In practice, metallic abrasion during the mixing of battery active masses can be reduced significantly, but it cannot be ruled out entirely through a single measure. A realistic approach is a holistic purity concept that minimises contamination at the source, separates critical particles as early as possible, and monitors the process analytically. Blanket ppb limit values for all cathode masses are not robust, however, because permissible residual contents, critical elements, particle sizes and measurement methods depend on the cell chemistry, the cell manufacturer's specification, the process stage and the specific quality agreement. For cathode active materials, the particularly relevant metallic impurities chromium, iron, copper, zinc and lead are frequently listed in relevant quality standards and analytical applications with target values below 1 mg/kg, i.e. below 1 ppm.

Metallic foreign particles are critical for lithium-ion cells because they can cause local defects, increased self-discharge, unwanted side reactions and, in the worst case, internal short circuits. Individual large or sharp-edged metal particles are particularly problematic, as they can damage separators or increase local current densities. The technical assessment must therefore not be limited to the total chemical metal content in ppm. It must also take into account the number, size, shape, magnetic property and spatial distribution of metallic particles. One industrial technical source classifies metallic particles above 5 µm as critical; other process chains apply different, product-specific size and acceptance limits.

The causes of metallic contamination occur along the entire process chain. They can originate from the raw materials, arise from the wear of milling, classifying, mixing and conveying components, or be introduced through the environment, maintenance activities and human intervention. In mixing and milling in particular, direct product contact with metallic tools, vessel walls or wearing sealing and conveying components can generate metallic particles. A recent review paper explicitly names the wear of metallic components in mixing and milling processes as a source of contamination and recommends, among other things, protective coatings for mixing and milling equipment, magnetic separators or screens, and strict operational cleanliness procedures.

The first and most effective measure is therefore an abrasion-minimised design of all product-contact components. Direct metal-on-metal contact in the product area should be avoided. Mixing tools, vessel wall, discharge elements, conveying bends, flaps, sealing areas and transfer points must be designed so that no grinding, striking or repeated impact of hard particles occurs at critical edges. Depending on the material system, technical ceramics, suitable coatings or high-performance polymers can be useful on selected contact surfaces. However, the specific choice of material must be checked against chemical compatibility, the abrasiveness of the active mass, electrostatic properties, cleanability, thermal load and the possible particle release of the respective lining material. A non-metallic material is not automatically suitable if it itself leads to abrasion, delamination or unwanted organic contamination.

A second important lever is process control. Low but sufficiently effective tool and conveying speeds reduce impact energy and therefore erosive wear. Sharp deflections, tight bends, uncontrolled drop heights, dead zones and repeated recirculation should be avoided. In pneumatic conveying, it is not only air velocity that matters but also solids loading, pipe routing, bend radius, particle hardness, pressure level and start-stop operation. When mixing, the rotational speed must be chosen so that the required homogeneity is achieved with the lowest possible mechanical stress. Unnecessarily long or overly intensive mixing can increase abrasion without further improving product quality.

Magnetic separation is a useful additional protective stage, but it is no substitute for a low-abrasion design. It can capture magnetisable foreign particles such as iron or certain ferritic steel fractions, but it does not reliably act against non-magnetic metals such as aluminium, copper or zinc. Demanding process chains therefore combine several measures: magnetic separators at suitable transfer points, screening or filter stages for coarse particles, controlled raw material release, separation ahead of critical process steps, and a suitable design of material transport. Non-magnetic particles must be controlled primarily by avoiding their entry, suitable filtration or screening, and analytical monitoring. According to the technical literature, non-magnetic foreign particles can be introduced into slurries especially during mixing, transport and storage, and are harder to remove.

An effective concept also requires a systematic maintenance and cleanliness strategy. Wear-critical parts should be monitored via operating hours, torque, vibration, differential pressure, product analytics and visual inspections. Not only the mixer but also charging, dosing, screening, conveying lines, filters, valves, maintenance tools and the ambient air belong in the contamination assessment. Preventive replacement of worn components is significantly more effective than subsequently cleaning already contaminated material. Because metallic particles can also enter the system through open interventions, maintenance or the production environment, closed transfers, defined cleaning zones, tool management, suitable protective clothing and controlled material pathways are important elements of the concept.

Verification should combine several analytical levels. ICP-OES and ICP-MS capture total elemental contents and can quantify metals in the ppm to trace range; ICP-MS offers particularly low detection limits. Both methods are used for analysing critical elements in cathode materials, with standards often specifying ICP-OES and citing limit values below 1 mg/kg for relevant contaminants such as Cr, Fe, Cu, Zn and Pb. Magnetic separation tests, microscopic particle analysis and SEM/EDX are useful in addition, in order to identify individual particles in terms of size, morphology and chemical composition. This particle analysis is indispensable, particularly for assessing process-related abrasion, because a low total metal value can mask individual safety-critical coarse particles.

A robust practical strategy begins with a contamination risk analysis. First, the critical elements and particle sizes are derived from the cell manufacturer's specification. All potential sources of contamination along the process chain are then mapped and assessed in terms of probability, quantity introduced and detectability. On this basis follow the choice of materials and construction, the definition of permissible process windows, the positioning of magnets and screens, and a test plan with raw material, intermediate product and final product analytics. It must then be demonstrated with representative active material that the plant meets the agreed limit values even after a defined operating period, after cleaning and after maintenance.