In the petrochemical industry, molybdenum, nickel catalysts are widely used in hydrodesulfurization of heavy oil. During the use of the catalyst, it is deactivated due to the influence of high temperature and the deposition and adsorption of harmful substances, thereby generating a large amount of spent catalyst. According to statistics, the annual consumption of catalysts in the world is about 800,000 tons, and the consumption of industrial catalysts in China is 70,000 tons. Recovered from spent hydrogenation catalyst useful metals, not only has good economic benefits, but also a very important social and environmental benefits.
Many domestic scholars have studied the recovery process of molybdenum. Generally, ammonium salt and molybdenum are used. The recovery rate of molybdenum is not high, the treatment of waste liquid is difficult, and the environment is polluted. There are also organic solvent extraction and separation of molybdenum, but organic solvents are highly toxic, complicated to extract, and difficult to recycle and reuse. In this paper, the alkali is roasting-water leaching to extract molybdenum, and then the molybdenum is precipitated by calcium salt to recover molybdenum. The recovery rate of molybdenum is above 80%, the process is simple and the environmental pollution is small.
First, the experiment
(1) Experimental materials
The spent catalyst used in the experiment came from a refinery and was black strip particles. The composition is shown in Table 1.
Table 1 Main element components (mass fraction)/% of spent catalyst
Mo | Ni | V | Fe | Al 2 O 3 | C | S |
10.45 | 4.54 | 0.30 | 0.0056 | 47.93 | 3.25 | 7.96 |
(Ii) Test Method
1. Alkali roasting The waste Mo-Ni/Al 2 O 3 catalyst is first calcined at a low temperature, and the sulfur and carbon therein are burned off, and then ground to 100 mesh (150 μm) in a molar ratio of Na 2 CO 3 to n (Mo). /Na 2 CO 3 ) = 1:1.8, the mixture is uniformly mixed, and the reaction is calcined in a high-temperature furnace to convert molybdenum sulfide and nickel sulfide into corresponding oxides, and molybdenum oxide is further formed into a salt, which is a blue mixture after calcination. The reaction formula is as follows:
2. Leaching The calcined mixture was dissolved in water and immersed for 5 hours at 90 ° C and a stirring speed of 400 r / min. At this point, sodium molybdate enters the liquid phase, and a very small amount of aluminum also enters the liquid phase in the form of sodium aluminate. The filter cake was filtered and washed to neutrality, and the nickel in the filter cake was recovered by acid dissolution.
3. The main impurity in the impurity removal leaching solution is aluminum. Since the vanadium content is very small (due to the deposition of the catalyst on the catalyst during use), the vanadium is substantially sublimated during the high temperature roasting process, so vanadium is not required.
PH was adjusted to 8 to 9, the silicon is removed in the form of magnesium silicate, the pH is adjusted with hydrochloric acid leach solution to 6 to remove the small amount of aluminum. Wash several times to reduce the loss of molybdenum when aluminum is deposited.
4. The molybdenum will concentrate the solution after the impurity removal, the molybdenum is present in the solution in the form of molybdate, the pH is adjusted to about 8, the calcium chloride solution is added dropwise, and the molybdenum is precipitated as calcium molybdate.
5. Filter residue treatment The filter residue obtained by leaching and filtration was reacted with NaOH solution at 100 ° C for 3 h, filtered, and washed to neutrality. At this point most of the aluminum is present in the solution as a metaaluminate. The resulting filtrate was then subjected to aluminum precipitation. The resulting filter residue is reacted with a mixed acid to convert the nickel oxide into soluble nickel ions, and a small amount of aluminum ions also enter the solution. Filtration, adjust the pH of the filtrate = 5 ~ 6, precipitate aluminum ions, dilute with heating water, filter while hot, wash the precipitate several times, reduce the loss of nickel ions.
The aluminum ion-removed cleaning liquid was added to a Na 2 CO 3 aqueous solution to adjust the pH to 8.5 to 9.0, and the nickel ions were precipitated as nickel carbonate. Filter and wash until neutral. The filter cake was dissolved in a metered ratio of sulfuric acid, then heated to evaporate, concentrated, and cooled to give a crystal of NiSO 4 ·7H 2 O.
(3) Analytical methods
The determination of molybdenum content is carried out according to the method of GB8638.10-88; the determination of vanadium content is carried out according to the method of GB4698. 12-84; the nickel content is determined according to the method of YS/T341.1-2006; the determination of aluminum content is carried out according to the method of GB4102.8-83. .
Second, the results and discussion
(1) Determination of roasting conditions
During the roasting process, the calcination temperature, the amount of alkali added and the calcination time have a great influence on the calcination results.
The effect of calcination temperature on molybdenum leaching rate is shown in Table 2. When the calcination temperature is low, the calcination is insufficient, and the molybdenum conversion is not high, which affects the recovery rate. When the calcination temperature is high, the diffusion rate of oxygen is increased, that is, the reaction rate is increased, and the reaction time is shortened. When the reaction temperature is too high, when it exceeds 795 ° C, the loss of molybdenum is severe due to the sublimation of molybdenum, which is disadvantageous for recovery.
