Radiological characterization of the phosphate deposit in Al-Jalamid phosphate mining area, Saudi Arabia
, , , and | Mar 06, 2021
Article Category: Original Paper
Published Online: Mar 06, 2021
Page range: 35 - 44
Received: Jun 29, 2020
Accepted: Oct 02, 2020
DOI: https://doi.org/10.2478/nuka-2021-0005
Keywords
© 2021 Abdulsalam M. Alhawsawi et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
The old Tethys Ocean, also known as the Tethys Sea, is the place from where the Mediterranean Sea originated and it separates Europe and Asia from Africa. The Tethys Sea flew across the Middle East and North Africa, an area that begins in northwest Saudi Arabia and extends across south Jordan, Egypt, Tunisia, Algeria and Morocco [1]. This area possesses the largest phosphate deposits worldwide. These deposits are composed of organic remains of plants and animals that decayed into a sedimentary series around 60–70 million years ago, which resulted in the formation of current dolomites, limestones, and shales. Further, organic ooze of this ocean became phosphorous deposits because of the process of diagenesis. Florida phosphate deposits are also found along a stretch on the southern Mediterranean. At the far eastern end of the Florida, phosphate deposit line is located in northern Saudi Arabia, which is one of the world’s largest reserves of phosphate ore [1].
It is well known that phosphate minerals usually contain naturally occurring radionuclides whose activity is enhanced by geological processes, as uranium replace calcium in the rock formation. However, several studies [2, 3, 4, 5, 6, 7, 8, 9] have been conducted worldwide to study natural radioactivity of the phosphate deposits, and its radiological impact on the workers and the surrounding environment due to mining, processing, packing, and other mining related activities. All these studies have indicated that sedimentary phosphate rocks of different origin contain high 238U activity concentrations ranging between 150 Bq/kg and 4800 Bq/kg, and much lower 232Th activity concentrations ranging between 16 Bq/kg and 78 Bq/kg [6, 10, 11]. Other studies have reported that concentration values for uranium are lower than 150 Bq/kg [12, 13, 14]. The main source of radiation in sedimentary phosphate rocks is the uranium series radionuclides. Most of these studies are consistent in reporting that the ratio of 238U to 226Ra activity concentration is almost equivalent to unity, showing radioactive equilibrium among radionuclides of the uranium decay series. Moreover, thorium concentration in phosphate rocks of igneous origin is often equal to the concentration uranium, although both are present in smaller quantities [5]. In his work, [5] reported typical concentration values for 238U to 232Th which are as follows: (a) 638 ± 153 Bq/kg to 258 ± 18 Bq/kg, (b) 298 ± 148 Bq/kg to 284 ± 20 Bq/kg, and (c) 344 ± 94 Bq/kg to 246 ± 17 Bq/kg in some Brazilian phosphate rocks of igneous origin. Generally, the levels of 40K and 232Th in all phosphates are similar to that in natural soil [15]. Furthermore, the International Atomic Energy Agency (IAEA) reported that natural uranium concentrations greater than 200 parts per million (ppm), which is equivalent to about 2460 Bq/kg, are of potential interest as uranium can be obtained as a byproduct economically rather than shipping it as a contaminant in the phosphate fertilizer [16]. This can be achieved by installing solvent extraction circuits in the fertilizer manufacturing facility [17].
Several investigations have been conducted to assess natural radioactivity in the phosphate deposits of the Tethys Sea [4, 18, 19]. Only one study is pertained to natural radioactivity in the phosphate deposits in northwestern Saudi Arabia [20]. The investigations now focus its attention on studying and characterizing natural radioactivity in the phosphate ore of Al-Jalamid phosphate mines located in northern Saudi Arabia. Data presented in those research efforts reveal a knowledge gap that requires additional investigation to further characterize the radiological levels in phosphate deposits at the Tethys Sea in northern Saudi Arabia. Additionally, results of this work will be compared to several phosphate deposit data worldwide. Therefore, the main objectives of this work can be summarized as: (a) characterizing natural radioactivity in phosphate deposits of Al-Jalamid phosphate mining area. Mining and processing activities started in 2010 in this mine, which is considered the first site when Ma’aden started the processing of phosphate; (b) characterizing the raw mineral phosphate ore, the beneficiated ore, reject, soil, and mining waste in terms of radioactivity levels; and (c) finally exploring whether the average uranium concentration in the beneficiated phosphate ore that is used as a feed for fertilizer production can be recovered economically as a byproduct from the local mining industry.
