 / 90 and 90 / 10. Without being bond to a specific example, the split FM / (FS+FT) may be equal to 10 / 90, 20 / 80, 30 / 70, 40 / 60, 50 / 50, 60 / 40, 70 / 30, 80 / 20 or 90 / 10.

[0108] After the splitting device, the fineness can be d(90) 20-30 pm less than with a normal process, or more in the case of ultrafine grinding.

[0109] According to an alternative embodiment, a portion (up to 5%) of the super fine particles in FE, with fineness below 45 pm, are diverted towards the dust collecting filter. They can then be incorporated in the final product, as explained in Figure 2 with the dust collecting filter represented by (10), and Figure 6. In this scenario, FE=FE’+FE”.

[0110] In step f) of the method of the invention, the particles in fraction FM return to the mill (c) for further re-grinding. This return is done via a return stream (j), a conveying system carrying the particles in fraction FM from the splitting device SD to the mill’s inlet (b), pneumatically or by belt conveying. Particles in fraction FM will join the fresh feed particles and the particles in fraction FSc that return to the mill (c) through stream / classifier’s return outlet (g). These three fractions of particles (FM, fresh feed particles and FSc) will together make the Ftotal fraction.

[0111] According to a particular embodiment of the invention, the circulation load is between 50% and 600%.

[0112] In step g) of the method of the invention, fraction FS is fed into a separation device (f) through stream (k) that will sort out the particles according to their size, wherein the sum of all particles FM+FS that exit the splitting device (SD) equals the fraction of particles FE that enter the splitting device (SD).

[0113] According to a particular embodiment of the method of the invention, the separation device is selected from a dynamic classifier, a static classifier, a vibrating classifier or an air classifier.

[0114] Dynamic classifiers utilize a rotating rotor with adjustable blades to create centrifugal force, effectively separating particles based on size and density. This type of classifier allows for precise control over particle size distribution, making it ideal for producing high- performance cements. In contrast, static classifiers rely on stationary vanes or blades to guide the air flow and separate particles through differences in settling velocity; they are simpler in design and require less maintenance but are less efficient than dynamic classifiers. Vibrating classifiers employ vibration to assist in the separation process, enhancing the movement of particles and preventing blockages, which is particularly useful for materials that tend to clump or have variable sizes. Air classifiers use air streams to classify particles based on their aerodynamic properties; the particles are suspended in the air flow, with finer particles carried upward to the classifier's outlet while coarser particles fall back for regrinding.

[0115] According to a preferable embodiment of the method of the invention, the separation device is an air classifier.

[0116] In step h) of the method of the invention, the mineral particles from fraction FS are sorted, in the separation device (f), into fine (FSf) and coarse (FSC) particles.

[0117] In step i) of the method of the invention, the fine mineral particles (FSf) are discharged from the separation device (f) into storage (i) (final product silo or dust collector filter) through stream (h).

[0118] In step j) of the method of the invention, the coarse mineral particles (FSC) are fed from the separation device (f) back into the mill (c) through stream / classifier’s return outlet (g) for further grinding.

[0119] According to a particular embodiment of the method of the invention, grinding aids may be added to the mill, such as ethylene glycol, diethylene glycol, triethanolamine (TEA), triisopropanolamine (TIPA), ethanolamines, polycarboxylate ethers, glycerol, lignosulfonates, calcium acetate, hydroxylamine hydrochloride, silicones and silicates, or a combination thereof.

[0120] According to a particular embodiment of the method of the invention, the mineral particles are cementitious materials, clinker or pozzolans.

[0121] In a traditional cement milling process, the returned particles are returned several times to the mill. As stated before, in normal conditions, the fresh feed represents only 1 / 3 in weight of the total material entering the mill whereas the other 2 / 3 in weight is represented by the return grits coming from the classifier.

[0122] With this new method, although the circulation load is high, particles are returned less times to the mill. Therefore, instead of impacting the throughput negatively as normally occurs in traditional milling process, in this case we can keep constant or even increase the fresh feed, increasing the throughput.

[0123] BRIEF DESCRIPTION OF THE FIGURES

[0124] Figure 1 represents an equipment for a ball mill grinding process.

[0125] Figure 2 represents a detailed description of a conventional milling process, with the mill here depicted as a double chamber ball mill.

[0126] Figure 3 represents a simplified schematic of a cement milling process.

[0127] Figure 4 represents a simplified schematic of a cement milling process, with the dashed lines representing the invention.

[0128] Figure 5 represents a simplified schematic of a cement milling process, with the splitting device SD splitting the stream (d) into 3 streams [(k), (j) and (I)].

[0129] Figure 6 represents a simplified schematic of a cement milling process, with a portion of the super fine particles in FE diverted towards the dust collecting filter.

[0130] EXAMPLES OF THE INVENTION

[0131] The following examples represents results obtained after applying the splitting device in the cement industrial plant with a two-chamber ball mill.

[0132] The splitting device consists of a gap on the air slide conveyor that deviates a limited amount of material to by-pass the separator. The flow of the material can be regulated by a rotary valve.

