Method for treating wastewater in rare earth production

According to the composition of the discharged wastewater in the production of rare earth, the treatment methods are also different. Generally, the radioactive component and fluorine in the wastewater can be treated by the precipitation method, and the neutralization method is adopted for the treatment of the acid and the alkali. The following principles should be followed in selecting wastewater treatment methods [1] .
1 The selected processing method is stable and reliable, advanced and reasonable, with good processing effect, convenient operation and high technical index.
2 The various equipments selected are simple and reasonable, easy to manufacture and easy to maintain.
3 The final discharge of wastewater should be guaranteed to meet the requirements of national emission standards.
4 Construction investment costs are small, and the cost of treating wastewater is low.
  Treatment of radioactive wastewater
It can be seen from Table 10-4 that the main source of radioactive waste water in rare earth production is the alkaline decomposition of monazite ore. Although the composition of the wastewater is relatively complicated and the radioactive elements exceed the national standards, it is still a low level of radioactive waste water. The treatment methods can be divided into two major categories: chemical methods and ion exchange methods.
(1) Since the chemical treatment of radioactive waste in the hydroxide, carbonate, phosphate and the like are mostly insoluble compound, thus chemically treated mostly low radioactive waste water precipitation method. The purpose of the chemical treatment is to move the radioactive elements in the wastewater to the precipitated enrichment, thereby allowing the large-volume waste liquid to reach the national allowable discharge standards. The chemical treatment method is characterized by low cost, remarkable removal rate of most radioactive elements, simple equipment and convenient operation. Therefore, the chemical precipitation method is adopted in the nuclear energy and rare earth plants in China to remove radioactive elements in wastewater.
â‘  In addition to precipitation and uranium and thorium caustic soda solution was added to the wastewater to adjust the pH value between 7 and 9, in the form of uranium and thorium places hydroxide precipitation, chemical formula:
Th 4 +4NaOH→Th(OH) 4 ↓+4Na +
UO 2 2 ++2NaOH→UO 2 (OH) 2 ↓+2Na+
Sometimes, the precipitation may also be used, and calcium hydroxide do neutralizing agent, may be added during the aluminum salt (aluminum sulfate), a salt form colloidal iron (floes) adsorbing radioactive precipitate.
2 Sulfate eutectic precipitation deionization In the presence of sulfate ions, 10% of barium chloride solution [1] is added to the wastewater after removing uranium and plutonium to form barium sulfate precipitate, and radium is also Radium sulfate is formed and precipitates as a crystal precipitate with barium sulfate. The chemical reaction formula is:
Ba 2 +Ra 2 + +2SO 2 - 4 →BaRa(SO 4 ) 2 ↓
3 Polymer flocculant in addition to suspended solids Most of the flocculants used in rare earth production plants are high molecular weight polyacrylamide (PHP). According to the molecular weight, it can be divided into PHP flocculant suitable for alkaline medium and PHP flocculant suitable for acidic medium. PHP is a surfactant that generates many reactive groups after hydrolysis. It reduces the potential between the ion diffusion layer and the adsorption layer in the solution. It can adsorb many suspended solids and gels and bind them tightly into a floc. Agglomerates, which accelerate the sedimentation of suspended solids and gums.
After removing most of our uranium, thorium and radium from the radioactive waste water, the PHP flocculant is added. After sufficient agitation, the PHP flocculant is evenly distributed in the water. After standing and settling, the suspended solids and gels in the waste water and residual A small amount of radioactive elements will make the wastewater clear and reach the discharge standard.
It should be pointed out that the treatment of radioactive waste water by polymer PHP flocculant requires no milky organic phase in the wastewater, otherwise radioactive sediment floating phenomenon will occur, which will affect the quality of radioactive wastewater treatment.
(2) Ion exchange method The ion exchanger used for ion exchange to remove radioactive elements in a solution is an ion exchange resin and an inorganic ion exchanger. The ion exchange tree fingering method is only applicable to the case where the impurity ion concentration in the solution is relatively small. When the solution contains a large amount of impurity ions, it not only affects the use period of the ion exchange resin, but also reduces the saturation exchange capacity of the ion exchange resin. Radioactive wastewater having a constant competitive ion concentration of less than 1.0 to 1.5 kg/L is generally considered to be suitable for treatment using ion exchange tree fingering, and it is often necessary to first remove the constant competing ions when performing ion exchange treatment. A secondary ion exchange column can be used for this purpose, wherein the first stage is primarily used to remove constant competing ions and the second stage is primarily to remove radioactive ions. Therefore, the ion exchange resin method is particularly suitable for treating radioactive wastewater after chemical precipitation, as well as radioactive wastewater with low salt content and low turbidity, and can obtain high purification efficiency.
Inorganic ion exchangers are also widely used in the treatment of low-level radioactive wastewater. More applications are various types of inorganic ion-exchange clay mineral (e.g., montmorillonite, kaolinite, bentonite, vermiculite, etc.), travertine, manganese ore. The composition of the clay mineral and its special structure make it possible to adsorb H + in water to form a substance capable of cation exchange. Some clay minerals, such as kaolin and vermiculite, have small particles and are in a colloidal state in the water. The radioactive wastewater is usually treated by adsorption. The treatment of radioactive waste water from clay mines is often treated with flocculation and sedimentation to make the radioactive clay easy to settle and obtain good separation effect. For low-level radioactive wastewater (containing a small amount of natural radium, thorium and uranium), some rare earth plants are treated with pyrolusite (pH=7-8), and good treatment results are also obtained.
  Treatment of fluorine-containing wastewater
It can be seen from Table 10-4 that the fluorine content and pH value of the wastewater generated when the mixed rare earth concentrate is treated by the acid method or the alkali method exceeds the national emission standard, and the fluorine content of the acidic wastewater exceeds 120 to 280 times, and the alkali The fluorine content of the wastewater exceeds 40 to 50 times. Such wastewater needs to be treated before it can be discharged.
(1) Treatment of acidic fluorine-containing wastewater [1] At room temperature, the concentration of CaO is 50% to 70%, and the lime milk solution is added to the fluorine-containing wastewater to precipitate the fluoride with calcium fluoride. 0.5 to 1.0 h, while sulfuric acid is neutralized and reaches the acidity requirement for discharge. The chemical reaction formula is:
Ca(OH) 2 +2HF→CaF 2 ↓+2H 2 O
Ca(OH) 2 +H 2 SO 4 →CaSO 4 +2H 2 O
The main equipment of this method includes wastewater collection tank, neutralization sedimentation tank, filter and waste water pump. After the wastewater is treated, the fluorine content is reduced to less than 10 mg/L, and the pH is 6-8, which meets the requirements of the discharge standard.
(2) Treatment of alkaline fluorine-containing wastewater [1] At the temperature of the temperature, a lime milk solution having a concentration (CaO) of 10% is added to the wastewater to precipitate the fluorine as calcium fluoride, and the fluorine content is from 0.4 to 0.5 g/L. Drop to 15 ~ 20mg / L, and then add sodium metaphosphate and aluminum salt as a precipitant, so that fluorine further precipitates fluoroaluminate, the chemical reaction formula is:
Ca(OH) 2 +2NaF→CaF 2 ↓+2NaOH
NaPO 2 +Al 3 + +3F - →NaPO 3 ·AlF 3 ↓
The processing flow is as shown below.
 

