الفهرس 
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Abstract
Lanthanides:
Rare earth elements (REEs)are the seventeen elements in the periodic table, including fifteen elements with atomic numbers from 57 to 71, i.e., from lanthanum to lutetium (“called lanthanides”), plus scandium (21) and yttrium (39).All of which, except promethium, occur in nature[1]. The rare earth elements, being chemically similar to one another, invariably occur together in the minerals and behave as a single chemical entity. Contrary their name, rare earth elements are not rare at all. They are found in 200 minerals and the earth crust contains moreof them [2] .The first part of their name, the word rare, relates to the considerable difficulties in separating one rare earth element from another because of their close similarity in chemical and physical properties, the second part earths, an old chemical term for oxides, was applied to as they were first identified as rare earth oxides. These elements are in fact fairly abundant in nature [3]. Cerium is the 26th most abundant element in the earth’s crust, neodymium is more abundant than gold and even thulium (the least common naturally –occurring lanthanide) is more abundant than iodine [4].
1.1.1 Abundance and Distribution of Lanthanide Elements In Minerals:
Table 1.1 presents the abundance of the lanthanides in the earth’s crust and in the solar system as a whole. (Although not in the same units, the values in each list are internally consistent.)
Table 1.1:Abundance of lanthanides:
Ln Elements Crust(ppm) Solar System*
La 35 4.5
Ce 66 1.2
Pr 9.1 1.7
Nd 40 8.5
Sm 7 2.5
Eu 2.1 1.0
Gd 6.1 3.3
Tb 1.2 0.6
Dy 4.5 3.9
Ho 1.3 0.9
Er 3.5 2.5
Tm 0.5 0.4
Yb 3.1 2.4
Lu 0.8 0.4
*Solar System with respect to 107 atoms Si
Two patterns emerge from these data.Firstly, that the lighter lanthanides are more abundant than the heavier ones.Secondly, that the elements with even atomic number are moreabundant than those with odd atomic number so lanthanides obey the Oddo-Harkins rule[5].Overall, cerium, the most abundant lanthanide.
In nature the rare earths do not occur in the elemental state, nor do they occur as individual rare earth compounds. The rare earths, scattered dilutely in the earth’s crust, occur as mixtures in many rock formations and are present in amounts ranging from 10 to 300 ppm[6] .The total REO contained in category world rare earth resources amounts to 93.4 million metric tons. Only 7% of these total REO tonnages are contained in placer deposits and 93% are in hard rock. Dividing in terms of minerals, 20% of the total REO tonnage is contained in monazite (present in both placer and hard rock), 77% in bastinasite, and 3% in other rare earth minerals, about 95% of all world rare earth resources occur in just three minerals: bastinasite, monazite, xenotime .
Monazite was the major rare earth resource from the beginning of the industry until 1965. Thereafter bastinasite production equaled or exceeded monazite production. At present, bastinasite is the world’s major source of rare earths. It constituted 62% of world output of rare earth minerals in 1989 and the proportion rose to very high levels starting in 1994–1995[7]. Incidentally, no bastinasite was produced before the 1950 .
The rare earth element distribution in bastinasite and monazite qualitatively reflect the relative abundance of rare earths in nature. The proportion of heavier rare earths (Sm–Lu, Y) content in bastinasite is considerably less than in monazite, which itself is less than that anticipated from crustal abundance data. Heavier rare earths are present in higher proportion in xenotime. While the minerals bastinasite and monazite are abundantly available in nature, xenotime occurrence and availability is small in comparison. Hence the rare earth composition in xenotime does not have much impact on overall rare earth availability.
1.1.1.i Monazite:
The mineral monazite is a phosphate, mainly of the cerium group rare earths and thorium [8]. Monazite is found in many geological environments. Monazite occurs widespread as a common accessory mineral in pegmatites, granites and gneisses. It frequently occurs as a detrital mineral in placer deposits, principally in river and beach sands. Large commercial detrital deposits are found in Australia, USA, India, Brazil and Egypt. Monazite is associated with other heavy minerals such as ilmenite, magnetite, zircon, and rutile. The monazite content of the black sands is 1.06%.[9].