[0133] The particle size distribution was measured using a laser diffraction analyser and was tested for Discharged ground mineral particles (FE), Fine mineral particles (FSf), Coarse mineral particles (FSc) and on the final product (final cement). Specific surface was measured using air permeability method (Blaine - method).

[0134] The PSD measurements were taken once steady process has been reached (minimum 5h after applying the splitting device). In the examples, the split ratio always represents FM / FS.

[0135] EXAMPLE 1 Table 1 - Results for Example 1

[0136] Results in Table 1 represent results after applying different ratios of splitting fractions compared to the system without splitting device (“Reference”). The initial circulating load was 200%. After applying the splitting device 5 / 95, 20 / 80 and 30 / 70, the mill feed was increased, while fineness was kept at the same level (controlled with Blaine and PSD).

[0137] It was possible to achieve 30% higher production when splitter device ratio was 30 / 70 without compromising the fineness of the final product - compared to the reference there was even an increase in fineness, mostly in d(50) value.

[0138] EXAMPLE 2

[0139] Table 2 - Results for Example 2

[0140] Results in Table 2 represent another approach to applying the splitting device. The feed was kept at the same level, while the splitting ratio was increased to increase the fineness of the final product. The initial circulating load was 200%. Ratio 5 / 95 and 20 / 80 show improvement in fineness in comparison to the reference, but the final cement fineness lies in range of conventional grinding.

[0141] Split 50 / 50 and above show more improvement in amount of fines (lower d(10)), and overall fineness of the final product allows to classify it as fine grinding. EXAMPLE 3

[0142] Table 3 - Results for Example 3

[0143] Results in Table 3 represent the effect of different % circulating load in combination with different ratios of splitting device.

[0144] When circulating load is as low as 50%, as shown in the example, the effect on the final product fineness is negligible, but in the fraction of Coarse mineral particles (FSc) the effect on particle size is significant, which implies enhanced classification efficiency.

[0145] To show improvement when applying splitter on low circulating load requires applying higher ratio of splitting device. As shown in the example, ratio 50 / 50 with circulating load 50% effects in improvement reduction of particle sizes. The same ratio 50 / 50 when higher circulating load is applied, such as 200% as show in the example in Table 3, allows to reach high fineness of cement, aiming in the fine grinding range (d(90): 30 pm).

[0146] Higher ratio of splitting device combined with higher circulating load allows to reach ultrafine grinding or even mechanical activation effect - for example, split 90 / 10 with 200% or higher circulating load.

[0147] EXAMPLE 4

[0148] Table 4 - Results for example 4

[0149] Results in Table 4 represent the effect of higher % circulating load.

[0150] When circulating load is as high as 450% or above, it is possible to achieve the fine or even ultrafine grinding range with splitting device ratio of 30 / 70, as seen on Example 4.

Claims

CLAIMS1. A method to increase the throughput of the grinding or milling process of mineral particles, using a single and / or multi-chambers ball mill, the method comprising the steps of: a) Feeding minerals (Ftotal) into a mill; b) Grinding the minerals in the mill; c) First discharging fine particles (FF) directly to the final product silo using the air stream from the mill; d) Second discharging the remaining mineral particles (FE) from the mill, wherein FE equals Ftotal minus FF; e) Splitting online the discharged ground mineral particles (FE) after step d) in at least two fractions (FM and FS) using a splitting device (SD); f) Feeding fraction FM back to the mill for further re-grinding; and g) Feeding fraction FS into a separation device, wherein the sum of all particles of said at least two fractions (FM and FS) equals FE; h) Sorting, in the separation device, the mineral particles from fraction FS into fine (FSf) and coarse (FSC) particles; i) Discharging the fine mineral particles (FSf) from the separation device into storage; j) Feeding the coarse mineral particles (FSC) from the separation device back into the mill for further grinding, whereas the splitting of the fractions FM and FS is operated using a splitting device SD that enables to set the ratio between FM and FS.

2. A method according to claim 1 , wherein the splitter in step e) allows to split the discharged ground mineral particles in a third stream of particles (FT) that can be sent to another mill or to be post processed.

3. A method according to any of claims 1 or 2, wherein the ratio between FM and FS (FM / FS) is comprised between 10 / 90 and 90 / 10.

4. A method according to any of claims 1 to 3, wherein the ratio FM / (FS+FT) is comprised anywhere between 10 / 90 and 90 / 10.

5. A method according to any of claims 1 to 4, wherein the circulation load is between 50% and 600%.

6. A method according to any of claims 1 to 5, wherein the separation device is selected from a dynamic classifier, a static classifier, a vibrating classifier or an air classifier.

7. A method according to any of claims 1 to 6, wherein the separation device is an air classifier.

8. A method according to any of claims 1 to 7, wherein grinding aids are added to the mill, selected from ethylene glycol, diethylene glycol, triethanolamine (TEA), triisopropanolamine (TIPA), ethanolamines, polycarboxylate ethers, glycerol, lignosulfonates, calcium acetate, hydroxylamine hydrochloride, silicones and silicates, or a combination thereof.

9. A method according to any of claims 1 to 8, wherein the mineral particles are cementitious materials, clinker or pozzolans.

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