Nitric acid pre-oxidation method for gold extraction-Nitrox method

In addition to leaching gold with nitric acid/sodium chloride, nitric acid oxidation is generally used for pretreatment. The Nitrox process and the Areno process for gold and silver concentrates are a step of oxidative pretreatment with nitric acid, which converts sulfides into oxides, making gold and silver suitable for use in certain methods (eg sulfur vein method). Or cyanide method) to extract. The Nitrox method uses atmospheric air, while the Ar-seno method uses pressurized oxygen. The Nitrox method is a nitric acid cycle, while the Arseno method controls the reaction with nitrous acid. The oxidation rate is faster than that of nitric acid. However, during the oxidation process, arsenic forms arsenite instead of arsenate and elemental sulfur is formed. Elemental sulfur is cyanated for the next step. Leaching gold is not good. In order to overcome the problems caused by the formation of elemental sulfur, a method (Redox method) which is carried out at a high temperature has recently been reported, which is characterized in that limestone is added to the reactor to remove various sulfates, and iron arsenate is precipitated to avoid generation. The trouble with elemental sulfur.
The extraction of copper , uranium , platinum , nickel , cobalt and silver with nitric acid has been completed. All of this work involves the dissolution of the element under study and its subsequent separation from other elements of dissolution. Since gold is not dissolved by nitric acid and is mixed with other acid insolubles, the nitric acid treatment of gold ore and concentrate is different from the above.
Gold in refractory ores and concentrates is often associated with pyrite, arsenopyrite and pyrrhotite. The Nitrox process involves the oxidation of these sulfide minerals to sulfates and arsenates. The pH of the solution is then typically adjusted with limestone to remove these sulfates and arsenates from the solution by precipitation. The behavior of various minerals in nitric acid varies from one to another, and the amount of elemental sulfur produced varies. Previous work has attempted to maximize the production of elemental sulphur, as doing so means lowering the reagents used for neutralization, reducing the load on the nitric acid recovery system, and facilitating disposal. Bjorling and Kolta (1964) studied various sulfides, found that low sulfur sulfides (e.g., pyrite and magnetic sphalerite) elemental sulfur yield, and the high sulfur sulfides (e.g., pyrite The yield of elemental sulfur is low. The performance of arsenic pyrite is similar to that of sulfides with low sulfur content, and the yield of elemental sulfur is about 70%. Medium concentrations of nitric acid react rapidly with arsenic pyrite and pyrite. A solution having a potential of 750 mV is equivalent to 12% by mass of nitric acid. It can be seen that arsenic pyrite is oxidized very rapidly, and its fine particle size is one of the reasons for rapid oxidation. However, even the coarse-grained pyrite can be used for 1 h under the selected conditions of 80 ° C, 750 mV and 10% solids. The reaction completely takes place inside.
In the Nitrox process, arsenic is left in solution under normal conditions. This creates conditions for various solutions for recovering gold and also provides a greater possibility for removing arsenic, iron and sulfur from the formation of precipitates in solution. If precipitates of arsenic, iron and sulfur are formed, the precious metals are mixed with these precipitates.
1) Recovery of gold and silver In the case where sulfides have been oxidized and major elements such as iron, arsenic and sulfur (and secondary elements such as copper, zinc, cobalt, nickel and magnesium ) are in solution, there are usually two process options. .
1 The first solution is in the oxygen leaching solution, sulfur is present in the form of sulfate, which can be removed by adding a calcium compound to the acidic medium:

