Introduction
Industrial process and mineral metallography is the niche field of metallographic analysis applied to mining concentrates, smelter products, refractory linings, and industrial ceramics. The samples are typically not engineered materials but byproducts or raw inputs in industrial chemistry, and the analysis questions are different from structural metallography: was the ore fully liberated during crushing? Did the smelter slag form correctly? Did the refractory lining wear evenly?
This guide covers two representative cases PACE Technologies handles: smelter slag (for metallurgical quality control) and mining concentrate samples (for separation efficiency analysis). The procedures here are smaller in scope than the engineering-material guides because the sample volumes are smaller and the analysis targets are coarser.
How this guide is organized
Two distinct procedures because the samples have very different physical character. Smelter slag is essentially a multi-phase glass containing oxide and sulfide inclusions; it prepares like a brittle ceramic with fine-grit sectioning and diamond polish. Mining concentrates are loose particulate aggregates of minerals that must be mounted intact and ground gently to avoid liberating locked mineral pairs.
Smelter Slag
Slag is the oxide and silicate byproduct that floats on molten metal during smelting and refining. Composition varies by process: ferrous slags contain Ca-Si-Al oxides plus entrained FeO and metal droplets; non-ferrous slags (Cu, Ni, Pb smelting) carry sulfide phases and entrained metal/matte droplets. Microstructural analysis verifies that the slag has the right phase content and that entrained metal is below acceptable thresholds.
Prep risks: Slag is fundamentally a multi-phase glass. Chipping during sectioning propagates conchoidal fracture through the bulk and ruins the analysis. Embedded metal droplets are much softer than the glassy matrix and produce hardness-mismatch relief during polishing. Sulfide phases can react with polishing chemicals.
Sectioning: Slag
- Blade: Diamond wafering blade, fine grit, low concentration. The fine grit is mandatory; medium-grit sectioning produces chipping that cannot be polished out.
- Wheel speed: 200-300 RPM.
- Feed rate: 2-5 mm/min.
- Cooling: Continuous water-based cutting fluid.
Mounting: Slag
- Castable epoxy or acrylic resin.
- Vacuum impregnation recommended for porous slag samples; some slags contain trapped gas pockets that emerge as voids during grinding.
Grinding: Slag
- 30 µm DIAMAT diamond on CERMESH metal mesh cloth, water, 5-10 lbs, 200/200 RPM, until plane. Semi-fixed diamond on CERMESH is the preferred plane-grinding approach for the glassy slag matrix.
Polishing: Slag
- 6 µm DIAMAT diamond on TEXPAN polishing pad with SIAMAT colloidal silica, 10 lbs, 200/200 RPM, 5 min.
- 1 µm DIAMAT diamond on GOLDPAD polishing pad with SIAMAT colloidal silica, 10 lbs, 200/200 RPM, 5 min.
- SIAMAT colloidal silica on TEXPAN polishing pad, 10 lbs, 200/200 RPM, 5 min.
The combination of mechanical diamond and chemical-mechanical colloidal silica throughout the polish suppresses the relief that hardness mismatch between glass matrix and entrained metal droplets would otherwise produce. The BLACKCHEM pad is an alternative to TEXPAN for the final SIAMAT step if more aggressive chemical action is needed.
Mining Concentrates & Mineral Samples
Mining concentrate samples are loose particulate aggregates of mineral phases produced by crushing and froth flotation. Typical examples: chalcopyrite (CuFeS2) and molybdenite (MoS2) concentrates from a copper-molybdenum porphyry deposit; galena (PbS) and sphalerite (ZnS) concentrates from a Pb-Zn deposit. The analysis target is mineral liberation: are the valuable minerals fully separated from each other and from gangue, or are they still locked together in composite particles that won't separate during further processing?
Prep risks: The opposite of slag prep. Mineral concentrates are loose particulate, so the prep must preserve the particle structure rather than reveal a bulk microstructure. Aggressive grinding tears apart locked mineral pairs that the mineralogist is specifically trying to count. Pull-out of the smaller particles is the dominant failure mode. SiC paper is used for initial grinding (not diamond) because diamond removes too much material too fast.
Sectioning: Mineral Concentrates
- Not required. Concentrate samples are loose particles; place a representative scoop directly into the mounting cup.
Mounting: Mineral Concentrates
- Castable epoxy or acrylic resin. Low-viscosity epoxy is essential so the resin flows around individual particles and locks them in place before curing.
- Vacuum impregnation recommended. Concentrate particles are typically porous or have surface-trapped air; without vacuum, the air pockets become voids during grinding and individual mineral grains pull out.
- Orient the sample so the analysis plane is perpendicular to the settling direction of the particles in the cup; this gives a representative cross-section of the size distribution.
Grinding: Mineral Concentrates
- 240 grit (P280) SiC paper, water, 5-10 lbs, 200/200 RPM, until plane (but stop as soon as the mineral grains are exposed). Use finer grit (320 grit or higher) for concentrates with abundant small particles.