Fig. 1
A map of Saudi Arabia showing the location of the mining site (A), the processing complex (B), and the fertilizer exportation marine port (C).
Al-Jalamid phosphate mining area lies in Hazm Al-Jalamid Quadrangle (long. 39°00′ and 40°30′E and lat. 31°00′ and 32°00′N), northwest of Saudi Arabia (see Fig. 1).
Location A comprises of two adjacent phosphate mines, physical beneficiation site and chemical beneficiation plant [21]. The phosphate concentrate is transported by rail and trucks from Al-Jalamid to the phosphate processing complex in Ras Al Khair, situated at the western coast of the Arabian Gulf at location B, shown also in Fig. 1, to manufacture phosphate fertilizers for exporting from the marine port in location C, shown in Fig. 1. Hazm Al-Jalamid Quadrangle contains early tertiary sedimentary rocks of Turayf Group along with other sedimentary deposits [22]. The sedimentary rocks exposed at the surface include Al-Jalamid, Mira, and Umm Wa’al formation of the Turayf Group. Al-Jalamid formation was deposited in a shallow marine environment. It is found mostly as pelletal phosphorite, dolomitic limestone, limestone, in addition to argillaceous, silicified limestone, and some dolostone. All the three formations of the Turayf Group (Urn Wu’al, Mira and Al-Jalamid formations) have the potential for commercial development. Other deposits in the surrounding area that might be suitable for commercial development include the Thaniyat, Turayf, and As Sanam deposits [22].
On-site observation of the mines showed that the exposed phosphate deposits exist in two layers namely upper and lower layers. The upper deposit layer is 5–8 m thick and the lower deposit layer is 1–3 m thick. The two layers are separated by a 0.1 m to 2 m thick waste deposit. The upper phosphate layer is topped up by an overburden layer; consist of mining waste mixed with soil that is around 18 m in thickness.
According to the documents of Ma’aden mining company, the raw ore contains 20–25% of P2O5 and 51–54% of CaO. On-site beneficiation activities, either by physical (sizing) or chemical (floatation) processes, increased P2O5 concentration to about 30%. Also, the radioactivity in phosphate rocks may vary from one location to another based on ground samples. Because of this reason, numerous samples representing the varieties of rocks and soils from all locations were collected to increase the probability of finding any radiometric anomalies that may exist.
Raw phosphate samples were collected from the upper and lower phosphate layers. Other samples were collected from the beneficiated ore (concentrate) and reject piles inside the beneficiation plant before transporting to the processing complex in Ras Al Khair. In the open mines, ground samples from the overburden layer composed of mining waste mixed with soil, and the lower waste layer were also used for samples. In all samples, large gravels, stones and other objects were removed by a 2 mm sieve and about 1 liter of the soil was labelled and packed in plastic bags.
Topsoil samples known as natural soil were collected from several undisturbed and non-mining sites. Topsoil samples were collected from an area of more than 0.25 m2 and from a depth of 5 cm into the ground.
All samples were labelled, sealed, and transferred to the laboratory for measurements and analysis. Then, the samples are prepared for the radio analytical procedure: they were dried at room temperature for several days, ground, and mixed. An aliquot of the mixed sample was heated in a muffle furnace at 550°C for about 20 hours to get rid of organic remains and eliminate carbonates. The resulting sample weight was corrected against the field sample weight. The samples were analysed for their uranium and thorium content by an α-spectrometer using radiochemical techniques. The detailed radiochemical technique is described in Appendix A.
Uranium was separated from other sample components by ion exchange technique and co-precipitated with Nd3+ as fluoride on a membrane filter for measurement [23]. About 0.5 g of each sample was digested in a 400-mL Teflon beaker for complete dissolution using concentrated HNO3/HCl. For samples containing silica, HF was used apart from the concentrated HNO3/HCl acids to attain complete dissolution. The sample was evaporated to dryness and about 3 mL of concentrated HCl was added and fully evaporated to convert the medium to the Cl– form. The final step was repeated twice to complete the conversion process.
The sample residue was taken up in 20 mL of 10 M HCl solution and loaded into 10 mL separation column (from Bio Rad) packed with AG 1-X8 anion exchanger (5 mL bed volume). The column was preconditioned by passing 20 mL of 10 M HCl. The trapped uranium was washed with another 20 mL of 10 M HCl and eluted from the column by passing 20 mL of 0.1 M HCl.
