1.1. Phosphate Rock Availability in Sudan
The only major phosphate rock reserves found up to now in the Sudan are the deposits discovered in 1983 during exploration work by geologists of the German Geological Group in the neighborhood of Jebel Kurun (Figure 1) on the eastern border of the Nuba Mountains  , at a distance of 690 kilometer from Khartoum and 1510 kilometer from Port Sudan.
Figure 1. Jebel Kurun phosphate rock availability in Sudan.
This phosphate occurrence with minor base metal and uranium content forms a low elongated hill rising to about 25 meters above the surrounding plain. The hill extends for about 500 meter in a WSW-ENE direction and its width increases from about 100 meters in the west to 200 meters in the east  .
A preliminary resource estimate at Kurun indicates 1.68 million tonnes of phosphate ore grading 20% P2O5, which amounts to about 336,000 tonnes of P2O5. The depth is calculated to the base of the hills (12 m in the west and 20 m in the east), but the true depth of the phosphate mineralization is not known  .
K. Brinkmann (1985)  indicates that during weathering, sulphides were dissolved, sulphate was removed away, and zinc and copper were held by limonite. The apatite was dissolved and since Al−3 ions were abundant, alumino-phosphates were formed. Woodhouseite was formed in the presence of sulphate released from the weathering of sulphide and in the absence of sulphate, crandallite was precipitated. Wavellite was formed by loss of calcium as an ultimate product and in the presence of copper turquoise was formed.
1.2. Visible Phosphate Reserve in Jebel Kurun
The arithmetic average of the chemical analysis of the 52 samples gave 20.1% P2O5, in tests with formic acids. Soluble phosphates were proved to be less than 20% of total phosphates, and the visible phosphate reserves of the sampled portion of Jebel Kurun down to the elevation of the plain were estimated at 336,000 ton of P2O5  .
The sampled area covered about 32,000 m2 i.e. 200 × 50 = 10,000 m2 in the west plus 275 × 80 = 22,000 m2 in the east. The average elevation above the plain was taken at 12 meters in the west and 20 meters in the east. The average phosphate content is 20% P2O5 and the specific gravity = 3 ton/m3. The limits of the phosphate mineralization were not determined laterally and in depth and therefore an increase in the amount of the reserves is possible  .
Geological Research Authorities of the Sudan expect the continuation of phosphate towards the depth; therefore they expect a total reserve of phosphate may reach 81 million tons with average content of 20% P2O5  . This estimation cannot be reliable without detailed drilling exploration.
Brinkman in 1986  discovered two types of phosphate deposits in Kurun and Uro areas in the center of eastern Nuba Mountains. However, using of these phosphates as fertilizers, it is necessary to check them chemically, especially for radioactive uranium presence  .
Twelve selected samples from Jebel Kurun phosphate―bearing rocks were analyzed by X-ray diffraction method and were found to contains the following minerals (Table 1)  .
Giad industrial group (Sudan) reported the analysis results in 2017 for two samples (Figure 2) and (Figure 3) for the characterization of Jebel Kurun phosphate rock (Table 2) for grain size distribution and (Table 3) for chemical composition.
Table 1. Phosphate minerals in Kurun-Uro rock phosphate  .
Table 2. Grain size distribution  .
Figure 2. LOT KR-AP.
Figure 3. LOT KR-AL.
The analytical results indicate the following observations:
• A very high content of alumina in the rock.
• Most of the phosphate present as fluorapatite, that has an excellent digestibility in hydrochloric acid.
Table 3. Chemical analyses  .
• Sand is present as quartz.
• Alumina and phosphate are present as Crandallite: CaAl3(OH)6(PO3(O0.5(OH)0.5))2.
1.3. The Objective of the Present Work Is to
• Assess the technical viability for the utilization of Jebel Kurun phosphate rock to produce phosphoric acid for fertilizer usage.
• Classification of J. Kurun phosphate rock and identification of types suitable for the production of phosphoric acid.
• Identification of the chemical characteristics and behavior of Jebel Kurun phosphate rock.
