Influence of manganese on vanadium precipitation and V2O5 purity based on different roasting methods of vanadium slag

Abstract

Manganese is a major impurity in acidic vanadium-bearing leaching solutions, but its effects on vanadium precipitation via hydrolysis and acidic ammonium salts remain unclear. In this study, vanadium-bearing leachates with varying manganese concentrations (VL-cMn) were prepared through calcium, a calcium–manganese composite, and manganese-based roasting of vanadium slag (VS) to investigate the influence of manganese on vanadium precipitation behavior during hydrolysis precipitation (HP) and ammonium salt precipitation (AP), as well as the microscopic characteristics and purity of the resulting V2O5 products. The results showed that increasing the pH mitigated the negative effects of Mn on the V precipitation rate during HP. However, as the manganese concentration increased from 5.69 to 15.38 g/L, the V precipitation rate gradually declined at higher temperatures and longer reaction times. The precipitates exhibited increased microstructural density, which might had contributed to the formation of Mn-bearing phases. Additionally, the average grain size of V2O5 was reduced and the particles were increasingly agglomerated, leading to a 2.55% decrease in product purity. For AP, as manganese concentration increased, raising the pH counteracted the negative impact of Mn on the V precipitation rate and reduced the required amount of ammonium sulfate. Moreover, Mn was unevenly adsorbed on the surface of the precipitates. Although V2O5 grains gradually shrank and became denser, there was no significant effect on the final product purity, which remained above 99.3%. In conclusion, roasting with added manganese salts influenced the hydrolysis of vanadium but had no significant effect on acidic ammonium salt precipitation.

1. Introduction

Vanadium is often referred to as the "vitamin of modern industry" owing to its distinctive and significant physical and chemical properties, which make it vital for applications in the steel, aerospace, and battery industries. The primary vanadium product, vanadium pentoxide (V2O5), is widely used as a raw material for producing energy storage materials, alloys, and photosensitive compounds. Approximately 88% of the global annual vanadium production originates from vanadium titanomagnetite (VTM). In China, the prevailing method for vanadium extraction from VTM is indirect, where vanadium is enriched in vanadium slag (VS) through blast furnace ironmaking–vanadium converter oxidation. Consequently, the extraction of vanadium from VS has consistently been the focus of research in the field of vanadium production.

As roasting is the most cost-effective pretreatment method, oxidation roasting is usually performed before the leaching of VS. The purpose of roasting is to decompose and oxidize vanadium–iron spinel in VS at high temperatures and finally convert V3+ into pentavalent vanadate. A number of roasting methods have been developed to aid the leaching of VS. Calcium salt brings significant advantages compared with conventional sodium vanadium extraction, for example, lower additives and wastewater treatment costs and easier tailing recovery. In addition, acid-soluble manganese vanadate has been identified in the calcified roasting of VS. Inspired by this, new technologies for manganese and calcium–manganese composite roasting have been proposed, in which manganese vanadate is generated as the target product and the acidic leaching solution contains a large amount of Mn2+. The introduction of manganese salt not only enabled effective vanadium extraction but also recovered most of the additives. Therefore, clean and efficient pretreatment with calcium and manganese is more suitable for the resource utilization of VS.

Precipitation is indispensable for obtaining vanadium-containing products from acid leaching solutions. The earliest application of hydrolysis precipitation (HP) in industrial production was the addition of inorganic acid to adjust the pH, and the vanadium-containing liquid was gradually hydrolyzed to produce hydrated vanadium pentoxide with increasing acidity. HP is simple and inexpensive but suffers from lower product purity. As a result, an acidic precipitation process using ammonium salt as precipitant was proposed, after which the V2O5 purity obtained by calcining the precipitated ammonium polyvanadate is as high as 99%, but the ammonia–nitrogen wastewater is difficult to treat. Owing to the advantages of both methods, many scholars have adopted them for preparing V2O5 products. According to previous studies, the V precipitation efficiency and purity of the V2O5 product are closely related to the types and concentrations of impurities in the vanadium-containing solutions.