Efficient Milling Technique

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The Brickworks challenge suggests jet milling, bead milling or ball milling for the milling of their various materials. Even though AVEKA has extensive commercial and R&D business with jet milling and bead milling, AVEKA thinks that bead milling or jet milling for large volume do not offer the economics desired. AVEKA believes that this industry would welcome additive changes with minimal equipment modifications through ball milling of Brickworks’ materials. Grinding aids for these large volume products will help in both grinding efficiency and in lower energy consumption. AVEKA has extensive experience with both commercial and R&D ball milling. AVEKA has been commercially ball milling ceramic materials at about 1 ton/day/mill for many years. Some of the ceramics that AVEKA has milled are Aluminum Titanate and Cordierite (Moh’s values of ~7). These materials are very similar to the pozzolans listed by Brickworks. AVEKA knows in some cases (as does the industry) that carefully used and selected grinding aids can greatly help size-reduction kinetics and particle size control to sub 20 microns. AVEKA has found that adding a small amount of a liquid grinding aid (about 1 – 20 ml per 100 kg of material) can significantly decrease the dry milling time. AVEKA proposes the R&D study of different grinding aids (GA’s) in the ball milling of Brickworks’ various pozzolans. GA’s have been used in milling for many decades but rarely have GA’s been changed during the milling process itself. Each of the pozzolans that Brickworks has listed have different surface chemistries and grain boundaries as well as different Moh’s hardness or particles in the same structure or conglomerate. As such, unique GA’s may be necessary for different materials at different times in the milling process. GA’s affect the surface chemistry of the milled particles as well as the cracking properties of the milled particles. The pre-milled pozzolans are weathered or calcined and are in the range of 300 µm to 10 mm in size and as such have low surface area. As materials are milled, fresh surface sites are exposed, and the surface area increases. The surface chemistry of the milled pozzolans can be significantly different than the initial starting material. The surface chemistry may change further as the particles are further milled. The temperature increases as the materials are milled and the surface chemistry can change as the temperature changes. AVEKA has found that different additives can change the particle morphology. AVEKA thinks that unique GA’s used in combination at different times in the milling process will decrease milling time as well as providing ultrafine particles. Various grinding aids (GA’s) have been used in ball milling such as ethylene glycol, stearic acid, magnesium stearate, boric acid etc. The GA’s reduce surface energy of the solid surface. In the following Tables, pozzolans from Brickworks’ list have been highlighted with an arrow. This recent article describes the use of grinding aids: Liquid grinding aids constitute another important additive class, especially in the field of dry fine grinding of inorganic materials. The additive molecules reduce the surface energy of the solid surface by adsorbing on the particle surface and prevent direct contact between the particles, which leads to a lower tendency of agglomeration and an improved powder flow behavior. Besides improving the final powder properties, the major aims of applying these additives are therefore to (a) increase the production capacities, (b) reduce the energy consumption or (c) enable the production of even finer particles. These additives are mainly applied for dry fine grinding in mills, such as tumbling ball mills or stirred media mills, and vertical roller mills. [Opposing Effects of Additives in Dry Milling and Tableting of Organic Particles; Lina Miethke, Paul Prziwara, Jan Henrik Finke, and Sandra Breitung-Faes; Pharmaceutics 2021, 13, 1434]. This recent review article describes the use of organic and inorganic grinding aids: The term chemical additives or grinding aids (GAs) refers to any substance which results in increased grinding efficiency and reduction in power consumption when added to the mill charge (amounts not exceeding 0.25 wt.% of the feed) during grinding. The use of grinding aids to increase mill throughput is quite common in the cement industry. In mineral beneficiation, wet grinding is much preferred compared to dry grinding, but the growing scarcity of portable water poses a threat to mining activities, especially in arid regions. The use of GAs improves material flowability, which presents an opportunity for the application of dry grinding. This ultimately reduces the environmental impacts such as CO2 emissions due to the energy intensive nature of grinding. GAs range from organic (e.g. polyols, alcohols, esters, amines) to inorganic (e.g. calcium oxide, sodium silicate, sodium carbonate, sodium chloride) chemicals. Despite the empirical evidence of the benefits for GAs in the cement industry, there is no agreed mechanism on their effects. However, suggested mechanisms are mainly based on two principles: (1) The chemical-physical effect on the individual particle such as surface energy reduction (2) The effect on the particle arrangement and material flow properties. [A Critical Review on the Mechanisms of Chemical Additives used in Grinding and Their Effects on the Downstream Processes; V.Chipakwe, P.Semsari, T.Karlkvist, J.Rosenkranz, S.Chehreh Chelgani; J. Mater Res Technol. 