THE ROLE OF FLUORINE IN VITREOUS ENAMELS BENEFITS AND PROBLEMS
Malcolm Heyes
Escol Products Ltd., England

Introduction
The positive attributes of vitreous enamel as a surface coating for steel have been appreciated formany years. It is a hygienic, and in most applications, a chemically inert material providing uses in many different applications. Despite these positive features, there are problems and difficulties in many areas of the frit manufacturing process. These health and safety, and environmental issues have become more prominent in specific areas, for example:

As frit manufacturers it is our responsibility in all of these areas, not only to meet but hopefullyexceed ever-tightening legislation, but also to protect of our environment. This is a major undertaking.
Environmental awareness is a global issue and is currently at different stages of developmentdepending on the geographic location; but sooner or later it will affect everyone, and in particular those of us in the business of frit manufacture. Our environment is our responsibility.
This presentation considers the role of fluorine, as a useful, but not always essential component in frit manufacture. It provides many beneficial properties but at the same time demands careful control. Unless properly managed, fluorine in any industry can pose a potential risk in terms of surface, air and groundwater contamination.
The enamelling industry is not alone of course in the use of fluorine; in addition to glass and ceramics production, fluorine has important applications in many other areas, such as:

So how do we continue to develop our process, in order to ensure survival of enamel as a versatile coating, and at the same time deal with these issues?
This paper considers the benefits of fluorine use, the problems and possible solutions.

The role of fluorine in vitreous enamels
In frit making, fluorine has traditionally been used as a refining aid, providing a rapid fluxing action during smelting. It also gives some benefits to the enameller in that a small amount incorporated into the glass composition can help assist in obtaining a smoother, fused enamel surface enabling more consistent fused results when applied over difficult substrates.
It also provides other properties in relation to surface performance of the fused enamelled surface.
Obtaining good opacity, in semi opaque compositions, for example, is helped by the introduction of fluorine into the glass during smelting. It is a useful ingredient in that it can assist opacifiers such as zirconium, alumina, and antimony to be taken into solution effectively.
High gloss levels are more easily achieved with the presence of some fluorine in the composition.
Increased levels of fluorine, which is undesirable in for example, high chemical resistant or easy to clean enamels, affect acid resistance. This negative aspect can however be minimized providing fluorine is at a relatively low level, and other oxides, for example silica and titanium, are adjusted to compensate.
Due to the increased reactivity of ground coat enamels with the steel substrate in the presence of fluorine, fired adhesion is also assisted by even relatively small additions. Conversely, removal of fluorine from a given recipe will result in the glass losing fluidity, so this may need to be recovered by adding extra lithium, boron or soda, or a combination of all three.
Although there are certain benefits from the use of fluorine within many frit compositions, its presence in enamels is not always essential. In some cases it is present as a result of the use of more traditional formulating methods. Research and development has provided ways of avoiding fluorine use, even in advanced pickle free solutions. This provides therefore, an opportunity to tackle some of the environmental aspects.

Problems
Impact on the environment and legislation
During the smelting process fluorine containing raw materials release hydrogen fluorine gas, which combines immediately with moisture in the atmosphere to form Hydrofluoric acid, HF. Hydrofluoric acid falls to the ground as acid rain, as it is commonly known.
At present, it is a mandatory requirement on all frit manufacturers to emit zero levels of fluorine into the atmosphere. Under the current regulations, a concession allows a maximum emission level of 5.0 mg/m³. This is the current European limit set by the EEC, but the impact of fluorine emissions on the environment, is of course global issue, which will come under closer scrutiny in all areas eventually.
More local effects concern workroom air. The Occupational Safety and Health Administration in the UK has set limits 0.2 mg/m³ for fluorine, 2.0 mg/m³ for hydrogen fluoride, and 2.5 mg/m³ for fluorine in workroom air. This is to provide a safe working environment for workers during an 8-hour shift over a 40-hour week.
The average loss of fluorine during the enamelling process is around 15 % of that remaining in the frit following smelting. This does not however create a significant exposure. This argument is supported by the fact that there are few known health problems in evidence that can be linked to excessive fluorine inhalation or ingestion.

Physiological effects
The EPA has set a maximum fluoride level in drinking water of 4.0 mg/l so that some protection is offered against the development of cavities in young teeth. Conversely, excess ingestion of fluorides can cause fluorosis, which results in mineralisation of teeth and bones causing osteoarthritis and/or osteoporosis. This paradox still today is the subject of major debate in the medical profession. There is also some evidence that fluorine can also contribute towards Attention Deficit Syndrome.

Solutions
Control during frit manufacture
In order to gain earliest possible compliance with regulations on emission to the atmosphere, Escol deployed fluorine-scrubbing equipment.
This is still the ‘best available technique’, and has been in use since 1995, with a second unit installed in 1998 to address even tighter emission limits, improve productivity, and also allow higher volume production.
This method of extracting the fluorine from furnace waste gases is achieved by the injection of calcium hydroxide (lime) into the furnace exhaust stream. The level of treatment is determined of course by the smelting programme, as different recipes vary in their fluorine content. Care must be taken not to introduce too much unnecessary draft into the furnace, thereby allowing undesirable nitrates into the stack emissions.
The reaction of fluorine-rich vapour with lime is as follows:
Ca(OH)₂ + 2HF = CaF₂ + 2H₂O
Hydrogen fluoride combines with calcium hydroxide to form fluorspar (calcium fluoride), and water.
The calcium fluoride is then collected in a hopper, and then transported to a bagging facility by means of a screw feed. Until recently this by-product was 100 % disposed to landfill, but more recently a programme of work has begun to allow its reuse as a raw material.
Chimneystack emissions are constantly monitored by means of a probe located at a strategic point in the chimney. This information is fed to a PC based data-logger for troubleshooting and archive purposes. This treatment means that fluorine emissions remain typically between 1.0 -1.5 mg/m³, however, it is a EU requirement that manufacturers should reduce this emission year on year.

Monitoring the local environment
We must be certain that any emissions however small, are not affecting the local environment in any way and so a secondary testing of the land in the locality of the factory is carried out annually whereby samples of grass are collected. These samples are tested for fluorine levels by an independent authority. This is an obligatory test in order to comply with EPA regulations.

R+D towards fluorine free compositions
Although maximizing reuse of fluorspar has obvious economic advantages, there is still a high cost carried in the process. So an ideal, but not total solution is to move towards fluorine free smelts. A large number of both recent and older frit compositions are already F2 free, but a programme of work is needed to increase the number of frits so that environmental targets are achieved

Reuse of waste product
The “spent” lime is analysed for its calcium fluoride (and hence calcium hydroxide) content, and compositions are melted utilizing these proportions of calcium and fluorine. To date, this works shows promise, and we anticipate incorporating higher levels of “spent” lime as the technique develops

Impact on enamellers
Although the removal of fluorine from all glass compositions is an ideal scenario, every enameller will be naturally cautious and concerned about the impact that reduced, or fluorine free enamels might have on the workability of long standing formulations. It will be vital therefore that any proposed changes in formulation are appropriately tested as part of a programme of work agreed between Escol Products and the enameller.
We all have a responsibility to contribute towards the goal of environmental protection and sustainability. Ultimately the use of hazardous or poisonous volatile materials in our industry alone will not be tolerated. As far as fluorine in concerned, the development of reduced, or fluorine free compositions, is an essential step towards meeting the targets set concerning this global issue. This is a direction must all take.