- 360 grit (P400) SiC paper, water, 5-10 lbs, 200/200 RPM, 1-2 min.
- 600 grit (P1200) SiC paper, water, 5-10 lbs, 200/200 RPM, 1-2 min.
SiC paper is used instead of diamond because SiC removes material slowly enough to expose grains without dislodging them. Diamond cuts faster but pulls grains out; do not use it for mineral concentrates.
Polishing: Mineral Concentrates
- 6 µm DIAMAT diamond on TEXPAN polishing pad with DIALUBE Purple extender, 5-10 lbs, 200/200 RPM, 3-5 min.
- 1 µm DIAMAT diamond on GOLDPAD or ATLANTIS polishing pad, 5-10 lbs, 200/200 RPM, 3-5 min.
- 0.05 µm NANO-W alumina or SIAMAT colloidal silica on TEXPAN, 5-10 lbs, 200/200 RPM, 1 min only.
The final step is brief (1 min) because extended alumina or colloidal silica polish will preferentially attack sulfide phases (chalcopyrite, galena, sphalerite) and bias the grain liberation count. Inspect after 30 seconds and stop as soon as the surface is clean.
Imaging & Contrast
Industrial process and mineral samples are imaged as-polished. Different mineral phases have characteristic reflectance values that distinguish them under reflected-light brightfield microscopy without any etching.
- Reflected-light brightfield: The standard tool for ore mineralogy. Sulfides like pyrite, chalcopyrite, and galena have distinctive yellow, brassy, and silver-gray reflectance values respectively; oxide gangue phases (quartz, feldspar) are dark gray to white.
- Polarized light (reflected, with rotation stage): Anisotropic minerals show pleochroism under crossed polars with sample rotation. Used for distinguishing minerals that have similar reflectance (e.g., chalcocite vs. covellite).
- SEM with EDS: Standard for complex multi-mineral analysis. Reflected-light imaging identifies most common minerals; EDS resolves the ambiguous cases.
- Automated mineralogy systems (MLA, QEMSCAN): For high-volume mineral concentrate analysis. These platforms use EDS-based phase identification on every pixel of an SEM image to give complete liberation data.
Troubleshooting
Chipping on slag after sectioning
Cause: Medium-grit wafering blade used instead of fine. Damage propagates into the bulk.
Fix: Re-section from the bulk with fine-grit blade at 2-5 mm/min. Damage from sectioning is not recoverable.
Small mineral particles pulling out during grinding
Cause: Diamond grinding instead of SiC paper, or SiC paper too coarse for the particle size distribution.
Fix: Switch to SiC paper. Use 320 grit or finer initial paper for concentrates with abundant fines. Reduce force to the lower end of the range.
Sulfide minerals tarnished or attacked after final polish
Cause: Final colloidal silica or alumina step ran too long; the chemical-mechanical action preferentially attacks sulfide phases.
Fix: Limit the final step to 1 min or less. Inspect at 30 seconds. Switch from SIAMAT to NANO-W alumina if sulfide attack is a recurring problem.
Entrained metal droplets producing relief in slag
Cause: Soft metal phase polishes faster than the glassy slag matrix.
Fix: Extend the SIAMAT colloidal silica final step. The CMP action flattens the relief.
Concentrate sample shows voids around every particle
Cause: Air pockets around particles weren't infiltrated by mounting resin. Either skipped vacuum impregnation or resin viscosity too high.
Fix: Re-mount the sample. Use low-viscosity castable epoxy. Apply vacuum at 25-100 mbar for 1-5 min before releasing to atmospheric pressure to drive resin into pores.
Additional Reading
- Zipperian, D.C. Metallographic Handbook. PACE Technologies, Tucson, AZ. House reference for industrial process and mineral preparation.
- Craig, J.R. and Vaughan, D.J. Ore Microscopy and Ore Petrography, 2nd ed. Wiley. The standard reference for ore mineral identification by reflected light.
- Picot, P. and Johan, Z. Atlas of Ore Minerals. BRGM / Elsevier. Comprehensive ore mineral atlas with reflected-light photomicrographs.
- Petruk, W. Applied Mineralogy in the Mining Industry. Elsevier. Applied reference covering ore mineralogy, comminution, and mineral processing analysis.
- Verein Deutscher Eisenhüttenleute (VDEh). Slag Atlas, 2nd ed. Verlag Stahleisen. The definitive phase-diagram reference for ferrous and non-ferrous slags.
- Schumann, R. Metallurgical Engineering, Volume 1: Engineering Principles. Addison-Wesley. Classic reference on smelting and slag chemistry relevant to interpreting microstructure.
- ASTM E1245. Automatic Image Analysis (particle size distribution and phase fraction measurement on mineral concentrates).
- ASTM E112. Standard Test Methods for Determining Average Grain Size (the standard reference for grain measurement; absorbs the largest-grain guidance previously in the now-withdrawn E930).
Explore More Procedures
For broader application-specific guides, see Failure Analysis and Heat Treatment Verification.