The so-called fire, characterized by a front half of a molybdenum concentrate roasting process to calcine the oxidized molybdenum. The production of ammonium molybdate from molybdenum calcination is still a wet process, and the basic process route is shown in the figure below. The whole process is divided into the following steps.
Figure Ammonium molybdate (fire method) production process
1. Ammonia dip
Calcine molybdenum trioxide in addition to a main component in an outer further comprising: not permeable calcined molybdenum dioxide and molybdenum disulfide, metal sulfates, metal molybdates, silicon impurities. The reactions of these different materials in the ammonia leaching process are also different.
Molybdenum trioxide is an acid anhydride which is very soluble in ammonia and reacts into the liquid phase as follows:
MoO 3 +2NH 4 OH =(NH 4 ) 2 MoO 4 +H 2 O
Molybdenum dioxide and molybdenum disulfide are insoluble in ammonia and remain in the solid phase. Copper , zinc , nickel sulfates, molybdates can be dissolved in ammonia water to form iron complexes, which occur as follows:
MeSO 4 +6NH 4 OH=Me[(NH 3 ) 4 ](OH) 2 +(NH 4 ) 2 SO 4 +4H 2 O
MeMoO 4 + 4NH 4 OH = Me [(NH 3) 4] 2 MoO 4 + 4H 2 O
Calcium sulfate can react with MoO 2- 4 :
CaSO 4 + MoO 2- 4 =CaMoO 4 ↓+SO 2- 4
The newly formed calcium molybdate and the calcium molybdate in the original calcine are insoluble in ammonia and enter the solid phase.
Although iron molybdate can be decomposed by ammonia, the reaction is slow. Because a film of iron hydroxide which is practically insoluble in ammonia water is formed on the surface of the iron molybdate, which hinders the further dissolution of the iron molybdate by the ammonia solution. Most of the iron molybdate remains in the solid phase. [next]
Ferrous sulfate or molybdate of ferrous iron forms ferrous hydroxide in ammonia solution, which is soluble in ammonia to form an ammonium complex:
Fe(OH) 2 +6NH 4 OH=[Fe(NH 3 ) 6 ](OH) 2 +6H 2 O
The silicon impurity is quartz (SiO 2 ) or silicate, which is the main impurity in the molybdenum calcine, and is insoluble in ammonia and remains in the solid phase.
The liquid-solid separation of the ammonia immersion liquid greatly reduces the amount of impurities in the obtained ammonium molybdate solution.
Molybdenum calcine was leached with 8% to 10% ammonia at room temperature or 50 to 60 ° C with a liquid-solid ratio of (3 to 4):1. The amount of ammonia added is 1.2 to 1.4 times the theoretical consumption value of the reaction. There is a residual concentration (25~30g/L) that prevents the formation of polymolybdate and ensures that ammonia must be retained in the final immersion.
The content of impurities in molybdenum calcine is different, and the leaching rate of molybdenum is also different. When the oxidative baking is insufficient, molybdenum dioxide or molybdenum disulfide may occur; when the content of calcium and iron is large, the leaching rate of molybdenum is lowered. Generally, the ammonia leaching rate of molybdenum calcine is between 80% and 95%.
The weight of the ammonia leaching residue is about 10% to 25% of the weight of the added calcined sand, and the molybdenum content is between 5% and 25%. It is also necessary to further recover the molybdenum therein.
In order to solve the interference of ammonia ions such as calcium and iron, in addition to improving the quality of molybdenum concentrate, the following measures are also taken:
(1) Adding ammonium carbonate to the ammonia immersion liquid, which reacts with calcium sulfate to form calcium carbonate (CaCO 3 ) which is more insoluble, can prevent calcium sulfate from forming calcium molybdate and increase the leaching rate of molybdenum. Ammonium carbonate can also react with iron sulfate and iron molybdate to form a precipitate of basic iron carbonate. Its adsorption capacity is smaller than that of iron hydroxide, which can reduce the content of molybdenum in the leaching residue.
