(262e) Fluoride Reuse in Aluminum Trifluoride Manufacture: Sustainability Criteria

Sustainable Development has been introduced in the chemical engineering objectives related to processes, plants and systems. It is a model of progress that links economic development, protection of the environment and social responsibility [1]. This concept aims to harmonize the economical, social and environmental dimensions of the development strategies and it is now a key feature of the policy making in the European Union (EU). Sustainable Development is related to: balanced and equitable economic development; high levels of employment, social cohesion and inclusiveness; and a high level of environmental protection and responsible use of natural resources.

The objective of the environmental policy in the EU consists of preventing, reducing and as far as possible eliminating pollution by giving priority to intervention at source and ensuring the sustainable management of natural resources, in compliance with the principles of pollution prevention [2]. The goal is an Integrated Prevention and Pollution Control Policy able to reduce the emissions in order to promote a Sustainable Development.

The design approach of pollution prevention consists in a hierarchy of different steps [3]: 1) minimize generation of pollution; 2) minimize materials and energy consumption. This hierarchy holds for preventing pollution during design or in fact any other engineering action. Every action should be focused on minimization of the generation of waste or the introduction of waste.

Until recently, environmental solutions to processing facilities occurred mainly in the form of end-of-pipe pollution control strategies. These solutions focus primarily on chemical, biological and physical treatment of waste streams leaving the plant, reducing the toxicity and volume of pollutants in industrial discharges. Although these pollution control strategies have often resulted in significantly reducing environmental consequences of processing facilities, they lacked cost-effectiveness and sustainability.

In this work it has been analyzed the AlF₃ production in a Spanish plant as basic inorganic product in terms of sustainability. The recovery of fluoride from industrial wastewaters as a product to be reused is presented as priority objective of the fluorine industry in order to contribute to the sustainable development. ALUMINUM FLUORIDE MANUFACTURE

Aluminum fluoride (AlF₃) is primarily used as a fluxing agent for the electrolysis of aluminum, but also in the glass industry and in the enamel industry for the production of white enamels.

The dry fluorspar process is the worldwide dominating process, counting for approximately 65% of total AlF₃ production. The main raw materials are fluorspar (CaF₂), aluminum hydroxide (Al(OH)₃) and sulphuric acid (H₂SO₄). The two main steps of the process are:

(1) the generation of gaseous HF from fluorspar and sulphuric acid

(2) the production of AlF₃ from gaseous HF and Al(OH)₃ (activated to Al₂O₃). Generation of gaseous HF

Dried fluorspar and sulphuric acid are preheated to 120-150ºC and fed into a rotary kiln reactor, the pre-conversion is usually 30-50%. The reaction is completed in a directly or indirectly heated kiln where the temperature of the reactants is raised to 200-300ºC at the outlet end of the kiln. The overall reaction can be described by the following equation:

CaF₂ + H₂SO₄ --> CaSO₄ + 2 HF                             (1)

Gypsum (synthetic anhydrite) is removed from the rotary kiln outlet end as a by- product and either transported to a landfill as a waste or reused as construction material. The anhydrite is then cooled down and traces of sulphuric acid are neutralized with lime before the product is ground to the required size for commercial purposes. Synthetic anhydrite from AlF₃ plants is used mainly for the construction of self-leveling floors, as an additive to cement production and in the fertilizer industry.

The effluent gas from the rotary kiln after separation/concentration contains 40-100% HF and it is passed through scrubbers to remove dust, elemental sulphur and the impurities before it is used in the AlF₃ manufacture step.

AlF₃ production

Al(OH)₃ is transformed to Al₂O₃ by heating it up to approximately 400ºC, and Al₂O₃ is the fed into a fluidized bed reactor where the reaction with gaseous HF takes place according to the following overall equation:

Al₂O₃ + 6 HF --> 2 AlF₃ + 3 H₂O                  (2)

This reaction takes place in a single or multi- fluidized bed reactor. Depending on the HF recovery system, a yield of 94-98% based on HF entering the aluminum fluoride reactor is achieved.

The flow sheet of aluminum fluoride production by the dry fluorspar process is given in Figure 1.

 

 

  Abatement techniques

Gaseous effluent from the production of AlF₃ are cleaned by passing the gas through one or several wet scrubbers for the removal of HF, sulphur compounds and dust before being emitted to the atmosphere. The emissions of HF are reduced to >99% in this type of treatment system and wastewater containing fluoride is produced in the gas cleaning operations.

