Number-sensitive particle size measurements for monitoring flocculation responses to different grinding conditions

Abstract

In gravity thickening, higher levels of fine particles within the feed slurry typically lead to reduced flocculation performance relative to that achieved at the optimal solids concentration and flocculant dosage. Grinding is applied prior to separation processes (e.g. flotation, leaching) to liberate valuable material, and the higher grinding required for finely disseminated minerals substantially increases the fines fraction. To quantify the potential impact on flocculation efficiency of excessive fines formation within feeds to gravity thickeners, slurries of one ore type with seven different particle size distributions (PSDs) were produced by a combination of grinding and blending different size fractions. Sizings by volume- or mass-sensitive methods for these broad PSDs are dominated by coarser particles, despite their contribution to particle numbers being very low. Chord length distributions acquired in situ by focused beam reflectance measurement (FBRM) give real-time indications of size at high solids concentrations, and the raw (unweighted) counts are sensitive to particle number. The results of experiments with the different feed PSDs showed flocculation was strongly affected by fines and thereby particle number. FBRM therefore offers considerable scope for monitoring feed properties in terms of potential flocculation performance.Many flocculation studies focus on a single solids concentration, but useful comparisons for different PSD feed slurries can only be made by testing across multiple concentrations and flocculant dosages, with settling data then converted to solids settling flux. Flux values varied by an order of magnitude at a fixed dosage across the PSD extremes. While tripling the dosage significantly increased fluxes for slurries containing high levels of fines at their optimum solids concentration (~100 kg m−3), much higher fluxes were attained with coarser PSDs at both higher concentrations and lower dosages. When flocculating slurries of the seven PSDs at a fixed dosage, the solids settling flux increased as FBRM counts <20 µm prior to flocculation declined; scope for using FBRM in feed-forward control is discussed.