(2) Before ammonia leaching, the use of acid "pre-dip" molybdenum baking is an effective method. At this point, the following reaction occurs:
MeSO 4 +2HCl=MeCl 2 +H 2 SO 4
MeMoO 4 +2HCl=MeCl 2 +H 2 MoO 4 ↓
Calcium, iron, copper, zinc, etc. enter the liquid phase in the form of soluble salts, and the molybdenum trioxide is decomposed by acid to form molybdic acid which is insoluble in acid (pH should be adjusted) and enters the solid phase. Thereafter, most of the impurity metal in the calcination can be separated by solid-liquid separation. The ammonia content in the ammonia leaching residue can be reduced to 3% or less after re-ammonia leaching of the purified calcine. When "pre-dip", molybdenum dioxide is soluble in the acid into the liquid phase:
MoO 2 +4HC1=MoCl 4 +2H 2 O
Therefore, when the molybdenum calcine contains higher molybdenum dioxide, the "pre-dip" waste liquid should be added with a process for recovering molybdenum.
The ammonia leaching process is usually carried out in an enamel reactor or a steel dip tank. These equipments have mechanical stirrers and steam heating jackets. The leaching process often has to be repeated 2 to 4 times. After a few times, the dilute ammonia immersion liquid can be recycled.
2, purification and removal
The ammonia molybdate solution obtained after ammonia leaching and filtration also contains a large amount of metal ammine ions. In particular, iron and copper have a large amount of ammine ions. In order to remove them, it is common to add ammonium hydrogen hydride (or ammonium sulfide, sodium sulfide) to the solution.
The ammonium complex ions of these metals are poorly dissociated except [Fe(NH 3 ) 6 ] 2+ , and other [Cu(NH 3 ) 4 ] 2+ and [Zn[Ni(NH 3 ) 4 ] 2+ are combined. Very stable, their PK instability is 13.32, 9.46. Thus, the concentration of positive divalent ions of copper, zinc, and nickel in the solution is very low.
Although [Cu(NH 3 ) 4 ] 2+ is very stable, the solubility product of CuS and FeS is lower. (L FeS = 3.7 × 10 -19 , L CuS = 8.5 × 10 -45 ) Therefore, the following reaction occurs in the solution until copper and iron are precipitated:
[Cu(NH 3 ) 4 ](OH) 2 +NH 4 HS+3H 2 O→CuS↓+5NH 4 OH
[Fe(NH 3 ) 6 ](OH) 2 +NH 4 HS+5H 2 O→FeS↓+7NH 4 OH
For zinc and nickel, although their sulfide solubility products are not high (L ZnS = 1.2 × 10 -19 , L CuS = 1.4 × 10 -24 ), their amido ions are relatively more stable. At this time, the very low [Zn 2+ ], [Ni 2+ ], and [S 2- 〕 in the solution are unlikely to reach the necessary concentration for producing zinc sulfide and nickel sulfide according to the solubility product. Therefore, most of the impurities of zinc and nickel remain in the solution. [next]
By liquid-solid separation, copper and iron impurities in the ammonium molybdate solution can be removed.
In production, the amount of ammonium hydrogen sulfide must be carefully controlled. If the ammonium hydrogen sulfide in the solution is excessive, thiomolybdate will be formed to contaminate the final product with sulfur. Therefore, ammonium hydrogen sulphide needs to be slowly added to the solution little by little and stirred constantly. After each addition, samples should be taken to check whether the sedimentation is complete. If it is found that the ammonium hydrogen sulfide in the solution is excessive, it needs to be replenished by adding fresh ammonia immersion liquid.
Ammonium hydrogen sulfide may also be replaced by ammonium sulfide or sodium sulfide, but sodium sulfide tends to cause an excess of Na 2 O in the final product and is less used.
Purification is carried out in an enamel reactor or a rubber-lined leaching tank. Also, a stirrer and a heated steam jacket are required.