Then, for further preparation of the uranium source, 50 μL of Nd3+ solution (1 mg Nd3+/mL, as chloride) was added to the separated uranium (the eluate) followed by an amount of TiCl3 (15%) drop-by-drop to change the solution colour to purple, and then an additional 1 mL was added to keep uranium in the tetravalent state. Five mL of 40% HF solution was added to the sample for uranium precipitation as fluoride and was left for 30 min prior to filtration using 0.1 μm polypropylene membrane filter. The precipitate was washed with 3 mL of 4% HF, then with 3 mL of 80% ethanol solution. The sample was dried at 50°C for 5 min, placing and fixing it on a stainless-steel disc using an adhesive tape, and counted.
Thorium was separated exactly as uranium except that the sample was spiked with 229Th radiotracer (instead of 232U), and 8 molar (M) and 1 M of HNO3 solutions were used in loading and eluting thorium instead of 10 M and 0.1 M of HCl solutions in case of uranium, respectively. Thorium source was also prepared similar to uranium except the addition of TiCl3 solution because thorium is permanently in the tetravalent state [23].
For quality control and validating the obtained radiological data, the analytical procedures of simultaneous determination of uranium isotopes and thorium isotopes in the environmental samples were tested against standard reference materials supplied by the IAEA. These references are RGU-1 and RGTh-1, which were prepared by dilution of uranium ore and thorium ore, respectively, with floated silica powder [24]. The radionuclides of uranium and thorium series are in radioactive equilibrium in these reference materials. Repetitive determinations of 238U, 234U, 230Th, from uranium series, and 232Th and 228Th from the thorium series were performed and the obtained results are presented in Table 1.
Results of analysing uranium and thorium isotopes in IAEA reference materials (RGU-1 and RGTh-1)
| Sample | Measured values | Reference value | Measured values | Reference value | |||
|---|---|---|---|---|---|---|---|
| 238U | 234U | 230Th | 232Th | 228Th | |||
| RGU-1 | 4649 ± 418 | 4893 ± 427 | 5004 ± 374 | 4940 ± 30 | 5 ± 2 | 6 ± 2 | <4 |
| 4557 ± 338 | 4777 ± 365 | 4811 ± 365 | 7 ± 3 | 6 ± 2 | |||
| 4797 ± 425 | 4723 ± 402 | 4886 ± 378 | 4 ± 3 | 5 ± 2 | |||
| 5288 ± 376 | 5489 ± 431 | 4768 ± 361 | 6 ± 2 | 5 ± 2 | |||
| Average | 4823 ± 39 | 4970 ± 40 | 4867 ± 38 | 6 ± 3 | 6 ± 3 | ||
| CV* | 0.81 | 0.82 | 0.78 | 50 | 50 | ||
| RGTh-1 | 75 ± 11 | 71 ± 11 | 70 ± 8 | 78 ± 6 | 3319 ± 206 | 3453 ± 214 | 3250 ± 90 |
| 88 ± 10 | 89 ± 9 | 73 ± 9 | 2987 ± 214 | 3173 ± 226 | |||
| 83 ± 9 | 84 ± 10 | 75 ± 8 | 3358 ± 231 | 2794 ± 205 | |||
| 79 ± 9 | 78 ± 9 | 69 ± 8 | 3335 ± 184 | 3252 ± 181 | |||
| Average | 81 ± 6 | 81 ± 6 | 72 ± 6 | 3250 ± 29 | 3168 ± 29 | ||
| CV* | 7.41 | 7.41 | 8.33 | 0.89 | 0.92 | ||
CV = coefficient of variation = (standard deviation/average value) × 100.
The obtained results in Table 1 for the repetitive determination of uranium isotopes in RGU-1 reference materials show that the precision in this activity level 4940 ± 30 Bq/kg, lies between 7.1% and 9%. Higher precision percentages imply an imprecise result. At uranium activity levels of 78 ± 6 Bq/kg in the RGTh-1 reference material, the obtained results lie between 10.1% and 11.9%, which is also an imprecise compared to the RGU-1 reference material due to the small sample size and low count rate.
Results from repetitive determination of thorium isotopes in the RGU-1 reference material show that the precision in the level of 4940 ± 30 Bq/kg for 230Th (a member of the U-series) lies between 7.5% and 7.7%. At activity concentrations of 3250 ± 90 Bq/kg of the other thorium isotopes (232Th and 228Th) in the RGTh-1 reference, precision ranges between 5.5% and 7.3%. At activity levels of thorium isotopes <4 Bq/kg in RGU-1, the obtained results are significantly less precise as shown in Table 1 due to the small sample size and the low count rate.
