الفهرس 
LITERATURE REVIEWOnly 14 pages are availabe for public view
 |  | from | 214 |  |  |
| | | from | 214 | | |
Abstract
Gold is one of the most important and valuable metal because it’s unique physical and chemical properties. It is used in high-tech industries and medical applications. Gold occurs in nature as metal or as alloys with other metals. It is usually associated with quartz or pyrite minerals. It is a rare element with a concentration of 0.005 ppm of the earth’s crust. The low-grade deposits contain 3 and 6 ppm, so it is a challenge to meet the commercial grade (3000-4000 ppm).
There are more than one hundred gold deposits are known in the Eastern Desert of Egypt. On the mineralogical basis, three gold- bearing formations are distinguished as sulfide, ferruginous quartzite, and quartz vein formations. The latter is the only source of gold in Egypt. This study was carried out on a sample from the El-Hudi gold deposit. Wadi El-Hudi lies in Egypt’s Eastern Desert, East of Aswan.
The gold-bearing mineral ore composed of a quartz mineral of about 96.5% SiO2. It contains 1.8% Fe2O3, 0.66% Al2O3, and 1% of other impurities with a trace of gold 12 g/t. Thus it must be upgrade to meet the commercial grade of 3000-4000 g/t. The gold grains ranged in their size from 10 to 40 µm. Sericite mineral is common in discrete gold-bearing veins. Pyrite is another main vein mineral that occured as fine to medium-grained.
The crushing and grinding carried out to liberate the gold from its associating gangues. Three feeds prepared by grinding the whole sample to less than 0.8 mm, less than 0.2 mm, and less than 0.075 mm in order to reduce the grinding cost.
Usually, the gold is upgraded using the gravity separation, flotation, cyanidation, or a combination of them. The gravity
concentration processes rely on the principal that the gold has higher specific gravity than its gangues. The gold has a specific gravity of 19.3 g/cm3, and typical ore has a specific gravity of 2.6 g/cm3. The shaking table employed as a conventional gravity process for coarse fractions while the Falcon concentrator as an enhanced gravity separator employed for processing of the fine fraction. The optimum conditions for shaking table were 4° for inclination, 200 g/min for feed flow-rate and 30 L/min for wash water flow-rate. The optimum conditions for Falcon concentrator were 0.5 L/min feed flow-rate, 6 Psi water pressure, 20% solid content and drum rotation speed of 1750 rpm (60 Hz) 176 g-force.
The gravity processing of the ground sample to less than 0.2 mm increased the gold content from 12 to 145 g/t with 78% recovery while; the recovery and content from the ground sample to less than
0.8 mm were 39 g/t and 28%. Although, the best recovery of 95% produced from the ground sample to less than 0.075 mm, its content was moderate (52 ppm).
The conventional flotation technique studied by using single quartz and gold samples. The quartz cannot be floated without being activated while the gold is natural floatable. Pine oil as a frother increased the floatability of both gold and quartz. Potassium butyl xanthate (PBX) which belongs to thiol collectors used as a collector for gold flotation.
The Bio-flotation is based on the selective bacteria adsorption onto either gold or quartz and exerts physicochemical differences to the surface then improves separation. The interfacial region of the mineral substrate and biofilm modified by the presence of microorganisms and their metabolic products. The strain that exhibited potential efficiency of gold separation was later identified as Bacillus cereus.
The gold surface strongly affected as a result of bacterial interaction while; there was no change of zeta potential of quartz mineral after treatment. The treatment of the single quartz with Bacillus Cereus decreased the contact angle of its surface while; it slightly decreased for the gold. Although, both bacteria and the mineral surfaces were negatively charged, the adhesion of similarly charged particles included surface heterogeneity of the bacteria with a polysaccharide envelope, hydroxyl, hydrophobic and ionic moieties.
It is well known that proteins can bind to the gold metal surface either through free amine groups or cysteine residues in the proteins; therefore, stabilization of the gold particles by surface- bound proteins is a possible. The FTIR showed that the Bacillus Cereus was adsorbed on the surface of gold particles through proteins while; there was no change of quartz spectrum. SEM images, proved the selective adhesion of Bacillus cereus cells on the gold surface rather than quartz mineral surface.
It was suggested that the attachment and agglomeration of bacterial cells to solid surfaces provided a stable growth environment for the cells and enhanced catalytic functions through the localization of cells into biofilms. The excretion of an extracellular polymeric substance composed of the macromolecules as polysaccharides, proteins and lipids promoted the development of the biofilms on the mineral surfaces. It is suggested that hydrogen bond formed between bacteria and mineral surface through the hydroxyl groups of the polysaccharide of metabolite. The total charges and the spacing between these charged groups control the bio-molecules interaction with the solid surfaces.
The floatability of gold and quartz in presence of potassium butyl xanthate (PBX), pine oil and Bacillus Cereus showed that the floatability was better than that achieved by treatment with Bacillus Cereus first. The maximum floatability difference (90%) occurred in the presence of 1×108 of bacteria cells. The conditioning with Bacillus Cereus prior PBX may enhance the xanthate-gold adsorption while it decreased PBX-quartz adsorption.
The flotation of binary mixture which composed of 0.05% gold with quartz in the presence of 108 bacteria cells, 5×103 M potassium butyl xanthate and 103 M pine oil at pH 7 and 35C for 10 min, a concentrate of 10612 ppm (g/t) gold with 95% recovery was obtained.
The pre-concentrate of 145 g/t which produced from the gravity separation employed as a feed for bio-flotation. At the other optimized parameters of temperature, pH and conditioning time, the gold content decreased with increasing the pine oil dose while the recovery increased. The gold content decreased with increasing PBX dose while the recovery increased.
A concentrate contains 3257 g/t of the gold and 90.2% recovery is obtained by using 80g/t potassium butyl xanthate, 80g/t pine oil and 1.5×108 bacteria cells at pH 7 and 35C for 10 min. The final concentrate meets the commercial grade is range between 3000 - 4000 g/t.