In any discontinuous or batch pyro-metallurgic smelting process there is a requirement to close and open the taphole to tap the molten metal and slag out of the furnace. The closing of the taphole is done by pushing a taphole clay (THC) into the taphole until after the molten metal and slag has stopped running out.
The objectives for the THC are: plugging the taphole safely until the next opening cycle to stop any unplanned metal tapping, which can result in a catastrophic furnace runout; easy opening of the taphole on the beginning of the taping cycle; and to maintain and possibly improve the integrity of the refractory structure of the taphole. These objectives should be met at the lowest cost possible and with the least environmental and health risk impact.
The Challenges For Taphole Clay
There are various requirements and challenges for THC to meet the objectives. A dual-binder system is necessary to ensure strength in the THC as the temperature in the taphole varies considerably. Positive expansion is also needed, to ensure good adhesion and grip between the THC and the refractory. Other challenges include the elimination of toxic substances in the THC, and to prevent low temperature bonding of the THC to avoid mud-gun blockage. Finally, the THC itself needs to boast a reasonable shelf-life and, naturally, be the lowest price possible.
Solving A Historic Problem
For a long period of time conventional THCs consisted of anhydrous tar that acted as a binder. Although effective, tar has two main problems concerning the environment and health. The first is that it releases volatile organic compounds (VOCs) into the atmosphere. These VOCs play a significant role in the creation of tropospheric (ground level) ozone and fine particulates, this in turn can also contribute to photochemical smog. Ground-level ozone gas has been shown to create a considerable amount of problems, which can include chest pain, coughing, throat irritation and congestion. It can worsen bronchitis, emphysema, and asthma. Ground-level ozone can also reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue. The same can be said for photochemical smog, which not only reduces visibility on days when atmospheric conditions are suitable, but also can irritate eyes and affect the respiratory system.
The other main problem with using tar in THC is that it contains benzo [a] pyrene, which is a polycyclic aromatic hydrocarbon; this binds to DNA resulting in mutations and eventually cancer. It is therefore listed as a Group 1 (agent is carcinogenic to humans) carcinogen by IARC (International agency for research on cancer). Other research has also shown that benzo [a] pyrene exposure effects memory and learning in rats, as well as has an effect on the number of white blood cells, inhibiting some of them from differentiating into macrophages, the body’s first line of defence to fight infections.
Due to these problems some THCs recently used resin instead of tar to act as binder, claiming to be a safer, more environmentally friendly option. Most of the resin used, however, contains formaldehyde as an ingredient. When formaldehyde is present at levels exceeding 0.1ppm some individuals may experience effects such as watery eyes, burning sensations in the eye, nose and throat, coughing, wheezing, nausea and skin irritations. Some people are very sensitive to formaldehyde while others show no reaction at exposure levels.
Long-term effects to formaldehyde exposure is less known but a study in 1980 showed that exposure to formaldehyde could cause nasal cancer in rats. More recently, in 2011 the National Toxicology Programme in the USA named formaldehyde as a known human carcinogen.
So what if you could create a THC with no tar or resin, making it a truly environmentally friendly THC without sacrificing the operational performance?
Environmentally Friendly Taphole Clay
This is exactly what Refraline has done, with its commitment to the environment and health and safety of the workers who work day in and out around the furnaces of its customers.
THCs are required to close the tap-hole in a safe way after the taping of the furnace is complete and then retain the metal that is building up until the furnace is ready to tap for metal again. This requires that the tap-hole clay be easily pressed into the tap-hole opening by using a mud-gun. The heat in the furnace lining has to set-off the bonding system to ensure sufficient strength in the THC to hold the metal trapped behind like water behind a dam wall.
The vast temperature profile in the refractory lining requires a dual-binding system to fulfil this task, ensuring hardening of the THC at lower temperatures as well as at higher temperatures. The low-temperature binding system has to set the THC at around 300°C and has to be in place until the sintering of the material takes place at temperatures above 900°C.
With these non-negotiable properties of the THC, Refraline developed an innovative, non-toxic, low temperature binding system that replaces the use of tar and resin, which was used for decades in the smelting industry. After years of development a high-tech binder system was found that could fulfil all the requirements. The newly developed non-toxic THC not only delivers on all aspects of health and safety (all toxins currently emitted when heating up a traditional tap-hole clay are eliminated) but also ensures that the THC is not more expensive for the end user.
Any variation for different metals can be accommodated using the same new binding system and only modifying the refractory aggregate to suit any furnace environment.
To the astonishment of Refraline’s clients, industrial trials have now proven that its non-toxic THC eliminates all toxins emitted by a traditional THC and could fulfil all requirements to close a tap-hole safely under normal furnace operating conditions, without any extra cost.
With this competitive advantage Refraline has since invested in a new production line at its factory in Meadowdale, South Africa to be able to supply the smelting industry with this sought after THC.
Implementation Of THC
Any changes in a critical process requires careful consideration and management of change (MOC), to evaluate and benefits and risks associated with the change. Refraline is assisting its clients with this by means of finite element analysis (FEM) of the client-specific taphole. In this FEA modelling the temperature profile in the taphole is siumlated for two scenarios: closed taphole and open taphole during tapping. The modelling is done time elapsing from the changed condition until steady state. The FEA modelling shows what temperatures can be expected at different time intervals during the opening and closing of the taphole. This information is needed to carefully fine-tune the bonding system of the THC to suit the specific furnace environment so a customer specific THC can be developed to account for all customer requirements.
Refraline believes it has set a new industry standard with this engineered solution of a well-known problem.
Manfred Rösch is MD of Refraline