I am writing this article to share my thoughts about the general methodology to be followed during a typical phosphate rock assessment.
The tests are carried out in a laboratory scale. This can be followed by a pilot plant investigations if necessary. The result of the rock assessment gives process engineers valuable input to evaluate the potential manufacture of phosphoric acid and fertilizers.
Why do we need Laboratory Investigation ?
Laboratory scale tests are carried out to provide preliminary but valuable representative indications of how a phosphate rock is likely to behave in full scale phosphoric acid process. This is particularly useful as a screening exercise when an indication of their relative performances is required.
Laboratory scale test also allow the most promising starting conditions to be identified for any pilot plant investigations that may follow. The tests are performed continuously to evaluate the performance and reaction conditions in the dihydrate (DH) and hemihydrate (HH) processes. The conversion of hemihydrate to dihydrate can also be carried to assess the potential use of a rock in the hemidihydrate process (HDH).
But first, let's make a chemical and physical analysis of our phosphate
Samples of rock are first analyzed to determine a basis for the subsequent testing. The analysis includes generally a chemical determination of the components P2O5, CaO, SO3, F, SiO2, Na2O, K2O, MgO, Al2O3, Fe2O3, CO2, H2O, Organic Carbon, Chlorides, Cadmium, and others minor elements if necessary. A full assessment of the rock particle size distribution is also performed.
Now, here is a general list of the test works to be performed
1. Foam Tests
Foam tests are carried out to assess the foaming characteristics of the rock when used for full-scale manufacture of phosphoric acid, which govern the need of any antifoam-addition. The rock is treated with acid under carefully controlled standard conditions of agitation in a calibrated reaction vessel. The height of foam produced and its persistence is measured at predetermined times after the addition of the acid. If required, different anti-foam agents can be tested.
2. Crystallization Tests
The crystallization tests are carried out in a continuous laboratory scale unit using a standard phosphoric acid composition for the initial start-up. To increase the representativeness of the test work, the reactors should be chosen to simulate the actual full scale flowsheet as close as possible.
The duration of each trials depends on the type of process being studied, but in all cases should be chosen to be sufficient to allow chemical equilibrium to be achieved.
In the initial test, the sulfate level and P2O5 levels in the liquid phase of the slurry are fixed. The fineness of the rock is also fixed by the requirements developed for each particular laboratory technique. It is sometimes necessary to examine the effect of different sulfate levels and different P2O5 levels.
As the test progress, samples of calcium sulfate crystals can be taken from the reaction vessel for microscopic examination and photography (see pictures below). Their specific surface area can also be determined. By monitoring any changes in the crystal structure, a confirmation is obtained that equilibrium has been reached.
Toward the end of the test, samples from the reaction vessel are filtered in a standard procedure that involves washing. This gives an indication of the filterability and the washing characteristics of the calcium sulfate.
At the end of the test, the product acid and calcium sulfate are analyzed for the major components and the more important minor components. The results of the acid analysis allow a preliminary assessment of its corrosive properties also.
The crystallization test should leave sufficient acid of representative quality, which can be used to perform other tests if necessary (test for example to produce ammonium phosphate)
Here is some examples of calcium sulfate crystals pictures, the shape and form will depend on whether it is HH or DH, but also on the rock phosphate source :
3. Corrosion Tests
Corrosion tests may be carried out on product acid. Materials of construction in common use in phosphoric acid production are tested and corrosion rates are compared with those found in full scale plants with other rocks.
4. Acid Concentration
Depending on the concentration of the product phosphoric acid and the required concentration of the phosphoric acid fed to downstream production, acid concentration may be required.
This acid is concentrated by vacuum evaporation. The concentrated acid is analyzed for the major components and the more important impurities. The fluorine balance is also examined.
5. Acid physical properties
The density, viscosity and vapor pressure of the product acid can be measured.
6. Post Precipitation Tests
If appropriate, the post precipitation characteristics are assessed for both the weak and strong acids. The post precipitated material is examined for quantity and composition.
7. Single Superphosphate (SSP) Tests
Samples of SSP are prepared in the laboratory from the phosphate rock and sulfuric acid. The effects of different acid to rock ratios are determined by a series of tests in which other parameters such acid temperature and rock size are maintained constant. The solidification time is noted in each of these tests and the physical conditions of SSP is examined after a given period. The rate of maturing is monitored by analysis to determine free acidity, water soluble P2O5 and total P2O5 at specific time intervals.
When samples of SSP have matured for 14 days they are analyzed for water soluble P2O5, total P2O5, acid P2O5 and citrate soluble P2O5. The results of these tests help process engineers to select the optimum acid to rock ratio which will give information on raw material consumptions.
Further samples are prepared at the optimum acid to rock ratio using ground rock to different degrees of grinding. These samples are analyzed after maturation. Selected samples are investigated for maturing rate. The results of these tests help process engineers to select the optimum degree of rock grinding.
8. Triple Superphosphate (TSP) Tests
Samples of TSP are prepared in the laboratory from the phosphate rock and phosphoric acid produced from the rock. The effects of different acid to rock ratios are determined by a series of tests in which other parameters such as acid temperature and rock size are maintained constant. The solidification time is noted in each of these tests and the physical conditions of TSP is examined after a given period. The rate of maturing is monitored by analysis to determine free acidity, water soluble P2O5 and total P2O5 at specific time intervals.
When samples of TSP have matured for 14 days they are analyzed for water soluble P2O5, total P2O5, acid P2O5 and citrate soluble P2O5. The results of these tests help process engineers to select the optimum acid to rock ratio which will give information on raw material consumptions.
Further samples are prepared at the optimum acid to rock ratio using ground rock to different degrees of grinding. These samples are analyzed after maturation. Selected samples are investigated for maturing rate. The results of these tests help process engineers to select the optimum degree of rock grinding.
9. Ammonium Phosphates (MAP/DAP) Tests
Samples of concentrated product acid are ammoniated to approximately pH 4 to produce MAP and approximately pH 7 to produce DAP.
These products are then analyzed for ammoniacal nitrogen, water soluble P2O5, citrate soluble P2O5 and total P2O5 and the more important impurities.
The results are used to predict the expected nutrient grade and further chemical composition of product in full scale industrial production of MAP and DAP. Test work gives also information on the expected consumptions of ammonia and phosphoric acid per ton of end product.
A comparison can be made with ammonium phosphates produced from other rocks to assess whether international standards relating to composition are being met.
Conclusion
Phosphate rock assessment is an important step especially when it comes to a new phosphate source. The results help engineers to provide a full assessment of the economic viability of the phosphate rock for commercial scale phosphoric acid production and fertilizers production. The optimum process route can also be selected.
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