The accuracy, which is a deviation from the mean value of the confidence range in determination of uranium isotopes in the RGU-1 reference ranges from 7.7% to 11.1%. This is equivalent to an activity level of 4940 ± 30 Bq/kg. On the other hand, the activity level of 78 ± 6 Bq/kg in the RGTh-1 has an accuracy value ranging from 9% to 14%. Despite the very low background level of the counting system, the determination of thorium isotopes of activity concentrations <4 Bq/kg was significantly less accurate due to the very small sample size and the sample low count rate.
The collected data is quite sufficient for analysing phosphate samples of activity concentrations between 300 Bq/kg and 600 Bq/kg of uranium series radionuclides, despite the small sample size 0.5 g. High activity concentration, long counting time, and the low detection limit of the counting system are the reasons for this level of accuracy and precision.
ORTEC ensemble was also used to measure the activity of uranium and thorium isotopes in addition to a high-resolution α-spectrometry system. The α-spectrometry system consists of eight independently operated, 450 mm2 silicon surface barrier detectors located in separate chambers connected to a vacuum pump. The efficiency of the detectors ranges from 20% to 21.5%. The background count rate was about 1 count/18 hours under each of the used energy peaks resulting in a minimum detection limit of 4 counts/18 hours, which is equivalent to 0.56 Bq/kg for the 0.5 g small sample size. This detection limit is appropriate for the phosphate samples with high activity concentration (>200 Bq/kg). An ultra-low-level liquid scintillation spectrometer, Quantulus model 1220 from PerkinElmer was used for measuring ground water samples in addition to the α-spectrometry system.
232Uranium and 229Th standard reference materials, SRM 4324 and SRM 4328C, respectively, were purchased from the National Institute of Standards and Technology (NIST). The standard materials were diluted and used as spiking radiotracers in uranium and thorium determinations.
The collected phosphate rock, phosphate waste, and topsoil samples from the mining site A, were analysed, and the obtained results are listed in Tables 2–4. Table 2 shows the activity concentration of the high-grade ore and its physical beneficiation products.
Activity concentration of uranium and thorium in high-grade raw phosphate ore and its physical beneficiation products (concentrate and reject)
| Sample | Activity concentration (Bq/kg) | ||||
|---|---|---|---|---|---|
| U isotopes | Th isotopes | ||||
| 238U | 234U | 232T | 230Th | 228Th | |
| Raw rock | 350 ± 35 | 396 ± 49 | 6 ± 1 | 388 ± 48 | 5 ± 2 |
| 351 ± 47 | 375 ± 50 | 9 ± 3 | 348 ± 53 | 8 ± 3 | |
| 367 ± 46 | 406 ± 41 | 10 ± 3 | 325 ± 42 | 7 ± 2 | |
| 454 ± 33 | 468 ± 38 | 8 ± 2 | 489 ± 36 | 8 ± 2 | |
| 389 ± 46 | 411 ± 42 | 8 ± 1 | 422 ± 43 | 9 ± 2 | |
| 364 ± 33 | 358 ± 32 | 11 ± 3 | 349 ± 52 | 12 ± 2 | |
| 353 ± 36 | 388 ± 45 | 4 ± 1 | 345 ± 43 | 3 ± 2 | |