2020; 9(4), 8148-8162] The authors of the above review show tables of the various organic GA’s used The authors of the above review showed tables of the various inorganic GA’s used The authors of the above review showed effect of GA’s on energy consumption Brickworks is asking for an ultrafine milled powder (<5 µm) of a variety of materials. Understanding the milling of each of the materials during the milling process is critical. The GA’s chemistry with the surface of the materials to be milled is important. As materials are milled, fresh surface sites are exposed, and their surface chemistry may be different than the initial starting material. • What type of GA should be used and when should it be added to the milling process? • Should different GA’s be added at different times during the milling process? Brickworks listed a variety of pozzolans; let us look at two of these pozzolans: 1. kaolin; and 2. sandstone. 1. Kaolin is a layered material that is made up of silica (SiO2) tetrahedra bonded to alumina (Al2O3) octahedra; these layers are then weakly hydrogen bonded to the next layer [see following picture]. Upon milling the first breakage is between the weakly hydrogen bonded area between the Al2O3-SiO2 layers which gives Kaolin a Moh’s value of ~2.5. However, upon further milling of the kaolin, the milling is of harder materials (Moh’s ~7-8). Also, the grinding will expose surfaces that are: silica-like [SiO2], alumina-like [Al2O3], and edges which are silico-alumina-like [SiO2-Al2O3]. Each of these surfaces has a different acid-base characteristic depending upon the temperature that they have seen. Surface Brønsted Acids are those that can dissociate H+ (hydrogen ions): M-OH  M-O- + H+. Surface Lewis Acids are coordinatively unsaturated sites that can accept a lone pair of electrons from a Lewis Base such as ammonia: OM + :NH3  OM:NH3. The different surfaces of the milled kaolin will have different amounts and strengths of Brønsted and Lewis acids. Basic material GA’s such as amines and alkanol amines can interact with either or both Lewis Acids and Brønsted Acids on the surface of particle undergoing milling: Similarly, various acid GA’s can react with the surface basic groups. The acid or base GA’s change the surface potential of the material being ground which changes the attraction/repulsion of particles aiding in the milling. 2. Sandstone is a sedimentary rock composed of grains of sand [Quartz – SiO2, Feldspar – KAlSi3O8] of various sizes held together by weaker cements. Sandstone usually breaks along the soft grain boundaries. Upon milling of sandstone the first breakage is with the weaker cement of the grain boundaries. Further milling requires the milling of the hard sand [quartz-Moh’s 7; or feldspar-Moh’s 6] particles. The milling of the spherical quartz grains causes the shape to become very sharp conchoidal pieces as shown we have observed under SEM. However, no matter how much the quartz is ground, the fine particles retain their very sharp conchoidal shape. The conchoidal shape can be seen in the following SEM [Scanning Electron Microscope] images below. We have samples and SEM images of in-process milling of quartz with a final product size < 5 µm. which is a grand challenge goal of this challenge. Grinding aids can interact strongly with the cracks in materials formed during the milling process and thus aid in further crack formation and milling. There is not one GA that works for all materials. AVEKA thinks that different GA’s will significantly help milling of the various Brickworks’ materials. As discussed for the milling of Quartz above to the desired < 5 µm size, it is important that the changes of the milled material during the milling process are analyzed. AVEKA [www.aveka.com] has R&D facilities that can test a variety of ball milling additives such as those listed in the above tables as well as other inorganic and organic compounds with Brickworks’ starting materials. Samples can be taken at various times during the milling process and analyzed. Particle systems such as milled powders are often complex and usually require several methods for analysis. AVEKA has a variety of analytical characterization tools that can be used to study how the various materials are milling: PSD [particle size distribution], shape and imaging [Camsizer, SEM (scanning electron microscope), EDS (elemental composition)], Thermal analyses [DSC (differential scanning calorimetry), TGA (thermogravimetric analysis), moisture content (Karl Fischer), Zeta Potential, Powder Flow Rheometer, and BET Surface Area. In addition to in-house analytical techniques, AVEKA can collaborate with the analytical characterization facilities at University of Minnesota for more in-depth characterization of the GA’s interactions with the pozzolans.