WASTE TREATMENT USING MOLTEN SALTS
  
The industrial applications of molten salts mainly cover metallurgy and in particular the extraction of  sodium, magnesium and aluminum. These are processes based on molten salts and are the most important industrially. Today this process, invented in 1886, has matured and is no longer very perfectible. 

Modern molten salts chemical technology was born in the USA in the 1960's  as part of the development of nuclear breeder reactors using molten salts as reaction medium either for fission or to extract plutonium and other fission products. Encouraged by these projects, large scale research was carried out in the field and led in the 1970's to a series of new processes outside the nuclear field (pyrolysis, gas combustion and scrubbing). Although none of them reached the industrial stage, the field of molten salts is very interesting as current technology makes them easy and cheap to use.

Applications in chemistry are the most wide-ranging: they particularly include catalysis, the treatment of heavy fuels,  gasification and liquefaction of coal. Other fields to develop range from pyrolysis and surface treatment combustion to refining processes and treatment of valuable waste. 

In 1997, the Rhone-Alpes Materials Agency carried out a study to evaluate the potential interest of molten salts technology in these applications and  particularly in material recycling and waste treatment. Further on a seminar was organized in February 1998 to present the technology during which exchange of views between laboratories and industrialists showed interesting applications perspectives. 

The Rhone-Alpes Materials Agency therefore decided to start a program to validate the most promising applications in collaboration with laboratories, interested industrialists as well as concerned public organizations.  		 

  MOLTEN SALTS, MATERIALS RECYCLING AND WASTE TREATMENT

 

Molten salts are exceptional reaction mediums that help transpose at high temperatures single chemical operations in water. Their productivity and selectivity mean they are particularly recommended in the implementation of reclamation and recycling processes. For this type of application, the technique puts materials or waste into contact with the bath of appropriately prepared molten salts brought to a given temperature. Toxic elements (chloride, fluorine, heavy metals) are separated from the others or fixed in a bath in the form of inert products. 

The processes are known for being clean and low energy consuming. They can help either recover cheap fuel (combustible gas, monomers) or purify a product.
The salts used in the bath (carbonates, sodium chloride, sodium, soda) are indeed in the environment are relatively inexpensive. The reduced size and the low fusion point of usable molten salts systems mean that in case of problem any leaks can be stopped by solidification. The main limit on using these processes rests on the need to avoid all contact of the bath with water. A molten salts industrial processing unit includes a continuous introduction system, the actual reactor and a system for recovering reclaimable products.

Details of possible uses

The variety of usable salt systems and the specific nature of each of the media means that it is possible to use molten salts in three quite distinct cases:

The separation of one or several constituents of waste. the salt reacts selectively on its constituents trapping them making it possible to recycle the reclaimable fraction or concentrate the harmful or undesirable fraction. 

The destruction of waste. Given the temperature and the reactivity of the medium, a pyrolysis can easily be carried out on the waste. 

Waste transformation. The salts' reactivity is then used to transform waste into reclaimed products. This is particularly the case for plastics treatment that leads to a de-polymerization with the recovery of the monomer.

Therefore, the molten salts technique may prove particularly interesting for treating products containing halogens or metals:

Processing plastic waste. Molten salts allow a de-polymerization of plastics and therefore a return to a monomer state. In the halogenated plastics (like PVC), we can supplement this action by trapping the harmful element (Chloride or fluorine).

Separation of metals/polymers playing on the selectivity of the molten salts medium with this material.

Treatment of paint residues. This waste contains a high proportion of metal pigments (titanium, chrome, cadmium or zinc oxide) mixed with an acrylic resin. Treatment in a molten salts bath will help separate the metal particles from their organic matrix and therefore lower the harm done by the waste.

Treatment of halogen products (Chloride, Fluorine, Bromine). The technique means that halogens from the waste can be put in the molten salt solution. The chloride may for example be trapped as NaCl in the bath. The applications aimed at cover the treatment of chloride solvents and distillation residues.  

De-metallization of used heavy oils and fuels. Full cutting oils, electric discharge machining oils or certain fuels that contain a large quantity of metal particles not retained by filtration systems. Treatment with molten salts helps recycle an organic product and the concentration of metals at the bottom of the bath. 

Processing the filtration resins. This means separating the resin from the heavy metals it contains to be able to re-use the product.

The applications that we have mentioned are specific treatment aiming at the elimination of metals and halogens. The reactivity of molten salts may however be used in other fields: smoke treatment, destruction of toxic organic products.

Other applications may be envisaged, particularly in the field of the recycling of tires and large size trucks. Indeed, these products are difficult to destroy with conventional mechanical means. Immersion in a bath of molten salts may allow recovery of the original monomer, the blacking and the metal structures.  

 

INDUSTRIAL DEVELOPMENT PROGRAM

 


The program of research envisaged by the Rhone-Alpes Materials Agency will be sub-divided into two main parts.

- A technical and economic feasibility study with the aim of validating the process in a laboratory. 

- The creation of an experimental pilot platform made available to industrialists so they can to large scale tests on their products and the validity of the process.  

 

The feasibility phase will mainly aim to acquire skills in the implementation and use of molten salts.

It will have three major sections:

- An extra bibliographical study aiming at greater knowledge of molten salts practice and comparing them to other waste treatment techniques. It would also be interesting to focus part of the research on the behavior of molten salts in the presence of organic elements.

- Feasibility tests that will aim particularly at defining the best adapted media and parameters (temperature, viscosity) for treating a type of waste. the study will lead to characterizations being made of the gaseous phase as well as calculations of the migration of products (loading baths in metal or halogens).

- An economic estimation that will give industrialists an idea of the investment necessary to operation an installation of this type. It would be particularly interesting to be able to calculate the thermal cost by the tonne of waste treatment.

 

The aim of the program's pilot phase is therefore to be able to offer interested companies the possibility of testing the validity of the process on their products. A pilot installation site is planned within a company that already has a certain amount of waste treatment equipment. We will emphasize the versatility of the system so that it allows work treating different types of waste, in several types of medium and in temperatures of up to 750°C. The Rhone-Alpes Materials Agency will monitor all the program and will check the suitability of the studies identified by industrialists. The Agency will be assisted in this by a steering committee  made up of representatives of public sponsoring organizations, industrialists and academics.


  
 THE PARTNERS  



A preliminary club of industrialists interested in the program has already been identified by the Agency. Several research laboratories will support the program.

Institut National Polytechnique de Grenoble (INPG) - Ecole Nationale Supérieure d'électrochimie et d'électrométallurgie (ENSEEG) - Université Claude Bernard Lyon I Laboratoire - Laboratoire de physico-chimie industrielle - Institut Universitaire des Systèmes Thermiques industriels de Marseille

 CONCLUSION  

Molten salts bring together the advantages of pyro-metallurgic processes (high productivity) and those of hydro-metallurgic processes (selectivity, cleanliness). 

Faced with new requirements for protecting the environment and the complexity of the materials to be recycled,  the molten salts seem to be a technique that is complementary to conventional processes that may be used in a certain number of fields where they are not entirely satisfactory.
 
The Rhône-Alpes Materials Agency