| 367 ± 36 | 375 ± 45 | 7 ± 2 | 382 ± 43 | 6 ± 3 | |
| 405 ± 58 | 423 ± 61 | 7 ± 2 | 395 ± 57 | 8 ± 2 | |
| Average | 378 ± 19 | 400 ± 20 | 8 ± 5 | 383 ± 18 | 7 ± 4 |
| CV* | 5 | 5 | 62.5 | 4.7 | 57.1 |
| Concentrate | 459 ± 47 | 471 ± 49 | 7 ± 1 | 463 ± 36 | 5 ± 2 |
| 542 ± 55 | 569 ± 57 | 5 ± 2 | 520 ± 41 | 4 ± 2 | |
| 557 ± 53 | 568 ± 55 | 11 ± 3 | 544 ± 49 | 7 ± 2 | |
| 442 ± 45 | 466 ± 45 | 10 ± 2 | 430 ± 47 | 9 ± 2 | |
| 475 ± 48 | 578 ± 58 | 6 ± 2 | 548 ± 54 | 8 ± 2 | |
| 622 ± 59 | 684 ± 61 | 7 ± 2 | 671 ± 66 | 8 ± 2 | |
| 399 ± 48 | 413 ± 46 | 4 ± 1 | 385 ± 50 | 4 ± 2 | |
| 555 ± 56 | 540 ± 54 | 5 ± 2 | 697 ± 56 | 4 ± 2 | |
| 439 ± 54 | 485 ± 59 | 6 ± 2 | 466 ± 49 | 4 ± 2 | |
| 488 ± 48 | 491 ± 51 | 8 ± 2 | 454 ± 50 | 6 ± 2 | |
| 539 ± 57 | 566 ± 59 | 5 ± 2 | 525 ± 47 | 4 ± 2 | |
| 503 ± 50 | 526 ± 57 | 9 ± 2 | 478 ± 47 | 7 ± 2 | |
| Average | 502 ± 25 | 530 ± 25 | 7 ± 2 | 515 ± 24 | 6 ± 2 |
| CV* | 5 | 4.72 | 28.57 | 4.66 | 33.33 |
| Reject | 327 ± 43 | 365 ± 43 | 5 ± 2 | 371 ± 45 | 5 ± 3 |
| 378 ± 42 | 383 ± 43 | 6 ± 2 | 376 ± 39 | 7 ± 2 | |
| 366 ± 47 | 412 ± 48 | 5 ± 2 | 346 ± 50 | 6 ± 2 | |
| 355 ± 41 | 403 ± 44 | 6 ± 2 | 563 ± 46 | 6 ± 2 | |
| 337 ± 44 | 394 ± 47 | 5 ± 1 | 386 ± 39 | 5 ± 2 | |
| 346 ± 42 | 344 ± 43 | 5 ± 2 | 361 ± 46 | 7 ± 2 | |
| Average | 352 ± 16 | 384 ± 16 | 5 ± 3 | 401 ± 16 | 6 ± 4 |
| CV* | 4.55 | 4.71 | 60 | 4 | 66.67 |
CV = coefficient of variation = (standard deviation/average value) × 100.
Activity concentration of uranium and thorium in low-grade raw phosphate ore and the chemical beneficiation products (concentrate and reject)
| Sample | Activity concentration (Bq/kg) | ||||
|---|---|---|---|---|---|
| U isotopes | Th isotopes | ||||
| 238U | 234U | 232Th | 230Th | 228Th | |
| Raw rock | 293 ± 30 | 260 ± 23 | 9 ± 2 | 267 ± 18 | 7 ± 2 |
| 243 ± 26 | 255 ± 30 | 6 ± 2 | 256 ± 19 | 7 ± 3 | |
| 268 ± 32 | 253 ± 27 | 6 ± 2 | 251 ± 22 | 6 ± 3 | |
| 316 ± 34 | 274 ± 31 | 8 ± 3 | 298 ± 18 | 7 ± 3 | |
| Average | 280 ± 11 | 261 ± 10 | 7 ± 3 | 268 ± 9 | 7 ± 3 |
| CV* | 3.93 | 3.83 | 42.86 | 3.36 | 42.86 |
| Concentrate | 393 ± 52 | 421 ± 54 | 6 ± 2 | 408 ± 58 | 7 ± 2 |
| 508 ± 55 | 534 ± 61 | 8 ± 2 | 601 ± 64 | 8 ± 2 | |
| 436 ± 63 | 416 ± 60 | 10 ± 2 | 430 ± 47 | 8 ± 2 | |
| 422 ± 52 | 414 ± 51 | 7 ± 2 | 437 ± 54 | 6 ± 2 | |
| 390 ± 46 | 417 ± 45 | 7 ± 2 | 382 ± 53 | 6 ± 2 | |
| 440 ± 46** | 457 ± 45 | 7 ± 2 | 446 ± 51 | 6 ± 2 | |
| 490 ± 55** | 486 ± 61 | 11 ± 2 | 430 ± 47 | 8 ± 2 | |
| Average | 440 ± 19 | 449 ± 19 | 8 ± 4 | 448 ± 19 | 7 ± 4 |
| CV* | 4.32 | 4.23 | 50 | 4.24 | 57.14 |
| Reject | 86 ± 7 | 82 ± 7 | 3 ± 2 | 92 ± 8 | 2 ± 1 |
| 97 ± 7 | 91 ± 7 | 4 ± 2 | 103 ± 9 | 5 ± 2 | |
| 62 ± 7 | 68 ± 6 | 3 ± 1 | 75 ± 7 | 3 ± 2 | |
| 113 ± 12 | 96 ± 9 | 5 ± 2 | 101 ± 10 | 4 ± 2 | |
| 78 ± 9 | 91 ± 9 | 4 ± 2 | 88 ± 11 | 5 ± 2 | |
| Average | 87 ± 6 | 86 ± 6 | 4 ± 3 | 92 ± 6 | 4 ± 3 |
| CV* | 6.9 | 6.98 | 75 | 6.52 | 75 |
CV = coefficient of variation = (standard deviation/average value) × 100.