3. Crystallization
The purified ammonium molybdate mother liquor often contains MoO 3 120~140g/L, and the mother liquor density is about 1.09~1.12g/mL. Usually, it is concentrated by pre-evaporation to a temperature of 280-300 g/L containing MoO 3 or a density of 1.20-1.23 g/mL of mother liquor. At this time, a small amount of CuS, FeS, and Fe(OH) 3 in the mother liquor are easily settled and can be filtered. In the future, there will be two processing options:
(1) Scheme I - Concentration-crystallization method: The pre-concentrated mother liquor is heated, evaporated, and concentrated in a stainless steel or enamel reactor with a mechanical stirrer, a steam heating jacket. The solution density was adjusted to 1.38 to 1.4 g/mL (corresponding to 400 g/L of MoO 3 ), and the hot solution was filtered and collected in a cooling and crystallizer.
Crystallization is carried out in a stainless steel or enamel crystallizer with a stirrer, cooling system. When the temperature of the mother liquid is cooled to 40 to 45 ° C, about 50% to 60% of ammonium paramolybdate is crystallized from the solution. The final product is obtained by centrifugal filtration, washing and drying. The remaining mother liquor is repeated several times by "concentration-crystallization". Finally, the tail liquid is evaporated to dryness and calcined at 350-400 ° C. The obtained molybdenum trioxide contains too much impurities and must be returned to ammonia leaching.
Operation should pay attention to: 4~6g/L free ammonia should be retained in the evaporation process; and to prevent local overheating, it should be stirred constantly, so as to avoid the precipitation of ammonium molybdate precipitated with strong acidity and fine grain.
"Concentration-crystallization" needs to be repeated many times, and the process lasts for a long time. After the second time, the crystallization of each batch is higher than the standard, and the crystallization is repeated to purify.
(2) Scheme II - Neutralization method: Neutralization of hydrochloric acid is added to the preconcentrated mother liquor, and different components of polymolybdate can be precipitated according to the final pH and temperature of the solution.
Carefully neutralize the ammonium molybdate mother liquor heated to 55-65 ° C with hydrochloric acid until pH = 2.3, and vigorously stir, 96% ~ 97% of the molybdenum precipitated as dihydrate tetramolybdate:
|
4(NH 4 ) 2 MoO 4 +5H 2 O
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PH=2~2.5
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(NH 4 ) 2 Mo 4 O 13 ·2H 2 O+6NH 4 OH
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→
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The precipitated crystals must be filtered immediately. Otherwise, it is easily dehydrated after prolonged contact with the mother liquor to form fine-grained anhydrous ammonium tetramolybdate and difficult to filter.
The ammonium tetramolybdate precipitate is highly pure, and impurities such as Ni, Zn, Cu, ..., AS, P, S, etc. remain in the weakly acidic mother liquor. However, it contains more chloride ions (0.2%~0.4%) which are not easily washed away by water, but need to be recrystallized to remove chloride ions.
First, ammonium tetramolybdate is dissolved in a solution containing 3% to 5% ammonia at 70 to 80 ° C until saturation (solution density 1.41 to 1.42 g/mL). The saturated solution is then cooled to 15-20 ° C, and 50% to 60% of the molybdenum is precipitated as pure ammonium paramolybdate ((NH 4 ) 6 Mo 7 O 24 · 4H 2 O). The mother liquor is then repeatedly dissolved in ammonium tetramolybdate, and then cooled and crystallized, which can be repeated for about ten times. The ammonium tetramolybdate is gradually converted into pure ammonium paramolybdate, and the impurities are accumulated in the mother liquor to a certain extent, and then sent to the purification treatment.
In the acidic mother liquor after separation of ammonium tetramolybdate, 3%~4% of molybdenum (equivalent to 6~10g/L) remains, and it is re-acidified to pH=2 to the sedimentation tank, from which various components of molybdenum can be precipitated. The acid salt is amorphous precipitated. The precipitate is sent to a purification treatment to remove impurities, and the tail liquid also contains about 1 g/L of molybdenum, which can be recovered by ion exchange.
