Copper is leached in a staged reactor system using nitric and sulfuric acids at 105°C. Iron is faraway from the pregnant liquor as a jarosite and cathode copper is electrowon instantly from the purified pregnant liquor. The spent electrolyte is recycled. Nitrogen oxides advanced from the leach are reconverted to nitric acid and are additionally recycled. Sulfur is rejected as S°, jarosite and gypsum. The method is very versatile and may get well valuable metals, molybdenum and zinc, when these parts are current.
Nitric Acid Processes
The use of nitric acid and combined nitric-sulfuric acids as a hydrometallurgical course of leach medium has been recognized because the early 1900’s. Current work, based mostly on the nitric-sulfuric leach methods has been described by Prater et al and Bjorling et al for copper concentrates and by Ouellet et al for nickel focus. In these processes, oxidant is provided by the breakdown of nitrate ion in acid answer.
1½ MeS + Four H+ + NO3 → 1½Me++ + S° + 2H2O + NO↑
S° + 2 NO3- → SO4= + 2NO↑, ΔH = -70 kcal/gmol
2NO + O2 → 2NO2, ΔH = -27.6 kcal/gmol
H2O + 3NO2 → 2HNO3 + NO, ΔH = -32.9 kcal/gmol
The regenerated nitric acid is returned to leaching together with sulfuric acid regenerated in the course of the profitable of the copper.
Course of Description
Feed focus is floor to an optimum particle measurement for the leaching and strong/liquid separation necessities of the leach step which follows. Within the leach step, primarily all of the metallic sulfides within the focus are dissolved whereas decreasing the focus of nitrate and free acidity within the pregnant liquor to low ranges. Nitric oxide from the leach reactors is oxidized within the nitric acid restoration step and absorbed to regenerate HNO3. If molybdenum is current, it’s utterly solubilized through the leaching and is faraway from the pregnant liquor within the subsequent step by liquid ion trade. The pregnant liquor is then fed to the nitrate removing autoclave the place residual nitrate is lowered to NO, and Fe+² is concurrently oxidized to Fe+³. Filtrate from the iron removing step is contacted with recycle cement copper to precipitate selenium and the purified liquor is fed to electrowinning the place cathode grade copper is recovered.
A mixed technical and financial evaluation of the method based mostly on the laboratory outcomes adopted. It confirmed that the method had about 10% decrease capital necessities and about the identical working prices as in comparison with steady smelting processes and 10 to 20% decrease capital and working prices as in comparison with developed hydrometallurgical processes.
A second part of laboratory work was then initiated based mostly on an built-in bench scale operation of the method utilizing Kennecott focus. It employed each step of the method proven in Determine 1 apart from nitric acid restoration and for purge copper and zinc restoration. Concentrate was floor in a batch mill. Options have been made as much as precalculated concentrations of the key constituents and operations have been steady for every course of step.
Course of Parameters
Grinding of ore provides considerably elevated preliminary leach charges which reduces reactor residence time and improves management of the primary stage leach reactor. Overgrinding can create solid-liquid separation issues. A restrict of 95% minus 325 mesh was discovered to be a suitable compromise of leach charges and management traits with filtration and settling unit areas. The leach step is designed to solubilize 99% or extra of the copper contained within the focus. The leach is carried out in a three-stage counter present association with solid-liquid separation between every stage.
The preliminary laboratory work which led to choice of leach circumstances demonstrated that leach charges improve with focus of NO3-, H+ and partial strain of NO. The response mechanism is complicated and for the step 1 reactor, cautious management of focus feed is required to maximise the utilization NO3- and H+. Response charges additionally improve with temperature however have a sensible higher restrict imposed by the 120°C melting level of sulfur. Nevertheless, acceptably quick response charges have been discovered for the 90 to 105°C vary which prevented strain reactors and allowed the use of brick lined vessels with chrome steel trim.
Removing of selenium is required to satisfy cathode copper high quality necessities. Selenium is partly rejected within the leach tails and partly precipitated with jarosite within the nitrate and iron removing steps. Roughly 40% of the selenium contained within the focus stays in answer after iron removing and is precipitated by response with recycle cement copper. The principal reactions are:
2Fe+³ Cu° → 2Fe+² + Cu+²
H2SeO3 + 4Cu° + 4H+ → Cu2SE + 2Cu+² + 3H2O
At a temperature of 70°C, response was discovered to be speedy requiring a holdup time of 10 minutes to finish the removing of the selenium. It will be important that acid content material be maintained above 30 g/l.
Typical electrowinning at roughly 200 amps/m² of cathode is used for restoration of cathode grade copper. Copper content material is decreased from a feed degree of 80 g/l to an effluent focus of 30 g/l. Important impurities are faraway from electrolyte previous to electrowinning. Cathode lead content material is managed by use of cobalt addition and by limiting present density to insure top quality deposits.
The bulk of the sulfur transformed to sulfate within the leach step is eliminated as jarosite within the iron removing step. The stability seems as extra sulfuric acid and is faraway from the copper depleted electrolyte by response with limestone to precipitate gypsum. The response is carried out in a crystallization reactor utilizing a holdup time of roughly 2 hours at a temperature of 70°C.
