Preparation of lightweight ceramsite from solid waste lithium slag and fly ash

Abstract

Lithium slag and fly ash are two types of solid wastes generated in large quantities in current industrial activities, which are often treated by storage methods because they contain harmful substances that cannot be used directly. According to the properties of the two solid wastes, this paper provided a novel eco-friendly utilization method of preparing them into small balls and roasting them at high temperature to produce lightweight ceramsite, and the properties and preparation mechanism of ceramsite were investigated. The results of the optimum conditions were as follows: the ratio of lithium slag: fly ash: kaolinite: albite, 40: 40: 10: 10; drying temperature 105 °C for 1 h; preheating temperature 500 °C for 15 min; sintering temperature 1240 °C for 20 min. Under these conditions, the apparent and bulk densities of ceramsite were 0.86 and 0.46 kg/m3, respectively, with a compressive strength of 1.76 MPa and a water absorption of 0.96%. Microstructural and crystalline phase analyses revealed that at the appropriate roasting temperature, the liquid phase on the surface of ceramsite and the generated gas acted to swell and form uniformly sized pores inside, and anorthite and mullite crystalline phases were formed. Thus, the prepared ceramsite was characterized by light weight, high strength, and low toxic leaching. This research filled the gap in the application of solid lithium slag in construction materials and innovatively combined lithium slag with fly ash to produce a new type of lightweight ceramsite. The use of solid waste to produce lightweight ceramsite provided a feasible way for harmless utilization and is of great significance to truly solve the environmental and reuse problems of solid waste.
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Introduction

In the process of manufacturing and construction, mankind has generated a large amount of industrial solid wastes, such as lithium slag and fly ash, which cannot be directly used and are disposed in large quantities. The storage and disposal of these solid wastes is one of the greatest environmental challenges facing the world today [1], [2]. With the development of society and the further depletion of natural resources, how to recycle solid waste in an environmentally friendly way is an important issue [3], [4].

With the widespread use of lithium batteries in electronic devices in recent years, the production capacity of lithium has also increased significantly [5]. Currently, lithium extraction from spodumene still occupies a dominant position in the production of lithium salts [6], [7]. The mature industrial process for the production of lithium carbonate from spodumene is the sulfuric acid production process, and the lithium slag is a solid waste product generated simultaneously [8], [9]. Statistically, in the production of lithium carbonate, the ratio of lithium carbonate and discharged lithium slag in lithium carbonate production is 1 to 9 [10]. For example, China produces about 80 million tons of lithium slag solids annually, which are difficult to recycle due to their low activity and high level of water absorption. The current common methods of lithium slag treatment are landfilling and dam construction, which pose high environmental safety risks. The study of the comprehensive utilization of lithium slag is of great significance for environmental protection and the healthy development of new energy industry [11], [12], [13], [14], [15].

Fly ash is a fine solid particulate matter collected from the flue gas produced by the coal combustion process, mainly from the production and supply of electricity, heat and other industries using coal-fired facilities, and is one of the major industrial solid wastes in the world [16], [17]. The amount of fly ash produced is huge, only China's annual production is more than 600 million tons, while the national average utilization level in recent years is at about 75%, and a large amount of fly ash is stockpiled every year [18], [19]. The stockpiled fly ash not only occupies a lot of land resources but also brings a series of environmental problems and many ecological and environmental safety hazards. Selecting suitable utilization technology and treatment method to efficiently convert fly ash into resources is an important research content.

Due to the large production of lithium slag and fly ash, it is a good choice to use them as raw materials for the construction industry. They have high contents of silica and alumina and some other metal oxides, which are suitable for the production of lightweight ceramsite [20], [21]. Lightweight ceramsite is a kind of lightweight aggregate, which is mainly used in the fields of building floor layer, heat preservation and insulation bedding layer, roof heat preservation and insulation bedding layer, waterproof and sound insulation bedding layer of the floor surface, etc [21], [22], [23]. It has excellent properties such as lightweight, good thermal conservation and insulation, sound insulation and noise reduction, and excellent frost resistance and durability. There have been many successful attempts by researchers to use solid waste as a raw material for the preparation of ceramsite. From the point of view of raw materials, the solid wastes that can be used to produce ceramsite include red mud [24], fly ash [25], [26], coal gangue [21], [27], incinerated municipal solid waste ash [28], sewage sludge [29], [30], and so on. From the point of view of the performance of the produced ceramsite, they are mainly divided into two kinds: high-strength ceramsite and low-density ceramsite.

Researchers in the past have used fly ash more frequently in the preparation of ceramsite, but no studies were found on the preparation of ceramsite with lithium slag [31], [32], [33]. Researchers in the past have used fly ash more frequently in the preparation of ceramsite, but no studies were found on the preparation of ceramsite with lithium slag. Lithium slag and fly ash are both high in silicon and aluminum, and when used together, fly ash compensates for the lack of foaming ability of lithium slag and stimulates its reactivity, which is a very compatible and creative choice. In this paper, the feasibility of using lithium slag and fly ash to produce ceramsite was investigated, and the effects of the ratio of lithium slag and fly ash and the roasting temperature regime were studied in terms of compressive strength, density, water absorption and leaching toxicity. This work will provide a practical method for the environmentally friendly reuse of lithium slag and fly ash, and will help to fill the gap in the application of solid lithium slag in construction ceramsite materials. It will enable the industrial solid waste to achieve economic benefits through green recycling, and promote the development of the whole industry.
Section snippets
Raw materials

Lithium slag, fly ash, kaolin and albite were used in the preparation of ceramsite. Lithium slag and fly ash were sourced from Pujiang and Meishan in Sichuan Province, China. Albite was taken from Tianjin Letai Chemical Co., LTD, China, and kaolin was purchased from Donggang Kaolin Development Company, Liaoning Province, China. The XRF analyses of the raw materials were listed in Table 1. The XRD patterns of lithium slag and fly ash were presented in Fig. 1. It should be noted that the carbon
Influence of raw material composition on ceramsite behavior

In general, the Riley phase diagram is used as a guide for the preparation of ceramsite with a raw material chemical composition of 40%-79% SiO2, 10%-25% Al2O3 and 12%-26% melt composition (e.g. MgO or CaO) [36]. The low content of SiO2 and Al2O3, which are the main components in the preparation of ceramsite, will lead to a low viscosity of the generated liquid phase and thus the generated gas will easily overflow and affect the foaming, while a high content of them will lead to a high
Conclusions

In this paper, the research on the preparation of lithium slag and fly ash as the main raw material has achieved satisfactory results. The results showed that the optimum conditions for the FL-based ceramsite were as follows: the ratio of lithium slag: fly ash: kaolinite: albite, 40: 40: 10: 10; drying temperature, 105 °C for 1 h; preheating temperature, 500 °C for 15 min; sintering temperature, 1240 °C for 20 min. Under these conditions, the newly formed anorthite and mullite crystalline
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.
Acknowledgements

The authors acknowledge the support of the National High Technology Research and Development Program of China (2022YFC2904601) and the National Natural Science Foundation of China (No. 52274284).