Field performance evaluation of comminution and gravity processing circuits for artisanal saprolite gold ores

Artisanal and Small-scale Gold Mining (ASGM) operations increasingly adopt mechanized comminution, assuming it universally improves gold recovery. This case study challenges that generalized approach through a comparative field performance evaluation of two gravity recovery circuits processing saprolite ore in Suriname: a traditional direct sluicing system (IST) and a mechanized hammer mill-centrifuge system (HS). Operational parameters and fractional gold deportment were evaluated using a reconstituted head grade methodology to mitigate the fundamental sampling error inherent to high-nugget deposits. Feed characterization revealed that approximately 80% of the gold is in the coarse fractions (> 100 μm), indicating natural amenability to simple gravity recovery. Results demonstrate both circuits severely underperformed due to operational misapplication. The IST circuit operated with extreme slurry viscosity (> 30% solids), causing hydrodynamic carry-over. Conversely, the HS circuit suffered from mechanical overloading and extreme hydraulic dilution (11–17% solids). Furthermore, deportment data suggests the hammer mill failed to achieve effective liberation, discarding ~ 70% of unrecovered gold in mid-size fractions (+ 250–1000 μm) and potentially flattening malleable nuggets. These combined inefficiencies project an estimated annual unrealized value of US$6.9 million (IST) and US$3.8 million (HS) under 2019 pricing. The findings strongly indicate that for gold in the coarse fraction of a saprolitic ore, the routine application of hammer mills may represent a metallurgical mismatch. Implementing strict process control over pulp density and feed rate, alongside proactive on-site ore characterization, are more critical drivers of operational profitability than deploying complex comminution technology.