Spent foundry sand may be a liability or an asset depending on how it’s handled.
We live in a world of increasing scarcity. There is limited raw natural resources, financial resources are often not enough, and acquiring land for waste disposal is getting even more difficult and costly. Government ought to set policy directions aiming for efficient use of resources based on society, keeping in mind that the end goal is to give a spotless, sound and attractive living environment to residents for present and for the future. They should likewise take a look at how to support re-use of by-products of industrial manufacturing in other processes to make other products, such as spent or waste foundry sand. Manufacturing is the backbone of any modern economy and the foundry industry is a very crucial part of manufacturing. It provides critical inputs to most of the manufacturing sectors, with mining, automotive, and general engineering being the largest industries it supplies. Challenges faced by the industry include waste management regulations.
Foundry sand is a high-quality uniform sand that is used to make moulds and cores for ferrous and non-ferrous metal castings. The metal casting industry annually uses millions of tons of foundry sand for production. Over time, foundry sands physically degrade until they are no longer suitable for use in moulds. Consequently, tons of sand is discarded each year.
Foundry sand and spent foundry sand composition
Foundries use sand moulds to cast metal. Silica sand is the most common sand that is used, but chromite sand is used at some foundries. In order to maintain the shape of the mould, a binder of some sort must be added to the sand. There are two main types of binding systems:
• Green sands use clay as the binder. The sand base can be silica or chromite sand
• Chemically-bonded sands use a chemical binder and a catalyst to bond the sands. Typically the sands are made up of 1.4% binder of which 30% is catalyst i.e. 0.4% catalyst
There are six main chemically-bonded systems with different additives which are used at the majority of foundries, namely:
1. Alkaline phenolic
2. Furane
3. Acid cured phenolic
4. Pepset (Phenolic urethane gas cured)
5. Cold box (Phenolic urethane)
6. Sodium silicate
The material that has exceeded its usefulness for casting moulds remains the responsibility of the operators to dispose of properly. The obvious solution, landfilling it, carries costs, so the availability of other options (e.g. road foundations, soil enrichment agents) means more flexibility for the foundry holding the sand bag.
However, the discarded foundry sands have remarkably consistent composition and are generally considered a higher quality material than typical bank run or natural sands used in construction.
Recycling of foundry sand can save energy, reduce the need to mine virgin materials, and may reduce costs for both producers and end users. Use of foundry sand as a fine aggregate in construction applications offers project managers the ability to enhance green sustainable construction by reducing their carbon footprint.
Based on the raw materials, additives, and the casting process, and information obtained through literature review, the spent foundry sand to be disposed of may be expected to consist of the following:
• Silica or chromite sand – the sand becomes physically modified through repeat heating and mechanical abrasion to the point where it is no longer suitable for use in the foundry. However, the sand would be expected to remain chemically inert, and not be a source of COCs
• Additives – Various chemicals or other components are added to the sand. The maximum concentration of these compounds in spent foundry sand is based on the percentage of compounds added to the original sand. The initial additives to the foundry sand comprise <1.5% of the total mixture. In addition, the chemicals that are added to the sand are expected to be largely thermally decomposed during the casting process. Organic compounds formed during thermal decomposition of the additives can condense within the moulding sand, while some of the additives away from the sand-metal interface may not decompose
• Metals and metalloids – Foundries cast a variety of different metals, including steel, iron, aluminium, brass etc. Some of the metals (and impurities within the metals) may be transferred to the moulding sands during the casting process. Metals may also be present within the raw material sand
Environmental protection agencies encourage foundries and foundry sand recyclers to consult regulations to ensure planned uses are consistent with government and local beneficial use and waste-management programmes and that the chemical and physical properties of the sand meet applicable environmental limits, engineering performance criteria, and other requirements.
Advancing the environmentally sound, beneficial use of industrial materials, such as spent foundry sands, provides substantial opportunities for addressing climate change and air quality, enhancing local partnerships, reducing costs, and working toward a sustainable future.
The development of SMEs is a vehicle for addressing the triple crisis of poverty, unemployment and inequality has been at the centre of government policy. The New Growth Path (NGP) and the National Development Plan (NDP) both identify SMEs as the main drivers of economic growth and employment creation. Using spent foundry sand can reduce the cost of input material. The reduced input costs have a potential to trigger new SMMEs. In the brick manufacturing industry, radical economic transformation should entail the systematic promotion of an entrepreneurial culture and creating a supportive environment through financial and non-financial support initiatives to ensure the production of mass quality bricks, efficient production techniques, access to markets and creation of decent jobs. It is therefore recommended that the legislation be reviewed and spent foundry sand delisted as hazardous waste. Regular testing of the sand from the foundries is recommended.
The SAIF and AFSA application to Department of Environmental Affairs to exclude spent foundry sand as a hazardous waste product is based on recent studies and assessments done locally where a human health risk assessment approach was followed to assess the risk related to re-use of spent foundry sand. The recommendation is that the spent foundry sand rather be used for making of concrete bricks and for constructing a road sub-base.
The application was submitted at the end of July 2020.