Monazite-bearing heavy mineral sand deposits are found in large quantities principally in Australia, Brazil, China, India, Malaysia, South Africa, and the United States. The rare earth content and individual rare earth element distribution in monazite are variable, as is its thorium content, depending on the location [10] . Usually monazite contains about 70% REO, and the rare earth fraction is constituted by 20 to 30% Ce2O3; 10 to 40% La2O3; significant amounts of neodymium, praseodymium, and samarium; and lesser amounts of dysprosium, erbium, and holmium. Yttrium content may vary from a trace to ~5% Y2O3, and thorium content of 4 to 12% is common. Some amount of uranium is also present in monazite [11].
Monazite was a declining source of rare earths with production practically coming to a halt in Australia—the principal monazite producer in the world [7]. The declining importance of monazite as a raw material has been due to growing supplies ofbastinasite from China and also due to problems associated with the disposal of the radioactive element thorium contained in monazite. There is similarly a decline in the production ofxenotime, as well, from the main producer Malaysia. The decreased supply of traditional minerals enhanced the importance of the “other ores” and their contribution to world production of mid and heavy rare earths.
There are actually at least four different kinds of monazite,depending on relative elemental composition of the mineral:
monazite-Ce (Ce, La, Pr, Nd, Th, Y)PO4
monazite-La (La, Ce, Nd, Pr)PO4
monazite-Nd (Nd, La, Ce, Pr)PO4
monazite-Pr (Pr, Nd, Ce, La)PO4
1.1.1.ii Bastinasite:
The mineral bastinasite is a fluorocarbonate of the cerium group rare earth metals and hardly contains any thorium. As regards the geological environment, bastinasite is found in vein deposits, contact metamorphic zones, and pegmatites. Bastinasite occurs as veins or dissemination in a complex of carbonate-silicate rocks, occurring with and related to alkaline intrusives, for example, in California.
Bastinasite occurs in quartz veins [12] that cut micaceousschists and quartzite, in Burundi. It is in fluorite-bearing veins and breccia filings in Permian sandstone .The rare earth content of bastinasite is approximately 70% REO, mostly of the lighter elements. Bastinasite is a primary source of light REO in the enormous deposit in Bayan Obo in China (800 million metric tons; 6% REO) and at Mountain Pass, Californiain the U.S. (3.3 million metric tons; 7.7% REO). In addition, bastinasite is also the main REO mineral at Brockman in Australia, Pocos de Caldas in Brazil, Thor Lake in Canada, and Karonge in Burundi. Bastinasite is chemically susceptible to weathering and this causes REO to dissolve and combine with available phosphates..The rare earth content ofbastinasite concentrate is constituted by as much as 99% of light rare earths (La–Nd), almost no heavy rare earths, and very little yttrium .
1.1.1.iii Xenotime:
Xenotime is an yttrium phosphate containing about 67% REO, mostly of the heavier elements [13]. Xenotime can appear as inclusions in the garnets and in the rock matrix, and reacts out as garnet grows[14].Having undergone a weathering, transportation, and concentration process similar to that of monazite, xenotime co-occurs with it in placer deposits, but such deposits are relatively few. Usually the content of xenotime may range from 0.5 to about 5% of the monazite present. Xenotime occurs in the placer cassiterite deposits in Malaysia and in certain Australian heavy mineral.
