As of 2003, China has accumulated more than 4.5 million tons of chromium slag, and it is still increasing at a rate of more than 400,000 tons per year, which has become the first serious pollution in China's chemical industry. For half a century, many methods have been proposed for the harmlessness and resource utilization of chromium slag. These methods can be roughly divided into two categories: detoxification treatment (ie, harmless) and comprehensive utilization (ie, resource utilization). Detoxification treatment is divided into dry detoxification and wet detoxification, but they are not widely used due to many problems such as incomplete detoxification, high cost, small processing volume and low efficiency. The utilization usually (such as cement, smelting iron, calcium, magnesium, phosphorus fertilizer, glass and glazed tiles, refractories, etc.) convergence with other related businesses, otherwise it will transport and protection and other issues that it is not economical.

The Institute of Process Engineering of the Chinese Academy of Sciences has been researching and exploring for many years in the field of green clean production, and has proposed a number of basic new technologies represented by the new process of clean production of chromium salt “sub-molten salt”. After more than 10 years of research, the “acid-base co-production” research group proposed a new system of “acid-base co-production and acid-base salt regeneration cycle”, and has been committed to applying this basic technology to comprehensive utilization of resources and waste resources. The process of chemicalization and ecologicalization of the mountain; after research, a new process has been proposed for the recycling of chromium slag, providing a new alternative for chromium slag treatment and secondary utilization of resources.

First, the test part

(1) Reaction principle

Chromium is strongly alkaline, which represents the main elements available oxide, chromium slag can react with ammonium chloride liberated, and chloride ions are bound to the metal chloride. The chemical reaction formula is as follows:

The chromium residue is leached with ammonium chloride, and the pH of the system is about 4. At this time, most of the Fe and Al chlorides remain in the slag as hydroxide, and the slag can be further used as a cement raw material. Adding ammonia water and carbon dioxide to the leachate can obtain Ca, Mg, and Cr hydroxide precipitates, and return to the red strontium sodium production process for recycling; the ammonium chloride solution is concentrated and recycled.

(2) Test methods

The chromium slag used is provided by Henan Yima Chromium Salt Factory. The main components are shown in Table 1. The ammonium chloride leaching chromium slag test device is shown in Figure 1.

Table 1 % composition of chromium slag

Na

Ca

Mg

Fe

Al

Si

Cr 6+*

1.54

20. 13

10.01

9.19

5.33

9.61

1.48

∑Cr *

Na20

CaO

MgO

Fe203

Al 2 0 3

Si0 2

4.35

2.08

28.18

16.68

13.13

10.07

20.59

* : Calculated in Cr 2 0 3 .

After the chromium slag is ground, it is sieved, mixed with a certain amount of ammonium chloride solution, and added to the reactor, sealed, stirred, and programmed. After reaching the set temperature, it starts to discharge inert gas, CO 2 , ammonia, and the like. The evaporation of ammonia entrains a certain amount of water, so it is necessary to quantitatively replenish water to maintain the liquid-solid mass ratio of the system constant. After the reaction was completed, it was filtered while hot and the filter cake was washed. The filtrate is combined with the washing water, the volume and pH are measured and sampled for analysis; the filter cake is dried in a drying oven for more than 2 hours, weighed and sampled for analysis.

Inductively coupled plasma generation spectroscopy (ICP-AES) was used to analyze the elemental elements. The leaching rates of Ca, Mg, Na and Cr were mainly investigated. The slag phase analysis results were used as the basis for calculation. Calculated as follows:

Where: Me is a metal element (Ca, Mg, Na, Cr, etc.); mi is the mass of the metal element in the chromium slag, g; m o is the mass of the metal element in the slag leached from the chromium residue, g.

Second, the results and discussion

(1) Effect of temperature on metal leaching rate

Chromium slag mass 100g (particle size 100 mesh), ammonium chloride mass 192g (formed 300g / L aqueous solution), FeCl 2 · 4H 2 0 mass 18g, stirring speed 300r / m, reaction time 4h (when the set temperature is reached) Timing). The effect of reaction temperature on the leaching rate of metal elements is shown in Fig. 2.

It can be seen from Fig. 2 that the leaching rate of Na and Cr 6+ does not change much with the increase of reaction temperature; the leaching rate of Ca increases with the increase of temperature; the leaching rate of Mg increases first and then decreases with the increase of temperature; the leaching rate of Fe and Al Both are lower. Considering comprehensively, the leaching temperature is preferably 120 to 140 °C.