Dust samples.
Activity concentrations of uranium and thorium in the topsoil and waste layers
| Sample | Activity concentration (Bq/kg) | ||||
|---|---|---|---|---|---|
| U isotopes | Th isotopes | ||||
| 238U | 234U | 232Th | 230Th | 228Th | |
| Topsoil | 47 ± 8 | 51 ± 8 | 137 ± 13 | 46 ± 4 | 129 ± 11 |
| 89 ± 9 | 82 ± 9 | 66 ± 8 | 63 ± 7 | 67 ± 7 | |
| 61 ± 8 | 66 ± 10 | 112 ± 10 | 71 ± 8 | 104 ± 10 | |
| 44 ± 7 | 40 ± 7 | 135 ± 11 | 47 ± 6 | 111 ± 10 | |
| 96 ± 10 | 110 ± 10 | 86 ± 9 | 114 ± 10 | 96 ± 9 | |
| 59 ± 7 | 55 ± 7 | 97 ± 10 | 56 ± 7 | 95 ± 10 | |
| Average | 66 ± 7 | 67 ± 7 | 106 ± 8 | 66 ± 6 | 100 ± 8 |
| CV* | 10.61 | 10.45 | 7.55 | 9.1 | 8 |
| Overburden waste layer | 64 ± 6 | 68 ± 7 | 12 ± 3 | 63 ± 5 | 13 ± 3 |
| 52 ± 6 | 57 ± 6 | 9 ± 2 | 48 ± 6 | 9 ± 2 | |
| 55 ± 7 | 62 ± 9 | 7 ± 2 | 56 ± 5 | 7 ± 2 | |
| 85 ± 7 | 86 ± 7 | 9 ± 2 | 82 ± 9 | 8 ± 2 | |
| 71 ± 6 | 76 ± 6 | 12 ± 3 | 77 ± 8 | 11 ± 3 | |
| Average | 65 ± 6 | 70 ± 6 | 10 ± 3 | 65 ± 6 | 10 ± 3 |
| CV* | 9.23 | 8.57 | 30 | 9.23 | 30 |
| Intermediate waste layer | 312 ± 25 | 311 ± 25 | 5 ± 2 | 290 ± 27 | 5 ± 2 |
| 177 ± 18 | 189 ± 19 | 4 ± 2 | 168 ± 11 | 4 ± 2 | |
| 150 ± 14 | 160 ± 16 | 5 ± 2 | 174 ± 12 | 5 ± 2 | |
| 214 ± 21 | 234 ± 23 | 6 ± 2 | 222 ± 13 | 5 ± 2 | |
| 263 ± 22 | 276 ± 26 | 4 ± 2 | 271 ± 15 | 5 ± 3 | |
| Average | 223 ± 10 | 234 ± 10 | 5 ± 3 | 225 ± 9 | 5 ± 3 |
| CV* | 4.48 | 4.27 | 60 | 4 | 60 |
CV = coefficient of variation = (standard deviation/average value) × 100.
Data in Table 2 indicate that the activity concentration of 238U in the high-grade phosphate ore and the beneficiation products (raw rock, phosphate concentrate, and phosphate reject) ranges from 350 Bq/kg to 454 Bq/kg in raw rocks, 399 Bq/kg to 622 Bq/kg in phosphate concentrate, and 327 Bq/kg to 378 Bq/kg in phosphate reject. The average value of activity concentration is 378 ± 19 Bq/kg in raw rocks, 502 ± 25 Bq/kg in phosphate concentrate, and 352 ± 16 Bq/kg in phosphate reject. The values indicate that there is a difference in uranium concentration between the raw ore, concentrate, and reject samples. This trend of values is in agreement with that of P2O5 concentration in the samples indicating that uranium is distributed homogeneously in different components of the phosphate deposits around this area. The observed average 234U/238U activity ratio is slightly greater than unity (about 1.1) in almost all samples, which is commonly found in phosphate deposits of sedimentary origin [13, 15, 25, 26]. The range of activity concentration of 232Th is between 4 Bq/kg and 11 Bq/kg with an average value of 8 ± 6 Bq/kg, which is far below that of 238U. Additionally, 234U and 230Th almost attained radioactive equilibrium. These findings are common in most phosphate deposits of sedimentary origin.