Table 1.2: Elemental Proportions of Rare Earth-Content of Minerals
% of Ln as Monazite Xenotime Bastinasite
Y 3 61 0.1
La 22 0.5 32
Ce 45 5 49
Pr 5 0.7 4.4
Nd 17 2.2 13.5
Sm 4 1.9 0.5
Eu 0.1 0.1 0.1
Gd 2 4 0.3
Tb 0.2 1 -
Dy 1 8.6 -
Ho 0.1 2 -
Er 0.4 5.4 0.1
Tm - 0.9 -
Yb 0.1 6.2 -
Lu - 0.4 -
Xenotime occurs also in the placer cassiterite deposits of Indonesia and Thailand and in the heavy mineral sands of China, as well as in the alluvial tin mines of Brazil [15] . In addition to the three major minerals, there are several other rare earth minerals that are, or could be, of importance in the economic recovery of rare earths
The crystals of xenotime are similar to zircon and can easily be confused with duller lustered , less transparent samples of zircon However, the cleavage and the softness of xenotime are sufficient to distinguish them.
Finally, The minerals monazite ((RE,Th)PO4) and bastinasite (RE(CO3)F) are the main sources of RE metals in nature [7]. The lighter rare earth metals (e.g., La, Ce, Pr, Nd) occur in both minerals; however, bastinasite tends to be poor in the heavier rare earths (e.g., Ho, Er, Tm, Yb, Lu, Y) as shown in Table 1.2.The most common metals in both ores are (in order to decreasing abundance) cerium, lanthanum, neodymium and praseodymium, with monazite also containing up to 10% ThO2 as well as smaller quantities of the later lanthanides.
1.1.2 General Properties of Lanthanides:
Lanthanides have electron configuration [54Xe],6s2,5d1,4f n , where n varying from 0 [for La(III)] to 14 [for Lu(III)]. Steady decrease in the size of atoms and ions with the increase in atomic number as the lanthanide series is crossed as shown in Table 1.3from lanthanum to lutetium is called lanthanides contraction [16]. Lanthanum has the greatest and lutetium the smallest radius. The cause of the contraction is stated to be the imperfect shielding of one electron by another in the same subshell. As one proceeds from lanthanum to lutetium, both the nuclear charge and the number of 4f electrons increase by one at each element. Owing to the shape of the orbitals, the shielding of one 4f electron by another is very imperfect [17].Atomic nucleus is poorly shielded by the highly directional 4f electrons and, as a result, at each increase of the atomic number the effective nuclear charge experienced by the 4f electron increases, resulting in a reduction in the size or contraction of the entire 4f shell. With successive increase in atomic number, such contractions accumulate and result in the steady decrease in size.
Table 1.3: Atomic and Ionic Radii Of Lanthanides( )
Ln Elements Atomic Radii Ionic Radii
La 1.88 1.06
Ce 1.82 1.03
Pr 1.83 1.01
Nd 1.82 0.99
Sm 1.80 0.96
Eu 2.04 0.95
Gd 1.80 0.94
Tb 1.78 0.92
Dy 1.77 0.91
Ho 1.77 0.89
Er 1.76 0.88
Tm 1.75 0.87
Yb 1.94 0.86
Lu 1.73 0.85
Consequences of Lanthanide Contraction:
Basicity of lanthanide ions decreases from La to Lu.
La3+>Ce3+>Pr3+>Nd3+>Pm3+>Sm3+>Eu3+>Gd3+>Tb3+>Dy3+>Ho3+>Y3+>Er3+>Tm3+>Yb3+>Lu3+.
These elements occur together in natural minerals and aredifficult to separate.
Theseelements are chemically classified by their ionic radii in threegroups: (a) light REEs from La to Pr, (b) medium REEs from Nd toGd, and (c) heavy REEs from Tb to Lu [18] . Since promethium is unstable and not well characterized.
Lanthanides generally favor tripositive oxidation state, thus being highly electropositive; their compounds are predominantly ionic in nature.Nevertheless, tetravalent and divalent forms exist as well, yet only Ce4+ and Eu2+ are stable enough to persist in aqueous solution, whereas their trivalent forms still present higher stability [19].
All lanthanides closely resemble lanthanum .Like most metals, the lanthanides have a bright silvery appearance. Five of the elements (lanthanum, cerium, praseodymium, neodymium, and europium) are very reactive. When exposed to air, they react with oxygen to form an oxide coating that tarnishes the surface. For this reason these metals are stored under mineral oil. The remainders of the lanthanides are not as reactive, and some (gadolinium and lutetium) retain their silvery metallic appearance for a long time.
