An innovative process for dealkalization of red mud using leachate from Mn-containing waste

https://doi.org/10.1016/j.jece.2022.107222Get rights and content

Highlights

  • An innovative dealkalization process on red mud of using Mn-containing leachate is proposed.

  • Mn-containing leachate can effectively remove Na (80%) and K (73%) from red mud.

  • The loss amounts of Si, Al and Fe from red mud are slight avoiding affecting subsequent utilization.

  • The dealkalized red mud products have low solubility of heavy metals.

Abstract

Red mud (RM) is an alkaline industrial solid waste discharged from Bayer process, and its high alkalinity is considered as a serious influence on environment and application. Dealkalization is prerequisite and basis for the complete utilization of RM. Mn-containing leachate is a weakly acidic wastewater generated during the storage of manganese sulfate residue. In the current study, Mn-containing leachate was used to investigate the multistage dealkalization behavior of RM. The results showed that Mn-containing leachate effectively lowered pH levels from 11.82 to 7.61 and the leaching efficiency of Na and K reached 80% and 73%, respectively, under the conditions of five-stage leaching with reaction temperature of 95 °C, reaction time of 2.0 h, and a liquid-solid ratio of 10 mL/g. X-ray fluorescence spectrometer analysis results suggested that the content of sodium oxide was reduced from 4.97% to 0.78% while the contents of SiO2, Al2O3 and Fe2O3 were essentially unchanged. X-ray photoelectron spectrometer and X-ray diffraction analysis revealed that hydroxycancrinite in RM was dissolved accompanying sodium released and Mn(II) appeared in the newly-generated form of charmatite in the dealkalized RM. The final dealkalized RM product was demonstrated to be with low soluble heavy metals from its water leaching test. This research provides a sustainable reference of disposal waste with waste for the dealkalization of RM, and the findings of this work demonstrate a new mechanism of replacement Na with Mn in RM dealkalization process.

Introduction

Red mud (RM), also known as bauxite residue, is an alkaline solid waste by-product derived in the extraction process of alumina from bauxite ore. With increasing demands for alumina, the global reserves of RM have reached 4.6 billion tons in 2018, and it is estimated that an annual increase was approximately 200 million tons [1]. The treatment and disposal of RM has been posing a huge challenge for alumina plants and the alumina industry, and the improper storage in bauxite residue disposal areas (BRDAs) can easily cause significant problems leading to environmental threats. In the downwind area of BRDAs, soil alkalinity is induced and heavy metals from RM can be transferred with the dust resulting in high concentrations distribution in the surrounding soil [2]. There are also several reports on contaminations/accidents issues caused by the discharge of high pH residue leachate [3], [4], [5]. Many efforts have been made to find cost-efficient and sustainable applications including construction materials, production of ceramics, environmental adsorbents [6], [7], [8], [9], and secondary source for metals recovery [10]. However, in addition to the small quantity of consumption, the high alkalinity of RM is an important and intractable obstacle which restricts its bulk application in the existing fields of soil aggregates and building materials [11], [12], [13], [14].

The high alkali contents and high pH values of RM are ubiquitous because bauxite is digested in a hot NaOH solution [11], [15]. Due to the high alkaline nature, it is possible to use RM for preparing alkali-activated materials or geopolymer [8]. On the other hand, in most cases for RM applications, neutralization to lower the initial pH and conversion of alkaline characteristics are necessary [16], [17]. Utilizing as building materials is one of the main ways to consume RM in large quantities. For applications in building materials, in addition to reducing the pH of the RM, the sodium content must be less than 1% due to low strength [13], [18] and inadequate durability caused by alkali efflorescence phenomenon [19]. Therefore, dealkalization is a prerequisite and basis for the complete utilization of RM.