Considering that 238U and 226Ra are in radioactive equilibrium, the average concentrations of 238U and 226Ra in this work are equivalent to 513.1 ± 2.5 Bq/kg and 39.1 ± 1.5 Bq/kg, respectively, as previously reported in phosphate samples collected from a site located northwest of Saudi Arabia [20]. This site is an extension of the phosphate deposits that spread from the north eastern to the north western region in Saudi Arabia.
The feed and products of the low-grade ore and the beneficiation plant were analysed, and their activity concentration results are shown in Table 3. All samples were collected at the mining site, site A. Data in Table 3 indicates that the activity concentration of 238U in the low-grade ore ranges from 243 Bq/kg to 316 Bq/kg, with an average value of 280 ± 11 Bq/kg. This average seems to be lower than that of the high-grade ore of 378 ± 19 Bq/kg. The average activity concentration of 238U in the produced concentrates and rejects ranges from 390 Bq/kg to 508 Bq/kg, and from 62 Bq/kg to 113 Bq/kg, with average values of 440 ± 19 Bq/kg and 87 ± 6 Bq/kg, respectively. The 238U activity concentrations in the concentrates produced from low and high-grade ores (Tables 2 and 3) are equivalent and almost the same. This trend of values is in agreement with that of P2O5 concentration in the samples (concentrate > ore > reject), because of the homogeneous distribution of uranium in the different formation of the phosphate deposits.
Even though the activity concentration of 232Th is far below that of 238U, it is similar to the values observed in the other published studies [13, 26]. The activity concentration of uranium listed in Tables 2 and 3 is less than by one order of magnitude when compared to the proposed IAEA value of 2460 Bq/kg. Therefore, the uranium content in the concentrate samples of Al-Jalamid phosphate deposits cannot be processed for economic benefits as it is not commercially viable. Additionally, the chemical phosphate beneficiation process produces reject with low uranium content as well.
Uranium and thorium were analysed in the topsoil and in the waste layers of the mining area in site A and the activity concentration results are given in Table 4.
The concentration of 238U is 223 ± 10 Bq/kg in the intermediate waste layer, which is higher than that of the topsoil layer with an activity concentration of 66 ± 7 Bq/kg, and the overburden waste layer with an activity concentration of 65 ± 6 Bq/kg even though the intermediate waste layer has the lowest 232Th content at 5 ± 3 Bq/kg. The intermediate waste layer is mostly carbonate with some phosphate and other waste materials. Topsoil material (natural soil) contains the highest 232Th content (106 ± 8 Bq/kg) compared to other layers of phosphate or waste (<11 Bq/kg), which is expected in natural soil. Usually, the concentrations of uranium and thorium are found to be significant in natural soil ranging from 25 Bq/kg to 60 Bq/kg [27]. This generalization is cannot be found in the present samples. Comparing the results of 238U and 232Th with that of normal soil listed in Table 4, it is clear that the samples are enriched in uranium and depleted in thorium. This shows the same trend in the phosphate ore despite the fact that it has lower uranium concentrations.
The 238U content in the topsoil and overburden waste layers is 65 ± 6 Bq/kg although their 232Th content was quite different. Additionally, data in Table 4 shows almost radioactive equilibrium between 238U and 230Th isotopes, both belonging to the uranium series, and between 232Th and 228Th, which are members of thorium series, in the topsoil and waste samples. The average concentration of 232Th is higher in the topsoil samples when compared to the ore and while in the waste samples, it is still high compared to its average concentration in normal soil. This is likely due to mixing of overburden waste and topsoil layers with phosphate and carbonate deposits.
This trend in results is consistent with that reported by [15], which reported that activity concentration of 238U and 232Th ranged from 684 Bq/kg to 2598 Bq/kg and from 0.8 Bq/kg to 2.5 Bq/kg, respectively, in ore samples. The study also reported that activity concentration of 238U and 232Th in ground samples were 16 Bq/kg to 77 Bq/kg and from 7.6 Bq/kg to 43.2 Bq/kg, respectively. The data trend shows lower concentration of 232Th in phosphate rock than in the ground soil, especially topsoil. Uranium and thorium data in the phosphate deposits are more meaningful when the obtained values are compared with other values for the phosphate sediments of the Tethys Sea from other countries. Table 5 summarizes most of these previous reported values.
