A purified ammonium molybdate solution basic principle impurity ions resulting calcine leaching ammonia ammonium molybdate solution containing copper and a small amount of divalent iron, nickel, zinc and the like. In order to remove these impurities, ammonium sulfide can be added to make it a sulfide precipitate by utilizing the property that their sulfide concentration is small. Its main reaction is:
                             [Cu ( HN 3 ) 4 ] ( OH ) 2 + ( NH 4 ) 2 S+4H 2 O ==== CuS+6NH 4 OH
[Fe ( HN 3 ) 4 ] ( OH ) 2 + ( NH 4 ) 2 S+4H 2 O ==== FeS+6NH 4 OH
Other divalent heavy metal ions can also generate MS precipitate. Divalent copper, iron, lead, and arsenic and antimony can be precipitated completely. Since the complex stability constant of Ni(NH 3 ) 4 2+ and Zn(NH 3 ) 4 2+ is large, and the solubility product of the sulfide is small, the effect of removing nickel and zinc is poor.
b Industrial practice The precipitation process is carried out in a stainless steel or enamel stirred tank. The mixing tank has a volume of 1 to 2.5 m 3 and is equipped with a serpentine heating tube and a stirrer. The stirring speed is 80 to 100 r/min. The concentration of (NH 4 ) 2 S solution is 8% to 12%, and the main amount is slightly higher than the theoretical amount required for precipitation of copper and iron, and the final pH is 8.5 to 9.0. After stirring at 85 ~ 90 °C for 10 ~ 20min, the purified (NH 4 ) 2 MoO 4 solution is colorless and transparent, the density is more than 1.16g / cm 3 , the copper and iron content should be low for industrial purity requirements. At 0.003g/L, it should be less than 0.0006g/L for high purity products. The recovery of molybdenum is higher than 99%.
The method for precipitating a molybdenum compound from a pure ammonium molybdate solution from a purified (NH 4 ) 2 MoO 4 solution mainly includes: 1) directly depositing a molybdenum compound from the purified solution; 2) purifying the solution first After concentration, low temperature acid precipitation, ammonia leaching, and then the molybdenum compound is precipitated from the ammonia immersion liquid. The latter, also known as the combined method, can obtain ammonium paramolybdate with high purity, uniform particle size and meeting calcination or reduction requirements.
a combined method of (1) was concentrated (NH 4) 2 MoO 4 solution was concentrated in a stainless steel stirred tank processes carried out for the technical conditions: stirring speed of 80 ~ 100r / min, boiled and evaporated to a density of 1.18 ~ 1.20g / cm 3 (heat When it is 1.20 to 1.23 g/cm 3 (cold), the pH is 7.0, or about 15 g/L of free ammonia, and then the agglomerated Fe(OH) 2 , Fe(OH) 3 and the like are filtered off.
(2) Low-temperature acid precipitation In the enamel stirred tank, the ammonium molybdate solution is neutralized with hydrochloric acid until the pH is 2 to 3, and the final temperature is 55 to 60 ° C. The ammonium diammonium diammonate is precipitated under vigorous stirring:
4 ( NH 4 ) 2 MoO 4 +6HCl ==== ( NH 4 ) 2 O · 4MoO 3 · 2H 2 O+6NH 4 Cl+H 2 O
Or 4 ( NH 4 ) 2 MoO 4 + 5H 2 O pH=2~2.5 →( NH 4 ) 2 O · 4MoO 3 · 2H 2 O+6NH 4 OH[next]
(3) Ammonia dissolution of ammonium polymolybdate Ammonia is dissolved in a stainless steel stirred tank. The technical conditions are as follows: ammonium polymolybdate crystal (kg): pure water (L): ammonia water (L) = 1: (1 ~ 1.1 ) 0.5 feeding. The stirring speed is 75 ~ 80r / min, the temperature is 70 ~ 80 ° C, the final pH = 6.5 ~ 7.0, the density of (NH 4 ) 2 MoO 4 solution after ammonia solution is about 1.4g / cm 3 , and then filtered, a small amount of impurity iron, etc. Enter the slag.