Table 2 Effect of Calcination Temperature on Molybdenum Leaching Rate
Calcination temperature / °C | Molybdenum leaching rate /% |
600 | 81.62 |
650 | 89.00 |
700 | 98.77 |
750 | 94.80 |
800 | 89.32 |
When the calcination temperature is 700 ° C, the effect of the amount of alkali added on the recovery of molybdenum is shown in Table 3. When the amount of alkali added is insufficient, the molybdenum in the catalyst cannot be decomposed into a soluble salt, so that the conversion of molybdenum is not high; when the amount of alkali added is too large, the carrier alumina of the catalyst also reacts in a large amount, causing filtration difficulties and carrying a certain amount. The molybdenum reduces the leaching rate of molybdenum. Therefore, a suitable base is added in an amount of n (Na 2 CO 3 / Mo) = 1.8 to 2.0.
Table 3 Effect of alkali addition on molybdenum extraction rate
n(Na 2 CO 3 /Mo) | Molybdenum leaching rate /% |
1 | 80.65 |
1.3 | 88.29 |
1.5 | 93.20 |
1.8 | 96.57 |
2 | 96.60 |
When the calcination temperature is 700 ° C, the effect of calcination time on the molybdenum extraction rate is shown in Table 4. The calcined material has a certain thickness. During the oxidation process, there is a diffusion of oxygen in the air from the surface and the sulfur dioxide gas generated by the reaction from the inside and the surface. The calcination time is insufficient and the inside of the catalyst particles is not sufficiently reacted. If the baking time is too long, it will increase the sublimation loss of the metal. As can be seen from Table 4, the suitable baking time is 4h.
Table 4 Effect of calcination time on molybdenum extraction rate
Roasting time / h | Molybdenum leaching rate /% |
2 | 85.21 |
3 | 87.32 |
4 | 91.37 |
5 | 93.05 |
(2) Determination of the condition of precipitation molybdenum
In the process of precipitation of molybdenum, the influence of the concentration of molybdenum in the mother liquor, the pH of the solution, and the amount of precipitant added on the precipitation rate of molybdenum are shown in Tables 5-7.
Table 5 Effect of Molybdenum Concentration in Mother Liquor on Molybdenum
Molybdenum concentration in mother liquor / (g·L -1 ) | Molybdenum recovery rate /% |
5.54 | 20.76 |
10.96 | 34.59 |
14.95 | 46.34 |
20.05 | 55.30 |
Test conditions: mother liquor pH = 4 to 5; n (CaCl 2 / Mo) = 1.2.
Table 6 Effect of pH on Molybdenum
pH value | Molybdenum recovery rate /% |
4~5 | 54.88 |
6~7 | 77.72 |
7~8 | 82.08 |
8~9 | 81.24 |
Test conditions: the concentration of molybdenum in the mother liquor was 20 g/L; n(CaC1 2 /Mo)=1.2.
Table 7 Effect of n (CaCl 2 /Mo) on molybdenum
n(CaC1 2 /Mo) | Molybdenum recovery rate /% |
1 | 74.51 |
1.1 | 79.89 |
1.2 | 80.72 |
Test conditions: the concentration of molybdenum in the mother liquor was 20 g/L; pH=8-9.
In the process of precipitation molybdenum, when the concentration of molybdenum in the mother liquor is low, the precipitate is dispersed in the solution and cannot be deposited, and the recovery rate is low; when the concentration of molybdenum is high, the aggregation rate of the precipitated ions is increased, and it is easy to precipitate. The mother liquor should be kept neutral to weakly alkaline before the addition of the precipitant, because under weakly acidic conditions, MoO 4 2 - will combine with H + to form HmoO 4 - , so that a part of the molybdenum remains in the solution and cannot be precipitated. In the process of adding the precipitant, the amount of the precipitant is 10% to 20% in excess. When the precipitant is added in a theoretical amount, as the concentration of the precipitant decreases, the reaction slows down, and a part of MoO 4 2 - is not precipitated. If the excess is excessive, the sulfate produced in the calcination process is brought into the molybdenum solution, thereby The formation of calcium sulfate precipitate affects the purity of calcium molybdate.
(III) Effect of alkali-soluble aluminum on nickel deposition
If the slag obtained after leaching is directly subjected to acid solution to remove nickel, it is difficult to filter after dissolution, because almost all of the aluminum ions enter the solution and are hydrolyzed to form aluminum hydroxide. When filtered, the fine aluminum hydroxide gel blocks the filtration. The holes cause filtration difficulties. First, most of the aluminum is eluted with a NaOH solution, and then nickel is dissolved, which makes the filtration after nickel dissolution easier, and also reduces the loss of nickel during the aluminum precipitation process.
Third, the conclusion
The process of extracting molybdenum from the hydrogenation waste catalyst is simple by the alkali roasting-water leaching method. The suitable extraction conditions are: spent catalyst particle size -0.154 mm, n (Na 2 CO 3 / Mo) = 1.8, calcination temperature 700 ° C, calcination time 4 h, and the leaching rate of molybdenum is over 90%. In the mother liquor, the molybdenum concentration is 20 g/L, and in the range of pH 7 to 9, the excess of the precipitant is 10% to 20%, and the recovery rate of molybdenum is 80% or more.
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