Part 2 of a 5-part blog series: “Minimizing Heavy Metals in Superphosphate Manufacturing”
Part 1 of the blog series introduced the problems that heavy metals present in the SSP manufacturing process as well as a refresher of SSP production in the Bradley Broadfield Process. Click Here to download the complete article in pdf format as published by BCInsight Ltd in Fertilizer International Nov|Dec 2023, issue 517, pp. 53-57.
Most phosphate rock sources used in superphosphate production worldwide contain concentrations of heavy metals (cadmium, uranium, radium, lead, mercury, etc.). Such elements are known to be toxic to both livestock and humans and injurious to health if ingested at high levels. High cadmium (Cd) levels tend to be most prevalent, making this element the heavy metal of most concern. Care is therefore taken to minimize its concentration in superphosphate fertilizers. Cd levels are typically kept to below a threshold of 280 ppm, although regulations do vary from country to country.
In superphosphate production, there is no simple or economically feasible way to remove heavy metals from the phosphate rock feed prior to the acidulation process, as these potentially toxic elements are locked within the crystalline structure of minerals. Fortunately, blending feed rock from multiple sources, as described in this article, can ensure that maximum Cd levels are not exceeded. Blending is a successful approach that can help prevent cadmium from accumulating in soils, crops, livestock and ultimately humans – this being the desirable long-term goal.
As well as potentially toxic elements, careful attention is also paid to the iron (Fe2O3) and aluminum (Al2O3) content of the phosphate rock feed. These two minor elements – collectively known as ‘R2O3 compounds’ – directly affect overall reactivity during the acidulation process and together are responsible for relatively high sulphuric acid consumption.
Iron and aluminum also have a strong influence on ‘retrogradation’, a phenomenon that occurs when unreacted acid is present due to a lowering of the reaction rate. This increases the liquid content of the mix and, together with iron and aluminum dissolution, causes processing difficulties and production headaches at superphosphate plants.
A phosphate rock feed with an iron and aluminum content of 1.5-2.5 percent is generally considered ideal, as it produces the best granules when processing with sulphuric and phosphoric acids. Levels above three percent, in contrast, will result in a sticky and difficult to granulate product, while, conversely, granules will degrade during storage if levels are below one percent. Running operations outside of this range is suboptimal and undesirable as, to offset inefficiencies in the acidulation process, it requires higher acid demand and energy consumption at the production plant. This result in both higher environmental impacts and lower production profitability.
Contact us to discuss the environmental concerns you may have in your SSP manufacturing process.