Study on synergistic leaching of potassium and phosphorus from potassium feldspar and solid waste phosphogypsum via coupling reactions

Abstract

To achieve the resource utilization of solid waste phosphogypsum (PG) and tackle the problem of utilizing potassium feldspar (PF), a coupled synergistic process between PG and PF is proposed in this paper. The study investigates the features of P and F in PG, and explores the decomposition of PF using hydrofluoric acid (HF) in the sulfuric acid system for K leaching and leaching of P and F in PG. The impact factors such as sulfuric acid concentration, reaction temperature, reaction time, material ratio (PG/PF), liquid–solid ratio, PF particle size, and PF calcination temperature on the leaching of P and K is systematically investigated in this paper. The results show that under optimal conditions, the leaching rate of K and P reach more than 93% and 96%, respectively. Kinetics study using shrinking core model (SCM) indicates two significant stages with internal diffusion predominantly controlling the leaching of K. The apparent activation energies of these two stages are 11.92 kJ·mol⁻¹ and 11.55 kJ·mol⁻¹, respectively.

1. Introduction

Phosphogypsum (PG), with main component as CaSO4·2H2O, is a solid by-product generated from wet phosphoric acid production process. The annual stockpiling is estimated to be exceeding 80 million tonnes in China [1]. The current status qua of bulk utilization of PG mainly relies on the preparation of building materials, geotechnical building materials, and chemical raw materials etc [2–8]. However, due to the presence of hazardous impurities such as P, F, and other organic matter (such as 2-methoxyethyl acetate) in PG, these substances will not only greatly impede the utilization of PG in the forementioned approaches, but also leads to stockpiling in colossal quantity along Yangzi River [9]. More importantly, those harmful and hazardous substances impose adverse environmental impact and jeopardize the health & wellbeing of local residences. Taking the F as an example, it is estimated to be approximately 400000 tonnes being discharged annually (with considering over 0.5% in PG on mass basis). The ionic F might evaporate if it encounters at relative high temperature. In addition, the leachate of PG, which is acidic, could potentially pollute aquatic system and cause acute, chronic diseases if it is not properly handled [10].

Albeit the potassium feldspar (PF) is a K rich resource [11], its chemical stable Al–Si–O tetrahedral ionic bond structure significantly prevents the leaching of K cations out of the matrix of PF using strong inorganic acids (i.e., sulfuric acid, hydrochloric acid, etc.) [12]. Although hydrofluoric acid (HF) can destroy the structure of PF, its notorious pricy cost and underlining environmental impacts makes it much less alluring for K leaching on large-scale PF processing.

In order to tackle the problem of utilization of industrial solid waste-PG and K-rich natural resources of PF, a process that considering intrinsic chemical features both of PG and PF rich in K is proposed in this work. By successfully combining PG (including F element) with PF (rich in K element) together with the aid of sulfuric acid, the proposed synergic reactions are found to achieve K leaching and elementary recovery (F in form of HF could be recycled in the process for multiple times). To depict some more appealing features of this proposed process, Table 1 summarizes the existing widely deployed PF and PG utilization methods. Through this proposed process, the removal of P and F from PG, and leaching of K from PF could be ideally achieved simultaneously, which perfectly aligns with circular economy and the philosophy of energy cascade and integrated material utilizations. The reports of using synergic reactions for simultaneous purification of PG and decomposition of PF, to the best of our knowledge, has not been reported before.