International values (averages) for 238U and 232Th in different phosphate deposits of the Tethys Sea
| Country | Activity concentration (Bq/kg) | |
|---|---|---|
| 238U | 232Th | |
| Al-Jalamid, Saudi Arabia, this study | 378 ± 125 | 8 ± 6 |
| Saudi Arabia [20] | – | 39 |
| Jordan [28] | – | 2 |
| Jordan [26] | 1300–1850 | – |
| Jordan [4] | – | 19 |
| Israel [28] | – | 11 |
| Israel [26] | 1500–1700 | – |
| Egypt [29] | – | 395 |
| Egypt [18] | – | 19 ± 2 |
| Egypt [30] | 408 | 23.7 |
| Tunisia [28] | – | 29 |
| Algeria [28] | – | 64 |
| Morocco [26] | 1700 | 30 |
| Morocco [26] | 1500 | 30 |
| Morocco [13] | 1700 | 20 |
| Morocco [19] | 1600 | 10 |
| Sahara Western [13] | 900 | 7 |
| USA (Florida) [19] | 1000 | – |
| USA (Florida) [13] | 1500 | 20 |
Values obtained from this work are about 2–3 times lower than those reported by other studies conducted in Morocco and Jordan [13, 19, 26].
More studies were conducted in various parts of the world and the results excluding the Tethys Sea are listed in Table 6.
International values for 238U and 232Th in phosphate sedimentary rocks of different countries excluding the Tethys Sea
| Country | Activity concentration (Bq/kg) | |
|---|---|---|
| 238U | 232Th | |
| Sudan (Kurun) [15] | 684 | 0.83 |
| Sudan (Uro) [15] | 2598 | 2.5 |
| Sudan [31] | – | 7.5 |
| Tanzania (Arusha) [25] | 4641 | 717 |
| South Africa (Togo) [13] | 1300 | 30 |
| South Africa (Togo) [26] | 1300 | 110 |
| USA (Western) [13] | 1000 | 20 |
| USA (South Carolina) [32] | 4800 | – |
| Brazil [33] | – | 3238 |
| Russia (Kola) [13] | 40 | 80 |
| Russia (Kola) [26] | 90 | – |
| Russia (Kola) [14] | 30 | 80 |
| Pakistan (Hazara) [4] | – | 48 |
| Pakistan [34] | 440 | 50 |
| China [26] | 150 | – |
| Finland [12] | 10 | 10 |
Activity concentrations listed in Table 6 indicate that values of Russia, China, and Finland, are far below the values reported for the phosphate deposits of Tethys Sea [12, 13, 14] and are comparable with that in natural soil. But, the values from South Africa, Sudan, Tanzania, and western USA are several times higher [15]. These values exceeded the IAEA threshold of 200 ppm (2460 Bq/kg) for uranium concentration in the phosphate rocks, which qualifies these ores for commercial and economical recovery of uranium. This wide range of uranium and thorium concentrations and 238U to 232Th activity ratios in the different phosphate deposits around the globe are mostly related to differences in the geological nature of the region/mine site.
As for the groundwater samples, they were analysed using the α-spectrometer and the liquid scintillation counter utilizing gross alpha and gross beta methods. Sample results demonstrate increased levels of natural radioactivity that requires treatment procedures before the water can be used for human consumption as reported in [35].
In conclusion, this work estimated radiation exposure and dose levels in Al-Jalamid site in northern Saudi Arabia. The activity concentration of 238U in Al-Jalamid phosphate deposits shows intermediate levels when compared with other reported values for sedimentary phosphate deposits worldwide. However, the activity concentration of the phosphate deposits in the site is of low value when compared to other reported values for the phosphate deposits in the same Tethys Sea area. The study concludes that the average activity concentration of natural uranium in phosphate concentrate is less than the value 2460 Bq/kg as set by IAEA, it cannot be recommended for its potential economic recovery. The levels of 232Th are found to be far below that of uranium in phosphate deposits, and lower than that of natural topsoil. Further, 238U and 232Th are in a state of radioactive equilibrium with their respective decay products. Ground water analysis revealed increased levels of natural radioactivity and must undergo a treatment procedure before it can be used for human consumption. Sample analyses determined that radiation exposure levels in the mining area are safe for personnel working at the mine site.