In general, the alkaline substances of RM can be mainly classified into soluble alkali and sparingly soluble structural alkali [1], [20]. In recent studies, the main dealkalization methods for RM were summarized as follows: acid leaching, acid gas sequestration, salt precipitation or ion replacement, and waste by-product leaching [20], [21]. Acid leaching mainly involves using mineral acids and organic acids [13], [17], [22], and acid leaching process also focuses on the secondary recovery of valuable metals [23]. However, strong acid leaching will also cause dissolution of silicon, aluminum and iron in RM, which tend to form colloids leading to difficulties for subsequent solid-liquid separation [20]. Furthermore, the yields of dealkalized RM are reduced [24], and the dealkalized products are acidic and are adverse to follow-up applications [20]. Carbon dioxide and other industrial exhausts have been validated to be of significance to RM neutralization with consumption of acid gases [12]. The method is sustainable due to disposal waste with waste, and it is included in the category of acid neutralization in a recent review publication [20]. Seawater/brine neutralization and gypsum remediation are based on the process of salt precipitation or ion replacement. Specifically, the mechanism of seawater and brine neutralization is the formation of hydrotalcite through the reaction of Ca2+ and Mg2+ precipitated alkaline anions, thereby reducing the pH value [25]. However, these methods need high liquid-solid ratio and are not available to industries in non-coastal areas. Gypsum is a slightly soluble calcium salt, and blending gypsum with RM can continuously release calcium ions to precipitate the alkaline anions through calcium and sodium replacement [26], [27]. Owing to gypsum’s slow dissolution rate and limitation of in situ application for RM [28], it is difficult to widely apply gypsum remediation or neutralization [29]. Consequently, dealkalization or neutralization treatment of RM should be economical and effective.

Manganese-bearing ores are extracted using sulfuric acid to obtain manganese sulfate product. Manganese sulfate residue (MSR) is discharged as a kind of industrial waste during the process of filtration and separation from the sulfuric acid leaching pulp [30]. The stockpile area of MSR would generate Mn-containing leachates. It is difficult to dispose of the Mn-containing leachates because impurities inside are relatively high and the low concentration of manganese is not expected to be recycled by mixing with the high-concentration manganese sulfate solution in the production line. In the current study, RM was leached using the mentioned Mn-containing leachate to reduce its sodium and potassium to meet the application requirements. The dealkalization efficiency of using Mn-containing leachate was compared with of using a prepared MnSO4 solution, and then the main experimental factors using Mn-containing leachate, including leaching temperature, leaching time and liquid-solid ratio, were investigated. In addition, the reaction mechanism referring to sodium releasing and manganese solidifying was discussed. The findings of this study provide a theoretical support for dealkalization treatment of RM using manganese ions and make the concept of disposal waste with waste be feasible in practical applications.

Section snippets

Materials and leachate preparation

The RM sample used was fresh and collected from the conveyor belt discharging to the disposal areas in an alumina refinery in Qingzhen, Guizhou Province of China. The main chemical and phase compositions of the RM were determined, and as reported elsewhere [31] the main chemical components in RM were Al2O3 (21.54%), Fe2O3 (18.43%), CaO (15.20%), Na2O (4.97%), K2O (1.04%) and SiO2 (16.69%).

Manganese sulfate residue was newly discharged in stockpiling site in a manganese sulfate production

Leaching efficiency of the Mn-containing leachate and MnSO4 solution

As has been widely recognized, different RM samples have a wide range of variation in the main chemical compositions, and are with complex mineral compositions [11], [23], [31]. According to the XRD analysis (Fig. S1), the main mineral phases of the original RM in this study were hydroxycancrinite, katoite, hematite, anatase, chamosite and calcite. Alkali existed as hydroxycancrinite in RM was sparingly soluble and difficult to be removed [16]. Each RM sample was leached for 5 stages using

Conclusions

The current work presented a sustainable new technology to dealkalize RM, which was based on using leachate from Mn-containing waste. When the dealkalization process was under the condition of leaching temperature of 95 °C, reaction time of 2.0 h, five-stage leaching, liquid to solid ratio of 10 mL/g, the Na and K leaching efficiencies reached 80% and 73%, respectively, with pH being reduced from 11.82 to 7.61. The content of sodium oxide in dealkalized RM was reduced from 4.97% to 0.78%.

CRediT authorship contribution statement

Zehai Li: Investigation, Resources, Data curation, Writing – original draft. Hannian Gu: Writing – review & editing, Conceptualization, Methodology, Supervision, Funding acquisition. Bing Hong: Supervision. Ning Wang: Conceptualization, Supervision. Mengjun Chen: Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work was financially supported by the National Key Research and Development Program of China (2018YFC1903500), the National Natural Science Foundation of China (U1812402), and The Youth Innovation Promotion Association CAS (No. 2021400).

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