35 3.1. Binder The CPB operation uses sulphate-resistant Portland binder, which meets ASTM C1157 standard. 3.2 Cerro Lindo tailings and existing backfill practice The actual backfill operation uses two paste plants to feed cemented paste to sublevel stopes via steel pipelines up to about 2 km long. Actual CPB production is approximately 250 ton/h, which is achieved with a continuous mixing and pumping process (Figure 1). Figure 1 – Cerro Lindo paste fill plant flowsheet Filtered tailings are delivered as a dense cake with solids contents around 90% by weight. X-ray fluorescence and mineralogical analysis indicate a material dominated by high sulfur contents linked to pyrite; quartz is the principal gangue mineral, with significant pyrite and minor barite, plagioclase, micas and chlorite (see below table 1 and Figure 2). Tailings particle size distribution (PSD) was measured using the laser diffraction method with a Mastersizer 3000 device, showing a fine material with more than 20% of particles smaller than 20 µm (see below Fig. 3). These characteristics are consistent with increased binder and water demand and with potential for sulphate-related durability issues if not managed. The pre-optimization backfill mix used the sulphate-resistant Portland cement as sole binder, with the following typical design values (see Table 2): • Solids content ≈ 74-75 %,
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