(4) Evaporation of pure molybdenum compound from pure (NH 4 ) 2 MoO 4 solution. Evaporation to remove part of ammonia, then MoO 4 2- is polymerized into Mo 2 O 7 2- , Mo 7 O 24 6- , Mo 8 O 26 4 - (or Mo 4 O 13 2- ) and produce the corresponding ammonium salt precipitated. The reaction of precipitating ammonium paramolybdate or ammonium dimolybdate is:
7 ( NH 4 ) 2 MoO 4 →( NH 4 ) 6 Mo 7 O 24 · 4H 2 O+8NH 3
           2 ( NH 4 ) 2 MoO 4 →( NH 4 ) 2 Mo 2 O 7 +2NH 3 +H 2 O
The evaporation process is carried out in a corrosion-resistant stirred tank under the technical conditions of a stirring speed of 75 to 80 r/min and a steam pressure of 0.1 to 0.15 MPa to keep the solution boiling in the tank. The evaporation process should maintain free ammonia 4 ~ 6g / L, mother liquor density 1.20 ~ 1.24g / cm 3 , cooled and crystallized and filtered. For products with high purity requirements, the crystallization rate is generally about 70%; for products of general purity, the crystallization rate is 85% to 90%.
The chemical composition of ammonium paramolybdate obtained by the combined method is shown in the table.
Chemical composition of ammonium paramolybdate obtained by combined method | ||||||
grade | Mo | Fe , Al , Si , Mn | Ca , Mg , Ni , Cu | Ti , V | Pb , Bi , Sn , Cd | W |
High purity | 5 | <0.0006 | <0.0003 | <0.0015 | <0.0001 | <0.15 |
Industrial purity | 55 | <0.003 | <0.003 or 0.01 | 　 | <0.0006 or 0.001 | 　 |
b Precipitating molybdenum compound directly from the purified ammonium molybdate solution (1) Evaporative crystallization method The density of the solution before evaporation is 1.09 to 1.12 gcm 3 (120 to 140 g/L MoO 3 ) and evaporated to a density of 1.2 to 1.23 g/cm 3 . The mixture was allowed to stand for filtration, and then the filtrate was evaporated to a density of 1.38 to 1.4 g/cm 3 (containing 400 g/L of MoO 3 ), cooled and crystallized. At this time, about 50% to 60% of the molybdenum is formed into ammonium paramolybdate crystals. [next]
(2) Neutralization crystallization method Neutralize with nitric acid or hydrochloric acid until the solution pH = 1.5 to 2.5, and precipitate a polymer of ammonium tetramolybdate (NH 4 ) MoO 13 or ammonium tetramolybdate (NH 4 ) 4 Mo 8 O 26 , (NH 4 ) 6 Mo 12 O 39 , (NH 4 ) 12 Mo 24 O 78 . The response is:
8 ( NH 4 ) 2 MoO 4 +12HCl ==== ( NH 4 ) 4 Mo 8 O 26 · 4H 2 O+12NH 4 Cl+2H 2 O
It is generally considered that the more suitable process conditions are temperature 45 to 55 ° C, pH = 1.5 to 2.5, the original solution specific gravity is 1.16 to 1.20, and β-type ammonium tetramolybdate is often obtained under the correct control.
The calcined ammonium paramolybdate of ammonium paramolybdate decomposes ammonia and water above 350 ° C to form MoO 3 :
3 ( NH 4 ) 2 O · 7MoO 3 · 4H 2 O → 7MoO 3 +6NH 3 +7H 2 O
The process of dehydration and ammonia of ammonium paramolybdate at different temperatures is:
3 ( NH 4 ) 2 O · 7MoO 3 · 4H 2 O 90 ~
~
Industrial calcination is carried out in a rotary calciner, which is made of stainless steel and is similar in structure and operation to the calcination of tungstic acid. The furnace temperature is (600±20) °C, (the surface of the material is 500-550 °C), the obtained MoO 3 powder should be light yellow-green, the bulk density is 1.20 ~ 1.60g/cm 3 , and the high-purity MoO 3 chemical composition is : Pb,Sn, Cd<0.0001%; ​​Mg, Sb <0.001%; ​​V, Co, Ti, Mn<0.0013%; Fe<0.003%; S, P, As, Ni, Bi<.0005%; Cu<0.0004 %; Ca, Si < .0008%; Al < .0006%; W < .15%.
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