SBE retains 25-40% of the oil so the mass of SBE disposed of is likely to be at least 25% higher than the bleaching earth input mass. This would suggest that around 3,600 – 4,000 tonnes of SBE is produced in Oman each year.

Currently the SBE from edible oil refiners in Oman is sent to landfill. SBE is classified as a hazardous waste in the EU as the residual oil in the clay is prone to auto ignition if left exposed to air for more than 48 hours. This is also likely to be a problem in Oman due to the higher ambient temperatures.

To address this problem, a significant amount of scientific research has been conducted into possible waste valorisation strategies for SBE that address the safety concerns but also maximise the value of the material. Malaysia is a hotspot for this type of research due to the huge quantities of palm oil refining conducted there. In the following section we highlight some of the most promising approaches to SBE valorisation from Malaysia and around the world.

As the residual oil has a very high free fatty acids and peroxide value, it can only be used for non-food applications, such as feed stock for biodiesel or bio-lubricants. For instance, Neste Oil has been using oil recovered from SBE as a raw material in its NEXBTL renewable diesel base since 2013^[148].

In the EU there are also efforts in progress to recover oil from spent bleaching earth. Within the port of Rotterdam, there are a number of edible oil refiners operating. Working with the Port Authority, they are developing an SBE reprocessing facility. Key figures from the project, initiated in 2018, include:

• 50,000 tonnes of bleaching earth used annually in the port of Rotterdam
• 15,000 tonnes of usable oil trapped in the SBE, of which 13,000 tonnes can be recovered
• €6.5 million estimated value of the recovered oil
• 12 jobs created for operating the plant
• 6-year payback period for the investment

Whilst the scale of this plant is considerably larger than that required in Oman, it is useful to note that, according to these figures, the recovered oil is worth €500 (OMR 212) per tonne in the EU market. The end use of the recovered oil is not described in detail, other than it will enter ‘secondary stocks’. Currently, it does not appear that a contractor has been selected to build or operate the plant.

In summary, it appears that there are now commercially available technologies for recovery of oil from SBE that can offer reasonable payback periods. It is recommended that further discussions be held with Mecpro to understand the typical characteristics of the recovered oil and potential applications as if there is no local customers for the industrial grade recovered oil then the financial feasibility of the application is likely to be limited.

One strategy is to reactivate the de-oiled SBE for reuse in the edible oil filtration application. In tests with soybean oil, Bachmann et al^[149] found that thermal reactivation of de-oiled SBE at 600ºC for 30 minutes was sufficient to provide 96-99% of the bleaching performance of the virgin bleaching earth (with or without oil recovery using n-hexane). The authors note that further testing is required to evaluate the quality of oil produced using the regenerated bleaching earth and to understand the number of regeneration cycles that can be completed with a batch of SBE before bleaching performance deteriorates.

In tests with an unspecified edible oil, Meziti and Boukerroui^[150] found that a combined thermochemical process involving impregnation of the de-oiled SBE with an ammonium chloride solution (3 M concentration), followed by thermal reactivation at 400 ºC for 60 minutes before a final washing step in hydrochloric acid solution (5%) led to SBE regaining the physical characteristics of virgin bleaching earth.

De-oiled SBE can be reactivated for reuse as an adsorbent in non-food applications. Plata et al found that de-oiled SBE could be used for palm oil bio-diesel filtration after reactivation through heat treatment at 500 ºC for one hour^[151]. Wambu et al^[152] found that thermal reactivation of de-oiled SBE at 370 ºC for 12 hours led to a good adsorbent for heavy metal removal. The treated SBE was able to recover >98% removal of Cu(II) ions from a solution. Marrakchi et al have also shown that de-oiled SBE can be used to remove chemicals. They used potassium carbonate (1:1 impregnation ratio) combined with thermal treatment (800 for 2 hours) to reactivate the de-oiled SBE. They found that the reactivated SBE was able to adsorb 84% of acid blue 29 and 96% of methylene blue. This application could therefore be useful for wastewater treatment for the chemicals industry.

The use of de-oiled SBE in building materials has also been investigated. Beshara and Cheeseman^[153] found that compaction of 100% SBE at 7 MPa followed by a heat treatment of 150ºC for 24 hours produced a brick-like material with a compressive strength of 54 MPa. The compressive strength increased linearly with the compaction pressure used in the preparation of the samples (from 1-7 MPa tested), which suggests that it might be possible to produce higher strength bricks if using compaction pressures greater than 7 MPa. Dr Beshara and Prof Cheeseman have confirmed that they would be interested in supporting the further development and application of this approach in Oman – see the Potential Partners section for further details.

Rokiah et al^[154] found that de-oiled SBE can be used as a partial replacement for cement in concrete. A mix with 30% substitution of cement with de-oiled SBE produced foamed concrete with improved compressive strength and similar performance in other key properties.

De-oiled SBE is also being investigated as a potential soil amendment. Loh et al^[155] stated that de-oiled SBE can be used as a soil amendment for sandy soils as it improves water holding capacity and can provide slow release of essential nutrients. An earlier study by Loh et al^[156] found that the use of de-oiled SBE as a soil amendment led to a 35-60% increase in the yield of okra, kangkong and groundnut magenta grown in Malaysia.

SBE has also been used to create mineral licks for ruminants. This application is currently being explored by the IIC within a separate project and so is not covered here.

Note that thermal reactivation of the SBE is also possible and was actually found to offer marginally better results in the study by Bachmann et al^[160] mentioned earlier, with 99% recovery of bleaching performance compared to 96% after oil recovery. However, if a profitable market for the recovered oil can be identified then reactivation after oil recovery should be prioritised.

Next steps for developing spent bleaching earth valorisation application

To progress this application it will be necessary to work closely with the oil refining companies in Oman to establish:

• Their objectives – do they just want a low cost, convenient means of disposing of SBE? Or are there strategic objectives that could be served by SBE valorization?
• What are the characteristics of the SBE they currently produce? E.g. What is the retained oil percentage? Have they experienced problems with auto-ignition of SBE waste? What contaminants are present? Etc.
• Are they willing to make a capital investment in developing technologies and systems for SBE valorization?

National priorities, such as improving food and water security should also be considered. This information can then guide the shortlisting of SBE valorisation options, which can then be investigated in more detail.

It should be noted that a previous project on SBE valorisation was initiated between Areej Vegetable Oils & Derivatives SAOC, Sultan Qaboos University and the Industrial Innovation Centre in 2011/2012^[161]. The results and recommendations of this project should be investigated as they might be useful in prioritising the various SBE valorisation approaches.

Beyond the oil recovery technologies offered by MecPro, none of the potential SBE valorisation technologies are available as commercial solutions. Further research and development will therefore be required. This R&D activity could be supported by academic researchers and the Industrial Innovation Centre. If the building materials application of de-oiled SBE is of interest then it is recommended to engage Dr Beshara and Prof Cheeseman have confirmed that they would be interested in supporting the further development and application of this approach in Oman – see the Potential Partners section for further details.