When contaminated with nonmetals, such as oxygen or nitrogen, the lanthanides become brittle. They also corrode more easily if contaminated with other metals, such as calcium. Their melting points range from about 819°C (1,506°F) for ytterbium to about 1,663°C (3,025°F) for lutetium. The lanthanides form alloys (mixtures) with many other metals, and these alloys exhibit a wide range of physical properties.
The lanthanides react slowly with cold water and more rapidly with hot water to form hydrogen gas. They burn readily in air to form oxides. They also form compounds with many nonmetals, such as hydrogen, fluorine, phosphorous, sulfur, and chlorine.
1.1.3 Lanthanides in Egypt:
In Egypt various activities are going to launch a national strategy for developing of both separation and subsequent production of lanthanides to meet the growing demand to get individual lanthanide suitably used in different industrial activities.
Abu Tartur is the most important locality in the New Valley,Egypt; it is situated in the Western Desert 50 km to thewest of El-kharga town and 275 km from Assuite phosphate deposit of the abu-Tartur plateau, Western Desert, Egypt. Extensive geological prospection and exploration, including field mapping, core drilling and sampling have been carried out in an area of 112 square kilometers of the plateau.
One of the most important characteristics of this deposit is its relatively high content of rare earth elements. Different investigations have contributed to the characterization of this deposit with respect to its lanthanides content [20].Inorder to achieve better information about the lanthanides distribution in the huge deposit, more data are required[21].
Egyptian Geological Survey and Mining Authority (EGSMA) planned a long-term program to assess the black sand deposits along the Mediterranean Cost of Egypt from Rashid to Rafah [22].The results achieved up to now can be briefly summarized as follows:
The reserves of black sands in the area investigated amount to 565 million tons in the Rashid area and 45 million tons in the Arish area representing concentrate reserves of 9.3 and 1.4 million tons in the two areas, respectively.
Ilmenite is the main constituent among the economic minerals in the areas investigated .Other economic minerals in decreasing order of abundance are magnetite, hematite, zircon, garnet, monazite and rutile.
The lanthanide ore monazite represents one of the most important economic minerals found in the black sands.
Preliminary investigations on the abundance and distribution patterns of REE in Abu-Tarturphosphorites have been carried out previously by researchers on a limited number of samples. One of the objectives of investigating the REE abundance in phosphorite deposits in general is the possibility of their extraction as byproducts during phosphoric acid and fertilizer manufacture along the Mediterranean Cost of Egypt from Rashid to Rafah . This program started in the area east of the Rashid where a 18-km plot was studied followed by a similar area west of Arish. Now, an area of 20-km at Rommana is under investigation.
In Egypt, black sands of economic importance are deposited in large stretches along the Mediterranean Cost extending from Abu Qir Bay, east of Alexandria to Rafah, the eastern boarder of Egypt. These black sands are derived from the upper reaches of the Nile, mostly from the Ethiopia Volcanic highland and Sudan and are deposited at the river mouths where the flow velocity is reduced. For several years, various studies have been carried out by different investigators and organizations to assess the economic mineral constituent of these black sands in different areas [23].The most attention was given to the Rashid (Rosetta) area as it has the richest occurrence of black sands with the highest concentration of heavy minerals. Later, other areas such as the Arish and Rommana areas were investigated.
1.1.4 Lanthanides in the World:
World demand for rare earth elements is estimated at 134,000 tons per year, with global production around 124,000 tons annually. The difference is covered by above – ground stocks or inventories. World demand is projected to rise to 180,000 tons annually by 2012, while it is unlikely that new mine output will close the gap in the short term .By 2014, global demand for rare earth elements may exceed 200,000 tons per year .China’s output may reach 160,000 tons per year [24].
As shown in Fig 1.1 , while world demand continues to climb, U.S. demand for rare earth elements is also projected to rise, according to the USGS Commodity