Valuable Element Resource Utilization Process of Vanadium and Chromium Wastewater

1 Background

Vanadium and chromium wastewater is industrial wastewater produced in the process of vanadium oxide production by vanadium slag through sodium roasting, leaching and filtration, and vanadium precipitation with acid ammonium salt. The content of hexavalent chromium is as high as 500×10-6~1000×10-6, far exceeding the national emission standard.

High-priced vanadium and chromium compounds, as heavy pollutants, such as efflux or leakage, will cause great pollution to water bodies and soil environment, seriously endanger human health, and cause waste of metal resources.

At present, the more effective methods for treating this kind of wastewater are:

(1) Reduction neutralization precipitation method, adding reducing agent (such as sodium metabisulfite, sodium bisulfite, etc.) to vanadium precipitation wastewater, so that pentavalent vanadium and hexavalent chromium ions are all reduced to trivalent, and then to the reduced waste water The alkaline solution is added to the aqueous solution to neutralize the waste water solution, and at the same time, the chromium and vanadium ions form hydrates and are precipitated from the waste water solution. The method is simple in equipment and large in processing capacity, but has the disadvantages of low recovery rate of vanadium and chromium, inability to directly recover valuable elements of vanadium and chromium from precipitation waste, difficulty in precise control of the dosage of chemicals, large consumption of chemicals, long treatment period and high processing cost, etc. defect.

(2) Conventional ion exchange method, that is, using ion exchange resin to recover the anion components in the vanadium extraction wastewater, and recover the vanadium and chromium in the vanadium extraction wastewater. The method has a simple process route and a high vanadium-chromium adsorption rate, but there are still many unavoidable defects. The concentrated solution of the analytical solution containing vanadium and chromium adopts the traditional ammonium precipitation process: ammonium salt precipitation of vanadium, filtration, and filter cake calcination to obtain a V2O5 product with a purity of 99.8%. Precipitated vanadium supernatant is treated by reduction, neutralization of precipitation chromium, and filtration. This process not only brings back ammonia nitrogen wastewater, but also cannot realize the resource utilization of chromium.

Aiming at the defects of the existing vanadium and chromium-containing wastewater treatment process, a technical scheme for resource utilization of valuable elements in vanadium-chromium wastewater is provided. Element product recycling.

Technical route

The technical route of barium and chromium wastewater treatment is shown in Figure 1.

Process research

3.1 Experimental steps

(1) Adsorption.

The vanadium and chromium ions in the wastewater containing vanadium and chromium are adsorbed by the adsorption medium to obtain the adsorption medium containing the vanadium and chromium ions;

(2) Analysis.

adding a desorption agent to the adsorption medium containing vanadium and chromium ions obtained in step (1) to obtain a desorption solution, and the adsorption medium can be reused after desorption;

(3) Shen vanadium.

Add alkaline substance to the analytical solution and filter after stirring to obtain calcium vanadate product and precipitation vanadium supernatant;

(4) Chromium crystals.

The vanadium precipitation supernatant that step (3) obtains is carried out evaporative concentration and cooling crystallization to obtain sodium chromate crude product and crystallization mother liquor;

(5) Recrystallization.

The sodium chromate crude product that step (4) is obtained is heated and dissolved, and cooling crystallization obtains sodium chromate product and cooling crystallization mother liquor;

(6) The crystallization mother liquor is returned.

The crystallization mother liquor obtained in the steps (4) and (5) is returned to the step (2) for repeated use as the ingredients of the analytical solution.

3.2 Experimental results

(1) The effect of temperature on the saturated adsorption capacity of the resin (see Figure 2) It can be seen from Figure 2 that as the temperature increases, the vanadium adsorption capacity of the resin first increases and then decreases, and the chromium adsorption capacity gradually increases, but the trend tends to be gentle. Therefore, the optimal adsorption temperature is 30–40 °C.

(2) The effect of NaOH concentration on the calcification rate of vanadium precipitation The calcification reaction generates a large amount of NaOH. The higher the initial sodium vanadate in the solution, the higher the generated NaOH concentration. If the NaOH concentration is too high, the calcification reaction will be hindered. In actual production, the initial reaction solution was neutral, and as the reaction progressed, the NaOH concentration in the solution increased. In the experiment, by adjusting the amount of sodium vanadate added, it was converted into the theoretical reaction end point NaOH concentration.

Sodium vanadate solutions with different NaOH end-point concentrations were prepared for the reaction respectively. The reaction conditions were as follows: the calcium-vanadium ratio was 1.2, the reaction temperature was 95°C, and the reaction time was 2h. The experimental results are shown in Figure 3. It can be seen from the figure that with the increase of NaOH concentration, the vanadium deposition rate gradually decreases. When the NaOH concentration is lower than 190g/L, the vanadium deposition rate is above 92%. In the experiment, it was found that when the NaOH concentration was greater than 160g/L, the viscosity of the material after the reaction was high, the water content was high, and the solid-liquid separation was difficult, which affected the yield and work efficiency. Therefore, the endpoint NaOH concentration should be less than 160g/L.

(3) Effect of CaO addition on vanadium precipitation rate

The calcium vanadium ratio refers to the ratio of the amount of calcium-containing substances in calcium oxide to the amount of vanadium substances in sodium vanadate.

Control the reaction conditions: the end point concentration of NaOH is 150g/L, the reaction temperature is 95°C, and the reaction time is 2h. With the increase of the amount of calcium oxide added, the conversion rate of calcified vanadium gradually increases, and it tends to be stable after being added to 1.8 times the amount of vanadium in the liquid phase. Continuing to increase the amount of calcium oxide will increase the calcium content in calcium vanadate and the vanadium content. decreased, resulting in an increase in the consumption of ammonium bicarbonate in the subsequent ammonium conversion. Therefore, the optimum calcium vanadium ratio is 1.8.

(4) Influence of reaction temperature and time on vanadium precipitation rate

Control reaction conditions: NaOH end point concentration 150g/L, calcium vanadium ratio 1.8, investigate the influence of reaction temperature and reaction time on vanadium precipitation rate, the experimental results are shown in Figure 5. With the increase of reaction temperature and reaction time, the conversion rate of calcified vanadium gradually increased. The optimal reaction conditions were as follows: the reaction temperature was 80 °C and the reaction time was 20 min, and the optimal calcified vanadium conversion rate could reach 98%.

(5) The effect of alkalinity on the crystallization of sodium chromate (see Figure 6)

With the increase of alkali concentration, the crystallization rate of sodium chromate gradually increases, but the crystallization quality gradually decreases, and the alkali entrainment increases. According to industrial application requirements, the remaining sodium chromate can be crystallized and precipitated in subsequent cycles without affecting the overall yield.

To sum up, the optimum process conditions for vanadium precipitation by sodium vanadate crystal calcification are NaOH terminal concentration of 160g/L, calcium-vanadium ratio of 1.8, reaction temperature of 80°C, and reaction time of 20min.

4 Conclusion

Through the implementation of the above process, the resource utilization of valuable elements in the vanadium-chromium wastewater can be realized, two end products of calcium vanadate and sodium chromate can be prepared by using the analytical solution, and the alkaline medium in the crystallization mother liquor can be recycled.