The production of amorphous mesoporous silica nanoparticles can be achieved using sodium silicate (Na2SiO3) solutions prepared from South African coal fly ash waste. The first part of this study compared two processes for the preparation of Na2SiO3 solutions. The first process, hereafter called sequential acid-alkaline leaching (SAAL), is a two-stage process, which involves (i) a H2SO4 leaching step for the preferential extraction of reactive aluminium over silicon, followed by (ii) the preferential extraction of silicon over aluminium from the resulting residues using NaOH. The second process is a direct alkaline leaching (DAL) process, which consists of a single-stage elemental extraction from ash using NaOH, i.e. without the preceding acid leaching step used in SAAL. The two processes generated Na2SiO3 solutions with identical pH (11.8), similar silicon (10.2–10.3 g/L), iron (ca. 200 mg/L) and potassium (ca. 800 mg/L) content, and low calcium concentrations (≤ 29 mg/L). However, the inclusion of the acid leaching step in the SAAL process yielded a Na2SiO3 solution with significantly lower aluminium content (166 mg/L vs. 1158 mg/L). The Na2SiO3 solutions obtained from the SAAL and DAL processes were then used as silica precursors to synthesise silica nanoparticles via a sol–gel method using polyethylene glycol (PEG) as surfactant and sulphuric acid as catalyst. All samples of synthesised silica nanoparticles were characterised by a high level of purity (up to 99.3 wt % SiO2). The insight gained is now being used to improve existing processes for the production of high-grade ultra-pure silica nanoparticles (i.e. ≥ 99.9 wt % SiO2) for catalyst support applications.