H ₂ SO ₄ +CaC0 ₃ +H ₂ O —→ CaSO ₄ ·2H ₂ 0+CO ₂ ↑

H ₂ S0 ₄ +Ca(N0 ₃ ) ₂ +2H ₂ 0 —→ CaSO ₄ ·2H ₂ 0+2HN0 ₃

H ₂ S0 ₄ +Ca(N0 ₃ ) ₂ +H ₂ 0 —→ CaSO ₄ ·2H ₂ 0+NO↑+NO ₂ ↑

The gypsum precipitated from the Nitrox solution is easy to filter, and the gypsum precipitate does not contain arsenic.
Iron and arsenic are usually precipitated at 80 ° C and pH 3-4. It is important to remove arsenic from the solution. The ratio of iron to arsenic is very important. When the Fe/As ratio is 4 or more, an alkaline arsenate which is extremely insoluble is formed. Precipitation of iron arsenate and gypsum also causes elements such as copper, zinc and nickel to decrease in different percentages. The recovery of gold from the precipitate of the oxidized pulp having a pH of 4 has a high recovery rate. The solution involves precipitating various dissolved substances in the presence of gold, the precipitate is easily filtered, and the filter cake is completely subjected to atmospheric recovery to recover gold and silver.
2 The second scheme is best to prevent these elements from entering the cyanide cycle when dealing with materials with high levels of elements such as copper. There are two differences between the second scheme and the first scheme. One is that there is a filtration process after the oxidation process, and the second is that the scale of the gold recovery cycle is relatively small because the amount of solids entering the cyanidation cycle is relatively small.
The concentrate is pulped with a returning Ca(N0 ₃ ) ₂ solution and then passed through a gypsum precipitation process. The amount of gypsum formed in this process varies with the amount of calcium returned from the precipitated iron process. The pulp enters the oxidizer after exiting the gypsum precipitation process. At this time, Fe, As, S, and Cu are dissolved, while the gangue and gold remain in the residue. After oxidation, the slurry is filtered, and part of the filtrate is returned to the gypsum precipitation step to regenerate the nitric acid. The remaining filtrate is sent to a precipitation step where appropriate reagents are added to produce the most desirable precipitate. It is easy to include the process of recovering metals in the process if it is considered cost-effective to recover dissolved metals such as copper, titanium and nickel.
The filter residue consists of gold, gangue, gypsum and some elemental sulfur. As the amount of oxidized slag decreases, the gold content increases accordingly. This shows that the second process produces low arsenic slag which can be treated in the cyanidation cycle or sent to smelting. The slag can be smelted to produce more than 98% gold.
The advantage of this solution is that it reduces the amount of oxidized slag and also removes arsenic, which is easily accepted by smelters. [next]
2) Recovery of nitric acid The oxidation of arsenic pyrite and pyrite requires oxygen, while in the Nitrox process oxygen is supplied by nitric acid. The oxygen demand for some sulfide minerals is shown in Table 1.

Table 1 shows the amount of nitric acid required to produce oxygen. Since nitric acid is regenerated from air, the amount of air required is also given. The reaction of reducing nitrate to oxygen is:

4HN0 ₃ —→ 2H ₂ 0+4N0+30 ₂

The reduction product is NO, and the valence of nitrogen is changed from +S to +2; for every 1 mol of NO produced, 3 electrons are lost. Regardless of the presence of oxygen or air, oxidation of pyrite in nitric acid can always proceed smoothly:

2FeS ₂ +lOHN0 ₃ —→ Fe ₂ (S0 ₄ ) ₃ +H ₂ SO ₄ +1ONO+4H ₂ 0

Therefore, the reaction kinetics does not involve the diffusion of oxygen through the gas-liquid interface through the liquid to the mineral surface. From the environmental and economic point of view, the NO formed by the decomposition of sulfides must be recovered, and the nitrate remaining in the solution after oxidation should also be recovered.
1 gas phase. The regeneration of nitric acid involves the oxidation of NO to the absorption of NO ₂ and NO ₂ . NO it will absorb 33% of the NO ₂ formed NO _2, without forming HNO _3. The NO produced must be oxidized and absorbed. The general equation for recovering 100% NO is as follows:

