Mining and Metallurgy

Caro's Acid 

The Caro’s Acid (H2SO5) is a liquid oxidant produced by the mixture of concentrated sulfuric Acid (H2SO4) and hydrogen peroxide (H2O2). In the cyanides oxidation, it reacts quickly and efficiently – both in clarified wastewater and in pulps.

Industrial Utilization

Caro’s Acid has been used in oxidant lixiviation of minerals in 1989, in Australia. Since 1997 it started being used also as in the detoxification of wastewaters containing cyanides, being already adopted in plants of gold extraction in France, Peru and United States. 

Generation of Caro’s Acid in situ 

The production of Caro’s Acid occurs according to the following reaction: 

H₂SO₄ + H₂O₂ → H₂SO₅ + H₂O

This reaction is essentially instantaneous and strongly exothermic, so that, in conditions in which the two reagents are mixed at ambient temperature, the resulting mixture containing Caro’s Acid reaches a temperature in the 110-120ºC (230-248 ºF) range.
 
Caro’s Acid is produced in the same application place, in a generator patented by Solvay/Peróxidos which is simple, safe, compact, versatile and doesn’t require any refrigeration. Caro’s Acid produced is fed hot and concentrated directly to the wastewater treatment tank.
 
Caro’s Acid is a potent oxidant, that must be used immediately after its generation, not being advised its storage, having in mind that, with time, it decomposes in:

H₂SO₄ + ½ O₂ (g)

Boletim Tecnico Mineria

Leaching of Gold and Silver Ores

In the heap leaching of gold ores, it may be advantageous to supplement the amount of dissolved oxygen in the leaching solution during percolation of the heaps. 

Depending on the type of ore and/or the altitude above sea level where the operation is conducted, it may be convenient to associate the addition of small doses of hydrogen peroxide to the agitated leaching solution or calcium peroxide to the heap. These will decompose during percolation, releasing the extra oxygen, necessary to the reaction of dissolution of the metal.

The main advantage that may be obtained by the supplementation of oxygen in cyanidation is the increased productivity (up to 50% reduction in leaching time). In addition, an increase in gold recovery, depending of the ore, may also be expected.

Other benefits that may be obtained through supplemental oxygenation with calcium or hydrogen peroxide are: 

• Maintenance of high gold leaching reaction rates in winter, by compensating the effect of low temperatures with extra oxidation.
• Maintenance of high dissolution rates of gold dissolution in the lower layers of heaps, or increase in height of leaching heaps.
• Reduction of the volume of cyanidation solution in the leaching circuit with consequent economy in water, cyanide consumption, and costs of discharging excess process waters. In addition, increase concentration of gold in the pregnant solution.

Agitated Leaching

In the agitated leaching of some types of ores, it may be advantageous to add a small dosage of hydrogen peroxide in the pulp preparation or conditioning step (pre-lime) before cyanidation.

In plants where this is being applied, the pre-treatment with H2O2 caused increase in gold recovery. It is believed that hydrogen peroxide may be acting in opening sulfide matrices or oxidizing organic matter that may be inhibiting the access of cyanide to the gold particles in the ore.

In addition, it is expected that addition of a small dosage of hydrogen peroxide or calcium peroxide in cyanidation may be beneficial to increase leaching speed and recovery of gold, especially in plants situated at high altitudes, where oxygen solubility in water is lower than that at sea level.

Effluents containing arsenic can be detoxified using hydrogen peroxide and iron sulphate.

The reactions occur as following:

HAsO₂ + H₂O₂ → H₃AsO₄ 

2 Fe²⁺ + H₂O₂ + 2 H⁺ → 2 Fe³⁺ + 2 H₂O

The need of an oxidization step arises from the fact that As V compounds are much more insoluble than those of As III.

Arsenic can be efficiently removed from aqueous solutions by precipitation of ferric arsenate slimes in open agitated tanks.

Fe³⁺ + H₃AsO₄ → FeAsO₄ (s) + 3 H⁺

Or, if there isn't enough Fe3+:

3 Fe²⁺ + 2 H₃AsO₄ → Fe₃(AsO₄)₂ (s) + 6 H⁺

Alternatevelly or additionally, the additions of Ca2+ ions (as in the addition slaked lime) to the wasterwater being treated will propitiate the occurrence of the reaction of formation of Calcium arsenate, also contributing to the removal of this metal:

3 Ca²⁺ + 2 H₃AsO₄ → Ca₃(AsO₄)₂ (s) + 6 H⁺ 

Wastewaters containing cyanides are generated mainly by hydrometallurgical operations of gold and silver extraction, galvanic industries, production of nitrated compounds, steel mills and oil refineries.

The current treatment options of include the use of an oxidizing agent that converts cyanide into the much less toxic cyanate. This one, in turn, hydrolyzes spontaneously, forming as final products of this operation ions of carbonate/ bicarbonate and ammonia/ ammonium.

The hydrogen peroxide is indicated for treatment of clarified wastewaters, which already present less than 10 mg/l of dissolved copper. The Cu2+ ion acts as a potent reaction catalyst. In case it’s insufficient, a CuSO4 solution is added to the treatment – The added Cu2+ precipitates itself at the end of the reaction as Cu(OH)2.

