Green template-free synthesis and characterization of mesoporous alumina as a high value-added product in aluminum black dross recycling strategy

Highlights

• This work studies template-free synthesis of mesoporous alumina from Al black dross.

• The as-synthesized mesoporous alumina had a specific surface area of 123–162 m²/g.

• The specific pore volume of the as-synthesized mesoporous alumina was 0.37 cm³/g.

• The as-synthesized mesoporous alumina possessed more than 98% purity.

Abstract

Aluminum black dross (ABD) is a solid waste of aluminum smelting process that causes serious environmental and public health problems. In addition, it imposes significant disposal costs to the aluminum industry. In this study, synthesis and characterization of mesoporous alumina (MA) from the ABD using a novel recycling procedure consisting of leaching at low temperature (85 °C) and atmospheric pressure, co-precipitation, separation through selective dissolution, re-precipitation, aging and finally calcination at 600 °C for 2 h. No hard or soft template was used during the formation of the gel. The as-synthesized MA had a specific surface area, total pore volume and average pore diameter of 123–162 m²/g, 0.37 cm³/g and 10.6 nm, respectively which were obtained by nitrogen sorption analysis. The as-synthesized MA possessed more than 98% purity. Its XRD pattern exhibited the characteristic peaks of gamma, eta, theta, and delta phase alumina. FESEM and TEM analyses revealed nanoscale morphology of the as-synthesized MA; having crystal size of ∼11 nm. The TGA analysis showed that the as-synthesized MA is thermally stable and exhibits only ∼5 wt% mass loss for heating from 30 to 800 °C. The characterization of solid residue at the end of the proposed process proved that it contains mainly silica phase which can alternatively be reused as filling agent or aggregate in concrete materials applications. It is quite clear that the process of production of MA from the ABD is feasible and practical and provides a novel mitigation to the environmental problems of the ABD.

Introduction

Mesoporous alumina (MA) has received considerable attention from the industrial and academic communities due to its great applicability in the preparation of catalyst/catalyst support, adsorption of various pollutants from wastewater (e.g. fluoride, dyes, heavy metals, organic compounds, etc.) [[1], [2], [3], [4], [5]], chemical and biochemical separation, and sensors [6,7]. This great deal of interest is due to special characteristics of the MA such as porosity, high surface area and chemical activity [[8], [9], [10], [11]].

The MA is prevalently fabricated through one of the following three pathways: 1) hard templating method (e.g. using carbon template) [12,13]; 2) solvent-free synthesis without the use of structure directing agents [14]; and (3) soft templating process with the contribution of ionic and nonionic surfactant [15,16], organic molecules [9,17], and ionic liquids [18]. The synthesis of the MA has also been reported by a combustion synthesis without template [19].

At present, there are many salts utilized as sources of aluminum for the synthesis of alumina, such as Al(NO₃)₃·9H₂O [20], AlCl₃ and NaAlO₂ [21], aluminum isopropoxide [22], aluminum ammonium carbonate hydroxide [23], and so on. These aluminum sources are expensive, making industrialization of alumina production is very challenging. Recently, the synthesis of alumina from industrial solid waste as secondary sources of aluminum has attracted the attention of researchers. For the synthesis, the researchers have proposed some processes for production of alumina from solid wastes such as the acid-base leaching [24,25], limestone sintering method [26], and lime-soda sinter method [27]. In addition to gaining economic benefits from material production, retrieving/recycling the wastes is a desirable solution for both industries and the environment to overcome disposal costs and environmental pollution [28].

Aluminum black dross (ABD) is the major aluminum production waste. Unfortunately, reliable statistics are not available on the utilization rate of the ABD, but as a rough estimation, the global ABD generation is more than one million tons per annum, of which around 95% is dumped in the vicinity of factories. In other words, this means a very low recycling rate [29]. The ABD is mostly discarded in landfills and thus imposes a severe menace to the environment [[29], [30], [31]]. The ABD is a compound inorganic waste consisting majorly of nonmetallic materials (oxides and salts) which is formed during the Al melting [32,33]. If an appropriate approach could be found to use and manage the waste, it will provide a three-fold advantage to both environment and industries. Firstly, the volume of the waste and consequently the corresponding disposal costs could be partly diminished. Secondly a cleaner environment can be achieved, and thirdly if possible, production and development of advanced materials from the waste at reasonable cost can lead to the increased economic benefit as well as the conservation of natural resources.

The aim of this work is to prepare MA with a relatively narrow pore size distribution using ABD as a cheap, alumina-rich inorganic material without using a template for the first time. The production process here includes a low-temperature and atmospheric pressure extraction process. The residual solids from the extraction process do not inflict environmental contamination and can be stacked at a much lower risk or alternatively reused as filling agent or aggregate in concrete applications. All these together evince the eco-friendliness of the proposed process within the framework of the principles of green chemistry [34].