2. Materials and Methods
Surface samples were collected (Figure 4, Figure 5) and analyzed by the Geological Research Authorities of the Sudan (GRAS), from Kurun area. A total of eleven profiles were covered. The distance between each profile is forty meters, and the distance among points inside each profile is about ten meters. Fifty six samples were taken in 2011. Thirty nine elements and Loss on Ignition (LOI) at 1000˚C were tested and reported for Jebel Kurun phosphate rock samples.
Samples were analyzed by using X-ray fluorescence spectrometry (XRF) after grinding, homogenizing and pelletizing process. The concentration of the detected elements was calculated from the measured net intensity of the corresponding fluorescent X-rays combined with elemental sensitivity factor and the coefficient of absorption in the residual matrix of the specimen through iteration procedure    .
Statistical analysis for the collected samples were carried on by using Microsoft Office Excel software program to calculate the average value, maximum value, minimum value, variance and correlation coefficient between tested elements and P2O5 concentration.
Correlation coefficient strength between each element and P2O5 concentration was divided into seven classes: No correlation, very weak, weak, moderate, strong, very strong and perfect correlation
Giad Industrial Group in Sudan, studied the analysis report with a conventional technology supplier and classified Kurun mining area phosphate rock into three main types (Table 9) as follows:
1) The first type: Phosphate rock including Lucinite or aluminophosphate ore
• Main component content:
CaO% ≤ 12%, Al2O3% ≥ 20%, P2O5% ≥ 18%
2) The second type: Apatite rock including aluminophosphate
• Main component content:
CaO% ≥ 25%, Al2O3% ≤ 10%, P2O5% ≥ 20%, 12% ≤ SiO2 ≤ 35%
Figure 4. Sampling site.
Figure 5. Sampling site.
3) The third type: Silica ore including phosphorus or iron ore including phosphorus
• Main component content:
SiO2% ≥ 40%, P2O5% ≤ 10%
Fe2O3% ≥ 20%, P2O5% ≤ 10%
The reserve portion that could be utilized to produce phosphoric acid by conventional processes has been estimated by identifying the number of samples representing type one and type two by using Microsoft Office Excel.
3. Results and Discussions
3.1. Jebel Kurun Phosphate Behavior
The statistical analysis results (Table 4 & Figure 6) shows the estimated phosphorus pentoxide ratio P2O5% in Jebel Kurun area has an average value of 19%, and ranges from 0.13% to 32.94% with a standard deviation of 9.05. There is strong negative correlation (Tables 4-8, Figure 7 & Figure 11) with value (−0.82) between phosphorus pentoxide and silicon dioxide, moderate positive correlation between phosphorus pentoxide and Aluminium oxide Al2O3, Calcium oxide CaO, Loss on Ignition LOI at 1000˚C, Vanadium V with values 0.66, 0.54, 0.63 and 0.61 respectively.
It is important to remember that the correlation coefficient between two variables is a measure of their linear relationship and that a value of r = 0 implies a lack of linearity and not a lack of association. a value of r equal to +1 implies a perfect linear relationship with a positive slope, while a value of r equal to −1 results from a perfect linear relationship with a negative slope    .
Uranium in Jebel Kurun area has an average value of 81.47 ppm (Table 7 & Figure 10) and ranges from 1.2 ppm to 208.3 ppm and a standard deviation of 53.89, a very weak positive correlation coefficient (0.16) (Table 7, Table 8 & Figure 11) with phosphorus pentoxide. Phosphate samples with an average uranium content lower than 60 ppm showed a higher correlation of 0.35 between uranium and P2O5 and represent 41.07% of total samples, a very weak negative correlation (-0.05) when uranium content is higher than 60 ppm for 58.93% of samples.
A research published by the Journal of the Argentine Chemical Society  stated that a positive relation exists between uranium content and P2O5 percentage
Table 4. Statistical analysis for major elements.
Table 5. Statistical analysis for trace elements.
Table 6. Statistical analysis for trace elements.
Table 7. Statistical analysis for trace elements.