In case of wastewaters in pulp, the best oxidant indication  is Caro’s Acid (H2SO5), that, for having a very quick action, eliminating the need of catalyst addition.

Process Chemistry 

With hydrogen peroxide, the action of cyanides occurs according to the following reaction: 

CN^- + H₂O₂ → CNO^- + H₂O

When using Caro's acid, sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) are mixed together to generate H2SO5:  

H₂SO₄ + H₂O₂ → H₂SO₅ + H₂O

The action of Caro’s acid over free cyanide produces the cyanate ion. It hydrolyses spontaneously to carbonate and ammonium ions. The reaction must be made in alkaline medium to avoid generation of hydrogen cyanide (HCN), usually by adding slaked lime or caustic soda:

CN^- + H₂SO₅ + 2 OH^- → CNO^- + 2 H₂O + SO₄^2-

CNO^- + 2 H₂O → NH4⁺ +CO₃^2-

In analogous reactions to the free cyanide, metallic cyano-complexes, moderably stable, such as copper, zinc and nickel, are oxidized, generating, besides carbonate and ammonium, precipitated hydroxides. This way, it is also achieved a removal of heavy metals from the effluent:

M(CN)_n^(2-n) + n H₂O₂ → n CNO^- + M²⁺ + n H₂O

n CNO^- + M²⁺ + (2n+2) H₂O → n CO₃^2- + n NH4⁺+ M(OH)₂(s) + 2 H⁺

where M = Cu, Zn, Ni, etc. 

In the case of iron-cyanide complexes, the removal is not by oxidation, but by precipitating insoluble complexes with  ions of transition metals:
 

Fe(CN)₆^4- + 2 M²⁺ → M₂Fe(CN)₆(s)

where M = Fe, Cu, Zn, Ni, etc.  

Process solutions and wastewaters are frequently contaminated with iron ions. Even tough the removal of this contaminant by precipitation is simple and well-known; it is convenient to assure that all the dissolved iron is in the 3+ oxidizing state, so that the precipitation is efficient from a pH starting on 3.5, with low base consumption.

The removal of iron by oxidation and precipitation with hydrogen peroxide is very fast, according to the following equation:

Fe²⁺ + ½ H₂O₂ + 2 OH^- → Fe(OH)₃ (s

Waters and wastewaters containing selenium are efficiently treated with hydrogen peroxide and Calcium hydroxide (slaked lime), leading to precipitation of calcium selenate, according to the following reaction:

Ca²⁺ + SeO₃^2- + H₂O₂ → CaSeO₄(s) + H₂O

Alternatively or additionally, the possibility of formation of precipitates of zinc and manganese selenates should be considered if the effluents treated with H2O2 contain these metals.

Process solutions and wastewaters from metallurgical processes are frequently contaminated with manganese ions.

The removal is eased by oxidizing the metal from the 2+ state to the 4+ state, which allows to reach, in a pH up to 9, a high level of precipitation. The same efficiency of hydroxide (Mn(OH)2) precipitation would only be possible with pH 10 or more.

The reaction occurs as following:

Mn2+ + H2O2 + 2 OH- → MnO2 (s) + 2 H2O

The chemical treatment of pieces of copper, brass and bronze is usually made to remove scales and superficial oxides that are formed during the thermal process to which the base metal is subjected in fabrication, and in many cases to give shine and uniformity. 

The traditional chemical process consists most of the times in a preliminary degreasing of the metallic surface, followed by pickling and oxidative polishes based on chromic acid, dichromates or nitric acid.

Our product METALPER® is used in the steps of pickling and pre-oxidative polishing, with great advantages over traditional methods.

METALPER® contains hydrogen peroxide, stabilizers and special complexants for treatment of copper leagues. Hydrogen peroxide is a powerful oxidizing agent, with oxidizing potential superior to those of dichromate, chromic acid and nitric acid.
 
Advantages of using hydrogen peroxide 

• Excellent quality of the treated surfaces
• Generation of effluents free of chrome and cyanide salts, considered harmful to the residual waters.
• Non generation of nitrous gases characteristics of the use of nitric acid
• Ease of handling and processing

The chemical treatment of the tin for clad and bijous is usually made to remove weld stains and superficial oxides present in the confection of chains, earrings, rings, pendants, etc.

The traditional chemical process consists, most of the times, in oxidative polishing based on chromic acid, dichromate’s, nitric acid or cyanides plus hydrogen peroxide. 

Hydrogen peroxide is used in the process of  pickling and pre-oxidative polishing, with great advantages over the traditional systems. Our METALPER® product contains hydrogen peroxide, stabilizers and special complexing agents for the treatment of copper and its leagues. 
 
Advantages of using hydrogen peroxide 

• Excellent quality of the treated surfaces
• Generation of effluents free of chrome and cyanide salts, considered harmful to the residual waters.
• Non generation of nitrous gases characteristics of the use of nitric acid
• Ease of handling and processing