Traditional chemical methods For the preparation of pure key compounds, traditional chemical methods are widely used. The principle flow is shown in the following figure:


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A calcine of a main reaction of ammonia leaching calcine containing Mo0 3, in addition, also calcium, iron, copper, lead, zinc and calcium salts of molybdenum, copper sulfate, MOS 2, MoO 2, Si0 2 , Fe 2 0 3, and the like. In ammonia leaching, Mo0 3 forms (NH 4 ) 2 Mo0 4 into the human solution:

Mo0 3 +2NH 4 0H====(NH 4 ) 2 Mo0 4 +H 2 0

Molybdates and sulfates of copper, zinc and nickel are also partially leached:

MMo0 4 + 4NH 4 0H ==== [ M (NH 3) 4 ] Mo0 4 + 4H 2 0
MS0 4 +6NH 4 OH====[M(NH 3 ) 4 ](OH) 2 +(NH 4 ) 2 S0 4 +4H 2 0

When the ferrous molybdate and iron molybdate react with NH 4 0H to form a cover film Fe(OH) 2 or Fe(OH) 3 , the reaction is slow. The ferrous iron part is taken into the solution by the iron-ammonium complex:
FeMo0 4 +2NH 4 OH====(NH 4 ) 2 Mo0 4 +Fe(OH) 2
Fe(OH) 2 +4NH 4 OH====[Fe(NH 3 ) 4 ](OH) 2 +4H 2 0

CaSO 4 reacts with Mo0 4 2- in the leachate to form a CaMo0 4 precipitate:

CaS0 4十 Mo0 4 2- ====CaMo0 4 +SO 4 2- [next]

CaMo0 4 does not react with NH 4 0H, but in the presence of CO 3 2- , the following reactions can occur:
CaMo0 4 +(NH 4 ) 2 C0 3 ====CaC0 3 +(NH 4 ) 2 Mo0 4
The presence of CO 3 2- also facilitates the conversion of Fe(OH) 2 to FeCO 3 , preventing it from forming a film covering unreacted particles. The MoS 2 and Mo0 2 in the calcination are insoluble in the ammonia water, and the residue is leached into the residue.
b Industrial practice (1) Traditional leaching process The leaching process is carried out in a steel stirred tank. The technical parameters are: room temperature about 50 ° C, solid-liquid ratio 1: (3-4), stirring rate 80-500 r / Min, the amount of ammonia is 1.2~1.4 times of the theoretical amount, the initial ammonia concentration is 8%~10%, the control pH=8~10, the molybdenum leaching rate is 80%~90%; the leaching slag contains molybdenum 10%~25% The slag rate is about 25%, the density of the leaching solution is 1.18~1.20g/cm 3 , and the Mo0 3 is 140~190g/L. The conditions for the three-stage leaching operation of a factory are shown in the table below.

Three-stage ammonia leaching conditions

Segment number

Material: water (or dilute solution): ammonia

Solution density / ( g · cm -3 )

pH

Temperature / °C

Stirring time / min

One stage of leaching

1 : 0.7~0.8 : 1.2~1.4

1.18~1.20

8.5~9.0

Room temperature

30~40

Two-stage leaching

1 : 1.5~1.7 : 0.17~0.2

≥ 1.05

8.5~9.0

70~75

30~40

Three-stage leaching

1 : 1.5~1.7 : 0.5

1.05

8.5~9.0

75~80

20~30

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The current improvements to traditional processes are:
1) Adding (NH 4 ) 2 C0 3 leaches CaMo0 4 . When the firing process output fluidized calcine molybdenum, the addition of (NH 4) 2 CO 3, molybdenum leaching rate from 83% to 85% to 93% to 96%.
   2) adding (NH 4) 2 S leaching the Cu 2+ is leached and other heavy metal ions precipitate as a sulfide, plus S 2- purification step is omitted. When leaching at industrial scale, a theoretical amount of 300% ( NH 4 ) 2 S was added and the temperature was controlled. 7 0 °C , the final leachate contains molybdenum 150.4g /L ; Nickel 0.001 ~ 0.005g /L ; copper, iron < 0.001g /L , the slag contains molybdenum of 8.04% .
    ( 2 ) Acid pretreatment - ammonia leaching process   The mother liquor of ammonium polymolybdate (including NH 4 Cl or NH 4 NO 3 ) prepared by the neutralization method is added with hydrochloric acid to pretreat the calcine, and the initial concentration of NH 4 Cl (or NH 4 NO 3 ) is 100g /L , HCl is 40~ 55g /L , solid-liquid ratio is 1 : 3 , 90 °C When the temperature is kept for 1.5~2h , the molybdate in the calcined sand is decomposed, and all kinds of heavy metals, iron, potassium, sodium and calcium are cations into the solution:

                             MeMoO 4 +2HCl ==== MeCl 2 +H 2 MoO 4

H 2 MoO 4 further forms a poorly soluble compound with NH 4 + , remains in the slag, and the slag is re-dipped to recover molybdenum. The obtained ammonia immersion liquid does not need to be purified, and the direct acid precipitation can obtain a product with higher purity than the conventional process, wherein the potassium content can be reduced to ( 3~5 )× 10 -3 % , which is 2-3 orders of magnitude lower than the conventional process. The comparison between the traditional process and the process is shown in the table below.

Comparison of traditional ammonia leaching process with acid pretreatment-ammonia leaching process (calcination containing 45.11% molybdenum)

Process name

Characteristics

Loss /% of each process

Direct recovery from calcination to ammonium molybdate /%

Pretreatment

Ammonia dip

Purification

Traditional crafts

Purify and remove impurities after ammonia leaching

　

The slag rate is 25~30 , the slag contains molybdenum 11~15 , and the molybdenum loss in slag is 3~15

Around 0.5

85 or so

Acid pretreatment - ammonia leaching process

Acid pretreatment destroys insoluble molybdate and removes impurities and then ammonia dip

0.6~1

The slag rate is 22~25 , the slag contains 2.5~3 molybdenum, and the molybdenum loss in slag is 1.5~3

　

95~96

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