Part 4 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. Part 2 provided a background to the environmental considerations that must be addressed in SSP manufacturing. Part 3 presented rock blending as the way to minimize heavy metals in the 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.
Determining the ideal rock blend
A stepwise approach for determining the optimal rock blend in superphosphate manufacturing is outlined below. Once the optimal blend is established, the production process tends to be straightforward and can run unimpeded with only relatively minor mechanical adjustments. However, any significant change in chemistry – due to a new phosphate rock source and/or sourcing from a new face in an existing quarry, for example – will potentially require a change to the blend.
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- Chemical analysis – This is used to determine elemental concentrations in each phosphate rock source. At a minimum, chemical analyses should be performed by the manufacturer every time a new rock source is introduced into the process, including potential source changes at existing mines and quarries. In continuous processing, analysis of the source rock(s) at regular intervals is also recommended to determine what process or mechanical adjustments might be required to maintain the highest end-product yields. Particular attention is placed on phosphorus, iron, aluminum, calcium, and fluoride concentrations, as these elements have the greatest influence on the reaction process and the specifications of end products. Levels of heavy metals such as cadmium are also verified at this point as, although having a minimal effect on the acidulation process, these are of primary importance to the commercial viability of the end-product.
- Determine ideal acid:rock (A:R) ratio – The information collected in step 1 is run through stoichiometric calculations to establish a theoretically ideal rock blend that will meet end-product specifications. This assumes ‘perfect-world’ conditions such as constant process conditions and uniform rock chemistry. A further quantitative analysis is then applied to establish the volume of acid required for optimal process conditions. This is known as the A:R ratio and, for the purposes of calculation, assumes 100 percent acid strength.
- Pilot-scale testing to confirm actual A:R ratio – Laboratory testing of phosphate rock sources only requires around 100 grams of material. The nearly instantaneous results obtained make this the ideal step for establishing the ’real world’ rock blend necessary for commercial-scale superphosphate production. The data gathered is used to determine the optimal phosphate rock blend and A:R ratio at production-scale needed to meet the target end-product specifications.
- Production testing – During production, the actual acidulation reaction conditions (e.g., time and volume) taking place in the exothermic conversion of rock to fertilizer cannot be replicated in laboratory batch tests. Production testing is therefore necessary. Conveniently, there is an opportunity to test both the physical and chemical properties of partially reacted superphosphate when this exits the den and enters the storage silos. Target values – for optimal maturation time and high-quality superphosphate – typically include 7-8 percent free acid and 11-12 percent water content. The target cadmium levels (<280 ppm) in the end-product are also confirmed at this stage. Further blending and testing, to keep within cadmium limits and achieve commercial acceptance, will be necessary if product Cd levels exceed this target value.
Continue to Part 5 – In-Line Optimization of SSP Manufacturing in the Broadfield Process
Back to Part 3 – Using Rock Blending to Regulate Heavy Metals in SSP Fertilizer Manufacturing
Contact us with questions about your rock blend and how to best optimize for optimal yields.