Table 8. Scaling correlation factors for phosphate rock components.
Figure 6. Mean values for major elements in phosphate rock samples.
Figure 7. Correlation coefficient for the major elements with P2O5 in phosphate rock samples.
Figure 8. Mean values of the trace elements in phosphate rock samples.
Figure 9. Correlation coefficient for the trace elements with P2O5 in phosphate rock samples.
Figure 10. Mean values of the trace elements in phosphate rock samples.
Figure 11. Correlation coefficient for the trace elements with P2O5 in phosphate rock samples.
in phosphate ore samples from Kurun area (r = +0.58), which contain 56.63 mg/kg uranium in average. The chemical analysis of phosphate ore samples indicated that in the case of phosphate samples that contain low uranium concentration, there is a strong positive correlation between uranium content and P2O5%, but if uranium concentration increases over 60 mg/kg, no correlation exits.
Samples with P2O5 content higher than 15% and an average uranium content 88.93 ppm, indicate a weak negative correlation (-0.37) between P2O5 and uranium, and represent 78.57% of phosphate samples. Accordingly uranium would not be expected to be of economic interest, considering that samples that have P2O5 content lower than 15% and average uranium content 54.13 ppm exhibited a moderate positive correlation 0.61 between uranium and P2O5 and represent 21.43% of total samples.
Statistical analysis for trace elements (Tables 5-7 & Figures 8-11) indicates the mean values and correlation coefficient with phosphorous pentoxide.
3.2. Jebel Kurun Phosphate Classification
Giad Industrial Group in Sudan admitted that the effective reserves for the type 1 and type 2 (Table 9) of phosphate rock in Kurun mining area should be more than twenty million tons to achieve economic viability.
4. Conclusions and Recommendations
• Phosphorus pentoxide P2O5 has an average value of 19%; the most abundant major elements for the studied phosphate rock are silicon, aluminum, phosphorus, calcium and iron. Strong negative correlation (-0.82) is between phosphorus pentoxide and silicon dioxide.
• Samples with P2O5 content higher than 15% and average uranium content 88.93 ppm, indicate negative correlation (-0.37) between P2O5 and uranium; they represent 78.57% of phosphate samples, and accordingly uranium would not be expected to be of economic interest, considering that samples
Table 9. Jebel Kurun phosphate classification.
that have P2O5 content lower than 15% and average uranium content 54.13 ppm expressed a positive correlation (0.61) between uranium and P2O5 and they represent 21.43% of total samples.
• Aluminophosphate ore (CaO% ≤ 12%, Al2O3% ≥ 20%, P2O5% ≥ 18%) is represented by 55.36% of sampled phosphate rock; this type can be used to produce phosphoric acid when P2O5 > 30%.
• Apatite rock including aluminophosphate CaO% ≥ 25%, Al2O3% ≤ 10%, P2O5% ≥ 20%, 12% ≤ SiO2 ≤ 35% is represented by 1.79% of sampled phosphate rock; this type can be used to produce phosphoric acid when it can meet the requirement (CaO% ≥ 30%, Al2O3% ≤ 7%, P2O5% ≥ 25%, SiO2% ≤ 30%).
• Silica ore including phosphorus (SiO2% ≥ 40%, P2O5% ≤ 10%) is represented by 16.07% of sampled phosphate rock, Iron ore including phosphorus (Fe2O3% ≥ 20%, P2O5% ≤ 10%) is represented by 5.36% of samples phosphate rock, and both types cannot be used to produce phosphoric acid.
• Study the statistical distribution of P2O5 in the size fractions of the mined and crushed rock is recommended and using a diamond drill for sampling deposits in order to calculate the reserve by grade.
I would first like to thank my thesis advisor Prof. Kamil M. Wagialla of the Chemical Engineering Department/Faculty of Engineering at University of Khartoum. I would also like to thank my colleagues in Giad Industrial Group (Sudan), Geological Research Authority―(Sudan), Arab Fertilizer Association (Egypt), Ecophos Technologies (Belgium) and Norinco Company (China) for their support and valuable inputs.