 

 

 

Materials and Energy Consumption

Typical materials and energy consumption in the dry fluorspar process are summarized in Table 1.

A range of specific emissions to air from an AlF₃ plant using the dry fluorspar process is given in Table 2. 

In turn, a range of specific emissions to water from AlF₃ plant using the dry fluorspar process is shown in Table 3. 

 

And Table 4 shows the residual solid materials involved in the process

 

FLUORIDE REMOVAL FROM INDUSTRIAL WASTEWATERS

Fluoride wastewater is an effluent requiring neutralization due to the acidity and fluoride concentration control. Chemical precipitation

Several methods to remove fluoride from industrial wastewater have been described and applied. Precipitation is the most common treatment technology. Chemical precipitation is a physico-chemical process, comprising the addition of lime or hydrated lime to wastewater, causing precipitation of an insoluble product that is settled out by sedimentation. Anionic polymers may be used to assist solid-liquid separation.

 

This fluoride removal generates huge amounts of a water rich sludge, which has to be disposed off with increasing costs. The high water content (60-80%) and the low quality of the solid material (30-60% of CaF₂) prevent technical and economically the recovery of precipitated fluoride [5].

Figure 2 shows the material flow of the AlF₃ production, including the removal of fluoride from wastewater by chemical precipitation. Crystallization in a pellet reactor

Crystallization is closely related to precipitation. In comparison to precipitation, the precipitated is not formed by chemical reaction in the wastewater, but it is produced on the seed material, working in a fluidized bed process. During the operation, the grains in the fluidized bed reactor increase in diameter and move down to the reactor bottom [5]. Due to the composition and the low water content, reuse of the recovered calcium fluoride is possible as synthetic fluorite, which can be reused in the first step of the process avoiding the sludge formation and leading to the reduction of solid waste.

In addition, the recovery of fluoride by crystallization is an example of process intensification. The idea behind process intensification is the optimal integration of energy, materials, and processing tasks with the goal of minimizing amounts of energy and materials needed and size of equipment required to produce a given quantity of product per unit time [6].

 

In this sense, the main advantages of the crystallization in a fluidized bed reactor to recovery of fluoride from industrial wastewater are: compact and flexible unit, thus enabling modular set-up and tailor-made materials selection; no sludge production; water-free pellets with high purity which enables recycling or further usage of the fluoride content in other sectors; raw material recovery/recycling; nearly waste-free process. From Table 5 it is possible to deduce that the crystallization in a pellet reactor allows to recover 115 kg of synthetic calcium fluoride per tonne of aluminum trifluoride produced. In these conditions, it is possible to save near 8% of fluorspar as raw material allowing the reduction of sludge.

Figure 3 shows the material flow in AlF₃ production including removal of fluoride wastewater by crystallization in a fluidized bed reactor.    CONCLUSIONS

As a conclusion it has been shown that the crystallization process to obtain synthetic fluorite can be considered a sustainable technology due to the intensification in the materials benefit and its application in the fluorine industry may contribute to the reduction of waste materials and the optimization of raw materials consumption. 

The economical benefits have been also evaluated and they allow to justify the new crystallization process investment.   REFERENCES

1.      The idea of Sustainable Development. European Commission, Sustainable Development. January 2005. http://europa.eu.int/comm/sustainable.

2.      Decision No 1600/2002/EC of the European Parliament and of the Council of 22 July 2002 laying down the Sixth Community Environment Action Program. OJ No L242.10.9.2002.

3.      Mulholland, K.L., Sylvester, R.W. and Dyer J.A. (2000), Sustainability: Waste Minimization, Green Chemistry and Inherently Safer Processing. Environmental Progress 19, pp.260-268.

4.      Draft Reference Document on Best Available Techniques in the Large Volume Inorganic Chemicals, Ammonia, Acids and Fertilisers Industries (LVIC-AAF). European Commission, European IPPC Bureau. Draft March 2004.

5.      Aldaco, R. Luis, P and Irabien, A. (2005). Fluidized Bed Reactor for Fluoride Removal. Chemical Engineering Journal, 107, 1-3, pp. 113-117.

6.      Korevaar, G. (2004). Sustainable Chemical Process and Products.  PhD Thesis, Technische Universiteit Delft, The Netherlands.