(2) Effect of leaching time on metal leaching rate

The mass of the chromium slag is 100 g (particle size below 100 mesh), the mass of ammonium chloride is 192 g (formed into 300 g/L aqueous solution), the mass of FeC1 2 · 4H 2 O is 18 g, the stirring speed is 300 r/m, and the leaching temperature is 120 °C. The effect of reaction time on the leaching rate of metal elements in chromium slag is shown in Fig. 3.

It can be seen from Fig. 3 that the leaching rates of Na and Fe are relatively constant during the leaching process, and the leaching rates of Ca, Mg and Cr 6+ increase first and then decrease with increasing temperature; the leaching rate of Al fluctuates greatly. This is mainly related to the rate of caustic ammonia. In the early stage of the reaction, the system is more alkaline and the reaction speed is faster; in the later stage of the reaction, the reaction power is obviously weakened until the dynamic equilibrium is reached. Considering comprehensively, the reaction time is preferably 3 to 4 hours.

(3) Effect of material ratio on metal leaching rate

Chromium slag mass 100g (particle size below 100 mesh FeCl 2 · 4H 2 0 mass 18g, stirring speed 300r / m, reaction temperature 120 ° C, reaction time 4h (starting timing when the set temperature is reached), ammonium chloride dosage to chromium slag The test results of the influence of the metal element leaching rate are shown in Fig. 4.

It can be seen from Fig. 4 that the ratio of ammonium chloride to chromium slag has a significant effect on the leaching rate of Mg and Cr 6+ , both of which increase with the increase of the ratio; the effect on the leaching rate of Na and Ca is not obvious. This is because the oxides of Na and Ca are more porous due to their higher alkalinity, and are easily reacted with NH 4 Cl. The Mg oxide is weakly alkaline, and Cr 6+ has a reduction process. According to the test results, it is determined that the appropriate amount of ammonium chloride is 1.1 to 1.3 times the theoretical amount.

(4) Effect of particle size of chromium slag on metal leaching rate

Chromium slag mass 100g, ammonium chloride mass 192g (formed into 300g / L aqueous solution), FeC1 2 · 4H 2 0 mass 18g, stirring speed 300r / m, reaction temperature 120 ° C, reaction time 4h (to start at the set temperature) Timing), the effect of chromium slag particle size on metal leaching rate is shown in Figure 5. It can be seen that as the particle size of the chromium slag decreases, the leaching rate of all elements increases, and the leaching rate of Ca and Mg increases particularly. This is because the particle size is reduced, the specific surface area is increased, and the mass transfer is greatly improved, which is favorable for the progress of the reaction. However, too small a particle size means an increase in operating load, so the particle size cannot be too small. According to the test results, the chromium slag particle size is preferably from 100 to 150 μm.

(5) Effect of stirring speed on metal leaching rate

Chromium slag mass 100g (particle size below 100 mesh), ammonium chloride mass 192g (formed into 300g / L aqueous solution), FeC1 2 · 4H 2 0 mass 18g, reaction temperature 120 ° C, reaction time 4h (to start at the set temperature) Timing), the effect of stirring speed on the leaching rate of metal elements is shown in Fig. 6.

It can be seen from Fig. 6 that the leaching rate of each metal element is basically increased as the stirring speed is increased, but the increase is not large, and it can be seen that the reaction is not controlled by diffusion. The effect of the agitation speed on the reaction is related to the form of the agitator and the form of the reactor, so that a suitable agitation speed can only be determined for a particular reactor and agitation. The test results show that under the test conditions, the stirring speed is more suitable at 200-300r/min.

Third, the conclusion

According to the test results, the leaching of chromium slag with ammonium chloride can achieve high-efficiency leaching of metal elements such as calcium, magnesium, sodium and chromium in the chromium slag. Under the test conditions, the optimum process parameters for ammonium chloride leaching are: reaction temperature 120-140 ° C, reaction time 3 ~ 4 h, chlorination dosage 1.1-1.3 times theoretical dosage, chromium slag particle size 100-150 μm, stirring The speed is 200 to 300r/m. After the treatment, the quality of the chromium slag is greatly reduced, and the chromium-containing calcium-magnesium precipitation and ammonium chloride can be recycled, and the leaching residue can be used as a cement raw material after further treatment, thereby realizing slag-free discharge.

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