6N0+30 ₂ —→ 6N0 ₂

6N0 ₂ +2H ₂ 0 —→ 4HN0 ₃ +2N0

4N0+30 ₂ +2H ₂ 0 —→ 4HN0 ₃

N0 is formed when NO ₂ is absorbed, which means that recovery of NH ₃ requires many stages. Many nitric acid plants require more than 20 grades in order to recover these nitric acids. Since NO is oxidized to NO ₂ in slower kinetics (this is because the concentration of NO and ₀₂ are thinning reason), the recovery of nitric acid required for later stages of the nitric acid recovered much more than the front part of the absorbent At the beginning of the column, the 7th grade recovers 90% of the nitric acid, and the remaining 13 grades recover the remaining 8%. To recover the last 2%, the required number of stages is even greater. The factory does not actually recycle all of the N0, and the remaining part of the NO is either vented or catalytically reduced to N ₂ and 0 ₂ to comply with environmental regulations.
The final NO is absorbed by the washing method, and the chemical reaction absorbed in the alkaline medium is different from the chemical reaction absorbed in the production of nitric acid. Only 50% of NO must be oxidized to NO ₂ , while N ₂ 0 ₃ is absorbed by nitrite. NOx is washed with NaOH, Ca(OH) ₂ and CaC0 ₃ , NaOH is the best, but CaC0 _{3 is} cheap, so generally It is washed first with CaC0 ₃ and then with NaOH or Ca(OH) _{2 to} ensure that the final concentration of NO is reduced to 1000 × 10 ^-4 %.
The advantage of using elemental sulfur as the oxidation product is that the size of the system can be greatly reduced. If 50% of the sulfur in the pyrrhotite is elemental sulfur, the amount of nitric acid required, as well as the formation and subsequent recovery of NO, can be reduced by 33%.
2 liquid phase. The recovery of nitrate in the solution varies with the concentration of nitrate in the oxidized pulp, which in turn depends on the amount of product to be washed and the amount of wash water that can enter the cycle. In the leaching solution at a potential of 750 mV at 85 ° C, the concentration can be changed from 2 mol/L (HN0 ₃ ) without sulfuric acid to 1 molL (H ₂ S0 ₄ ), and 0.7 mol/L when Fe ₂ (S0 ₄ ) ₃ is added (HN0) ₃ ). By adding an excess of FeS concentrate, the nitrate concentration can be further reduced to 0.15 mol/L. The addition of this concentrate also consumes the free acid, raising the pH to more than 1 at the time of precipitation. The change that occurs after using this process is that the amount of nitrate lost will be as low as 0.3% of the HN0 ₃ required for oxidation (50% pyrite concentrate). [next]
The filtration test of the precipitate obtained with CaCO ₃ at 80 ° C and pH = 4 showed that the material was easy to wash and the recovery of the solute was high (99.7%). The recovered nitrate is returned to the cycle and contacted with free sulfuric acid formed during the oxidation to form gypsum and nitric acid.
3) Circulation of the Nitrox Process In a hydrometallurgical system, oxygen is stored in the gas phase and is consumed in the liquid phase. The transfer of oxygen from the gas phase to the liquid phase is an important step in the oxidation of arsenopyrite. One of the most fundamental advantages of the Nitrox process is that oxygen in the air can be introduced into the slurry at a lower cost, which is achieved by an intermediate product (ie gaseous NO ₂ ). It is easily soluble in the liquid phase and makes it easier to treat oxygen in the oxidation process of sulfide ore. This is because in the slurry, NO ₂ reacts with water to form nitric acid (3NO ₂ +H ₂ 0 —→ 2HN0 ₃ +NO), and nitric acid reacts easily with sulfides and arsenides (for example, sulfur: S+2HN0 ₃ — → H ₂ S0 ₄ + 2N0 or S+3NO ₂ + H ₂ 0 —→ H ₂ S0 ₄ +3N0). Briefly, three NO ₂ molecules enter the liquid phase to oxidize sulfur in one sulfide to sulfuric acid and produce three NO molecules. Gaseous NO has a low solubility in water (similar to oxygen) and therefore immediately escapes from the solution and re-enters the gas phase.
However, NO is not an inert gas, and it easily reacts with oxygen in the air to form NO ₂ . This newly formed NO _{2 is} immediately absorbed by the solution to form nitric acid. The latter in turn reacts with the hard-impregnated sulfide to release NO. This establishes a Nitrox cycle that allows oxygen to be transferred very efficiently to the slurry of hard-to-dip sulfide. The oxidation reaction formula of NO can be written as:

3
3NO+——O ₂ —→ 3NO ₂
2
Or the total reaction formula is:
3
S+——O ₂ +H ₂ O —→ H ₂ SO ₄
2

It can be found that neither HN0 ₃ , NO or NO ₂ appears in the overall reaction formula. However, the nitrate content of the solution is such that nitric acid and NO should be considered as reaction products or intermediates in the Nitrox process and should not be considered as catalysts.
4) Research and application of Nitrox method Example 1 Nitrox process for processing gold-containing arsenopyrite. The Hydrochem Development Company in Brampton, Ontario, Canada, built a Nitrox demonstration plant on the banks of the Serpent River with a daily processing capacity of 100t Dickenson concentrate. The process flow chart is shown below.

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The arsenopyrite is mixed with the calcium nitrate containing the filtrate, and then contacted with a portion of the ferric sulfate, arsenic acid and nitric acid solution from the reactor to precipitate the gypsum and form nitric acid to acidify the slurry:

3Ca(N0 ₃ ) ₂ +3H ₂ S0 ₄ —→ 3CaS0 ₄ +6HN0 ₃

The Nitrox process uses the produced sulfuric acid to react with calcium nitrate in the filtrate to reproduce the nitric acid.
Since iron, arsenic and sulfur are all dissolved, leaving the reactor system and then filtered, the quality of the oxidized concentrate is significantly reduced. It has been found that precipitated gypsum greatly contributes to the filtration of residues substantially consisting of gangue, elemental sulphur and gold. The solution is sent to precipitate iron arsenate, and the remaining sulfate and nitrate are converted into gypsum and calcium nitrate, respectively. The calcium nitrate solution can be used to scrub the tail gas before returning to the reactor chute.
The airflow system is very simple. Oxidation only requires some air to enter the reactor system. This can deplete oxygen and NO in the gas stream. After the evaporated nitric acid and the trace amount of NO ₂ are washed by heat, a low-concentration nitrate aqueous solution condensed from the gas stream is used as the washing water of the system. The remaining air is then passed to recover additional nitric acid and the recovered nitric acid is returned to the reactor. Finally, the tail gas is washed with lime water to reduce the concentration of NO (i.e., NO and NO ₂ ) to the extent permitted by the chimney discharge.
The chemical composition of the Dickenson concentrate in the process is as follows: Fe 27%, As 10.8%, S 23.2%, Au 3.7 g/t, H ₂ 0 10%. All arsenic is present in the form of FeAsS. All magnetite is present in the form of FeS. All iron in the material is symbiotic with sulfide. A series of minerals can be calculated as: FeAsS 23.4%, FeS ₂ 29.3%, FeS 8.4% . Considering the heat and material balance, the oxidation reaction is assumed as follows:

