Chemistry
The main reactions of Spodofos are the aluminothermic reduction of phosphate and iron oxides to elemental phosphorus and iron, respectively.
Conversion is strongly exothermic. Phosphate to phosphorus conversion is complete.
The main reactions of Spodofos are the aluminothermic reduction of phosphate and iron oxides to elemental phosphorus and iron, respectively.
Conversion is strongly exothermic. Phosphate to phosphorus conversion is complete.
The phosphorus distills from the reactor as P2 gas. At lower temperatures, the P2 gas combines to the well-known P4 gas. At further cooling, it is condensed as white phosphorus, only requiring filtration before shipping. The liquid slag and iron are tapped from the reactor, separated by density difference, and solidified.
Some side reactions occur of which the partly dissolving of P2 gas into liquid iron is of most importance. This solubilization yields ferrophosphorus and lowers the recovery efficiency of the core process. In Spodofos this phosphorus sink will be minimized because of the very high temperature and specific provisions.
Optionally, the lean ferrophosphorus can be treated to harvest the phosphorus content as a phosphate that can be fed back into the process.
Including postprocessing of ferrophosphorus, overall P-recovery efficiency is very high.
Indeed, sewage sludge ashes can be reduced with (bio)cokes in a traditional electric arc furnace (EAF). The problem is the high concentration of iron oxides, typically present in sewage sludge ash. These oxides will be reduced, simultaneously with phosphates, to metallic iron. Phosphorus exhibits a high solubility in liquid iron, leading to a significant loss of P-recovery (up to 40%). That's why the former white phosphorus producer Thermphos (Netherlands, closed in 2012) only co-processed sewage sludge ashes with a very low iron-to-phosphate ratio.
In Spodofos, iron will be reduced as well. This accounts for much of the required energy evolved. However, due to higher temperature compared to EAF and special provisions in the Spodofos process, the phosphorus loss is limited considerably. Therefore, Spodofos does not require changes of coagulant at the waste water treatment plants.
Traditional reduction of sewage sludge ash scores low in sustainability. Because of the iron oxides present, this route requires double the electrical energy and leads to a doubling of the CO2-footprint per unit of P-mass when compared to carbon reduction of phosphate rock.
Unlike in EAF production, the phosphorus gas flow from the reactor is not diluted with carbon monoxide. Therfore, Spodofos doesn't require feedstock sintering.
Spodofos recovers phosphorus from virtually all ashes and minerals that contain phosphate, such as:
Sewage sludge ashes;
Bone meal ash;
Struvite;
Vivianite;
Sewage sludge char;
Neutralised phosphoric acid waste;
Phosphate rock fines.
Some moisture and/or organic material in the feedstocks is tolerated.
The aluminium to be used can be scrap of the lowest quality. All aluminium alloys and a wide variety of contaminations are allowed.
It's evident, that both phosphate and iron concentration in a feedstock, as well as the aluminium scrap used, affect economy of conversion.
Three products are formed in the Spodofos reactor:
Phosphorus gas, which after condensation and filtration yields valuable white phosphorus;
Slag, which contains a high amount of aluminium oxide (this enables high-end applications);
Metallic alloy, mainly consisting of iron and phosphorus.
White phosphorus (also called yellow phosphorus, elemental phosphorus or P4) is listed by the European Union as a critical raw material. On an industrial scale, white phosphorus has never been recycled from secondary sources only. Its market is demanding.
Slag can be produced in compact or expanded state. The compact slag can be applied as a granulate in high alumina cement as well in refractory materials. When finely milled, the expanded slag exhibits hydraulic activity and can be applied as a substitute of cement.
The metallic product has several direct applications, but can also be processed into a coagulant, for use at waste water treatment plants, and a valuable phosphate product. Optionally, the phosphate product can be fed back into the process to increase phosphorus yield.
A Spodofos plant is a small plant. The maximum volumetric throughput is the volume of the ash. Additionally, Spodofos comprises few process steps with very low hold-up. This all leads to a limited capex. A Spodofos plant in a base case, with a normal ash gate fee, provides for a prognotized payback period of less than 4 years.
Aluminium scrap is the main operational cost. White phosphorus is the main operational revenue. Their market prices are related to eachother by the energy price. Therefore, these prices exhibit a similar historical trend which dampens the economic impact of variations on Spodofos operation.
Only a neglectable part of the aluminium scrap market volume is required. Spodofos plants will not disturb this market.
Sustainability has been assessed independently by CE Delft (www.ce.nl) in December 2024.
The new European LCA guidelines (Product Environmental Footprint and Circular Footprint Formula) are applied. As a result, 39% of the primary aluminium footprint is allocated to the secondary aluminium, used in Spodofos (dark blue bar in the graph).
For the LCA a base case in applying the slag and ferrophosphorus are defined: as a cement substitute respectively as a high density agent in Dense Medium Separation processes (DMS). In the base case, Spodofos processing shows a footprint of around -1.400 kg CO2-eq. per ton of sewage sludge ash*, thus lowering climate impact compared to the production of equivalent primary materials.
Per ton of product the CO2-eq. footprint is:
White phosphorus: 44% less than primary white phosphorus
Slag: 80% less than cement
Ferrophosphorus: 69% less than ferrosilicon
All heavy metals are immobilized in the slag or in the metallic product. Leaching tests confirm this. The volatile heavy metals are separated from the gas exhaust of the reactor. It is expected that PFAS, if present in the feedstocks, will be destroyed at the Spodofos reaction conditions. This assumption will be checked through practice testing later.
The Spodofos core process only requires limited electricity for preheating aluminium and limited natural gas for preheating ash. This contrasts with traditional phosphorus production where 13MWh electricity per ton P4 is necessary for the reduction process. Spodofos can be designed completely fossil fuel free.
*Data are based on mean Dutch communal sewage sludge ash and are not referenced to another ash treatment method.