4. Ammonia leaching residue recovery
According to the different components of molybdenum calcination, the leaching rate of molybdenum is between 80% and 95%, and the rest remains in the ammonia leaching residue with a yield of 10% to 25%. The molybdenum content of the slag is as high as 5% to 25%. between. [next]
The phase of molybdenum in the ammonia leaching residue is: calcium molybdate or iron molybdate which is insoluble or insoluble in ammonia; molybdenum dioxide and molybdenum disulfide insoluble in ammonia; a very small amount of molybdate ion adsorbed on the surface of iron hydroxide . The author analyzed the crop phase of ammonia leaching residue in Luanchuan County ammonium molybdate plant. It was found that the adsorption of MoO 2- 4 was rare, while the content of CaMoO 4 and MoS 2 accounted for more than 80% of the amount of molybdenum in the slag. See the table below.
Distribution of molybdenum in surface ammonia leaching
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Molybdenum phase
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MoO 2- 4
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Fe 2 (MoO 4 ) 3
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CaMoO 4
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MoO 2
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MoS 2
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total
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Molybdenum partition rate (%)
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4.19
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9.33
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35.75
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4.67
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46.06
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100.00
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There are many processes for recovering molybdenum from ammonia leaching slag. Many processes are similar to the decomposition process of molybdenum concentrate. This is only a brief introduction. These processes are also divided into fire and wet methods.
Common fire processes are: (1) secondary roasting - ammonia leaching; (2) sodium carbonate roasting - water immersion; (3) sulfuric acid roasting - ammonia leaching. The latter two are suitable for ammonia leaching slag containing various molybdenum compounds. Among them, the sodium carbonate roasting method is used most.
Secondary roasting method: Richard roasts the leaching residue in oxygen-rich (or pure oxygen) at 600~650 °C, and the total leaching rate is over 99% after 15~30min.
Sodium carbonate roasting-water dissolution method: The wet ammonia slag is mixed with sodium carbonate powder, placed in a baking furnace, and calcined at 700~750 °C for 6-8 h. At this time, various molybdenum compounds in the ammonia leachate are converted into soluble sodium molybdate. The residue is dissolved by heating with water, and sodium molybdate is dissolved in the liquid phase and separated by filtration. Iron molybdate was precipitated from the immersion liquid with ferric chloride in a slightly acidic medium having a pH of 3.5 to 5. The ratio of FeO 3 /MoO 3 in the precipitate is indefinite, usually not in agreement with Fe 2 (MoO 4 ) 3 , and can be dissolved in aqueous ammonia to obtain an ammonium molybdate solution.
Sulfuric acid roasting-water immersion method: The ammonia leaching residue is mixed with sulfuric acid and calcined at 600 ° C, and various molybdenum compounds are converted into molybdic acid. The residue is leached with ammonia water, and the molybdic acid is converted into ammonium molybdate into the solution for recovery.
Common wet processes include: (1) alkaline hydraulic cooking; (2) acid decomposition; (3) sodium hypochlorite decomposition.
Alkaline hydraulic boiling: When the molybdenum in the ammonia dipping slag is mainly in the form of molybdate, and the content of MoO 2 or MoS 2 is very low, the ammonia dipping slag is leached with the sodium carbonate solution in the autoclave. At 180~200 °C, 1.2~1.5MPa leaching, other molybdate can be converted into soluble sodium molybdate for separation and recovery.
Acid decomposition method: When the tungsten grade of ammonia leaching residue is high (3%~5%W), it is difficult to separate W-Mo by other methods. At this time, 20~30% hydrochloric acid is heated to about 100 °C to leaching ammonia dregs, and the molybdate can be completely decomposed to form molybdic acid which is easily soluble in hydrochloric acid, and most of the tungstate does not decompose and is combined with impurities. Residue in the solid phase, separating the molybdenum acid solution to recover molybdenum. The residue can be recycled with tungsten and MoS 2 , MoO 2 .
15% concentration of nitric acid, 10% concentration of sulfuric acid, when the liquid-solid ratio is 3:1, heating to 70-80 ° C, leaching ammonia leaching slag for 2h, can convert various molybdenum compounds in ammonia leaching residue into molybdic acid, The residue contains only 0.44% molybdenum.