A evaluate of the power necessities (on a gasoline foundation) for hydro and pyrometallurgical processes exhibits that, generally, the hydrometallurgical processes require extra power than the pyrometallurgical processes. The nitric-sulfuric leach course of requires about 2.5 occasions as a lot power as flash smelting. A big portion of the extra power is consumed within the copper electrowinning step. A typical flash smelter requires about 15,000 Btu/lb to course of focus to cathode, whereas, the nitric-sulfuric leach course of requires about 35,000 Btu/lb. Electrowinning energy accounts for 45% of the whole.
Additional improvement of the Nitric-Sulfuric Leach (NSL) course of has led to an improved design of the leach and nitric acid restoration steps. NO produced within the leach step is reacted with oxygen, regenerating NO2, which is sparged instantly into the leach reactor. Leaching is simplified to 2 steps and the nitric acid plant is changed with an oxidation recycle fuel system. Laboratory work substantiated the idea and led to choice of working circumstances.
Within the seek for a hydrometallurgical course of to switch smelting for the manufacturing of copper, the nitric-sulfuric leach course of described by Brennecke et al was developed instead which meets all the necessities for processing most copper concentrates. The key technical benefits of the method are reviewed under.
- It makes use of a strong and regenerable oxidant, nitric acid, to utterly (99%) leach copper and different heavy metals from all sulfide concentrates.
- It takes benefit of sulfuric acid course of traits by rejecting iron as jarosite and recovering cathode copper immediately from purified pregnant liquor.
- By-product metals are recoverable.
- Course of wastes are secure and must be suitable with disposal necessities.
Financial evaluation confirmed that the method had comparable capital prices ($2400/AT) and decrease working prices (34¢/lb Cu) as in comparison with different hydrometallurgical processes. It in contrast favorably with flash smelting for crops smaller than about 50,000 TPY of copper; nevertheless, the prices weren’t sufficiently engaging for NSL to exchange smelting until another crucial resembling environmental constraints, low grade concentrates, ores not amenable to smelting, or distinctive by-product restoration functionality would justify the danger of commercializing the brand new know-how.
NO2 Leach Course of
Within the revised leach-NO restoration course of, NO2 is sparged instantly into the leach reactor the place it contacts a slurry of mineral focus in combined nitric-sulfuric acid leach liquor. The mineral is leached in accordance with the stoichiometry proven in response.
The leach is carried out in a 2-stage (as in comparison with the unique Three-stage) countercurrent association with solid-liquid separation following every leach reactor. As with the unique NSL course of, the aim of the first stage leach is to react recent focus with 2nd stage liquor to supply a pregnant liquor with lower than 10 g/l every of NO3- and H2SO4.
When NO2 is sparged into an aqueous liquor, it reversibly absorbs and reacts in line with the next reactions.
Leaching was carried out in a 1.52, resin kettle reactor usually utilizing 600 ml of leach answer and 100 g of focus per experiment. Fuel flows out and in have been monitored utilizing Matheson digital mass circulate meters. An automatic fuel chromatograph and a chemi-luminescent NOx analyzer have been used to find out NO, NO2, O2 or Ar, N2, N2O and H2O within the offgas.
The outcomes confirmed that absorption was not depending on acid focus over the vary studied (zero to 74 gms HNO3/liter). Absorption fee of Three.zero x 10 -Four g/min/ml answer for run 9-1 is roughly half that required for a business reactor. In a second run, 9-2, 150% extra oxygen was added with the NO2 and quantitative absorption resulted. The info indicated that NO2 absorption wouldn’t be an issue both within the check work or in a business reactor the place reactor depth is favorable to absorption effectivity.
Concentrate leaching experiments have been carried out in a fashion analogous to the NO2 absorption checks besides that NO2, and in some instances NO2+O2, was sparged into an agitated slurry quite than into clear answer.
When NO2+O2 was used because the oxidant, a modest improve in sulfide oxidation charges was noticed. This most probably arises from a rise within the partial strain of NO2 within the reactor. A rise in last [HNO3] can also be noticed, however most of the leach fee enhancement happens whereas [NO3-] is sort of low.
- Leaching at 90°C was simpler than leaching at 100° C.
- Growing preliminary acidity from 20 to 100 g/l H2SO4 inhibits sulfide leaching to a small however vital extent.
- Beneath most circumstances, the improved leaching charges within the presence of NO2 + O2 aren’t very delicate to modifications within the NO2/O2 ratio.
In air agitation electrowinning, air induced convection of electrolyte will increase the diffusion price of dissolved copper to the cathode floor and permits use of larger present densities whereas producing bodily sound deposits. The excessive fee of convection additionally sweeps impurities from the face of the cathode such that positive solids impurities don’t grow to be occluded within the deposit. The result’s a bodily sound cathode of excessive purity.
The Kennecott air agitation course of makes use of shut anode-cathode spacing (2.5 cm) each to extend convection per unit quantity of air and to scale back cell voltage. Cathode blanks of chrome steel or titanium are sufficiently bigger than the anode in order to remove edge deposits and the necessity for plastic edge strips on the clean.
Air agitation electrowinning was initially examined in a liberator cell at our Baltimore refinery alongside cathodes with out air agitation.