3 FeAsS+lOHN0 ₃ —→ 3Fe ³⁺ +3As0 ₄ ^3- +H ₂ SO ₄ +2S+4H ₂ 0+lONO↑

6FeS ₂ +30HN0 ₃ —→ 3Fe ₂ (S0 ₄ ) ₃ +3H ₂ S0 ₄ +12H ₂ 0+30N0↑

2FeS+4HN0 ₃ +H ₂ SO ₄ —→ Fe ₂ (S0 ₄ ) ₃ +3H ₂ 0+4N0↑

As for the precipitation reaction of dissolved iron, arsenic (V) and sulfur, the assumed chemical reaction formula is:

Fe ₂ (S0 ₄ ) ₃ +3CaCO ₃ +5H ₂ 0 —→ 2Fe(OH) ₃ +3CaS0 ₄ ·2H ₂ 0+3C0 ₂ ↑

Fe ³⁺ +As0 ₄ ^3- +2H ₂ 0 —→ FeAs0 ₄ ·2H ₂ 0

H ₂ SO ₄ +CaCO ₃ +H ₂ 0 —→ CaSO ₄ ·2H ₂ 0+CO ₂ ↑

It can be seen from the above six reaction formulas that except for the formation of a small amount of elemental sulfur in the arsenic pyrite, all the sulfur in the material needs to be neutralized with calcium carbonate, which makes CaC0 ₃ or limestone form one of the main consumables in the Nitrox method. The cost is an important factor in the production cost. Therefore, you need to consider this issue.
There are no large deposits of calcite in northern Ontario, but there are two limestone production plants in Peninsla, northern Michigan, USA. Plants are built near Huron Lake or a port in Lake Superior. By water, Ni can be made in northern Ontario. The production cost of the -trox method was reduced, and the plant was selected near the Serpent River.
In the material balance calculation, the equipment, investment and production costs are estimated. The processing cost per ton of ore is 106 Canadian dollars, which is much lower than the cost of pressurized oxidation and microbial oxidation.
2 Treatment of gold-bearing sulphide ore and concentrate. The process of hydrometallurgical treatment of gold-bearing sulphide concentrates has been tested in semi-industrial applications in the rare metals of Irkutsk.
The experiment was carried out under relatively mild conditions, the temperature was 40-80 ° C, the concentration of HNO _{3 was} 20-100 g/L, and the time was 3 h. Oxygen was oxidized by the introduction of oxygen, and no harmful gas was emitted during the oxidation process. In the case of acidic oxidation leaching, it is ensured that the negative pressure in the reactor is 50 to 100 kPa, and the required oxygen consumption is close to the theoretical value of oxidation of the sulfide ore. The results of acidic oxidation leaching of sulfide concentrate are shown in Table 2. [next]

It can be seen from Table 2 that in the hydrometallurgical oxidation process, most of the iron, arsenic and non-ferrous metals are transferred into the solution, while Au and Ag are enriched in the solid residue (leaching residue).
In order to recover gold and silver, the acid oxidized leaching residue was cyanated, and the results are shown in Table 3. At this time, the leaching rates of gold and silver are 93.6%~94.8% and 86.4%~90.4%, respectively. After alkali treatment of the refractory sulfide concentrate (using lime or lime soda solution), it can be supplemented and recovered 2%~6. % of Au and 10% to 20% of Ag to cyanide solution.

The refractory pyrite concentrate is treated intermittently with lime and soda solution at a temperature of 70-80 ° C, a liquid-solid ratio of 2:1, and a time of 3 h. The amounts of Na ₂ C0 ₃ and CaO were 60 kg/t and 30 kg/t concentrate, respectively. Lime and soda were neutralized to treat the residue, and two-stage cyanidation (24 h per section) was carried out at a concentration of 2 g/L NaCN. The consumption of NaCN is 4.6 kg/t concentrate. The leaching rates of gold and silver in the cyanidation solution were 92.0% and 73.6%, respectively. The pretreated slurry is acidic and has Fe ³⁺ , which is the two main technical requirements for sulfur leaching of gold.

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