Lime milk solution

 

Fluorine-containing wastewater

 

 

 

 

 

↓

 

 

 

 

↘

 

Primary defluoridation

 

 

 

 

 

↓

 

 

 

 

 

Settling filtration

 

 

 

 

 

↓

 

 

 

 

 

Sodium metaaluminate and aluminum salt

 

 

 

 

Filter residue

 

 

  filtrate

 

 

(discarded)

 

    ↓

 

 

 

 

 

 

Secondary defluoridation

 

 

 

 

 

↓

 

 

 

 

 

Settling filtration

 

 

 

 

 

↓

 

 

 

 

 

 

 

 

 

Filter residue

 

filtrate

 

 

           (discarded)

 

↓

 

 

 

 

Detection

 

 

 

 

↓

 

 

 

 

emission

In the figure, the flow chart of alkaline fluorine-containing wastewater treatment
When a fluorine, lm 3 waste water was added a solution of 0.025m 3 operation, the reaction time for 45 min, sedimentation time 0.5 ~ 1.0h. When the fluorine is removed twice, the lm 3 wastewater is added with 40 g of sodium metaaluminate and 160 g of aluminum salt, and the final pH of the wastewater is 6-7. The main equipments include wastewater collection tanks, fluorine removal reactors, and filters. After the defluoridation of the wastewater twice, the fluorine content is generally less than 10 mg/L, and the pH is 6-7, which meets the discharge standard.
  Treatment of acid-containing wastewater
When rare earth oxides are used to prepare rare earth oxides, the mother liquor after oxalic acid precipitation of rare earths has higher acidity (pH ≤ 1.5), mainly hydrochloric acid and oxalic acid, which can be discharged after treatment. Such wastewater treatment is relatively simple, and the neutralization treatment with waste caustic soda or lime emulsion can reduce the acidity, and the chemical reaction formula is:
Ca(OH) 2 +2HCl→CaCl 2 +2H 2 O
Ca(OH) 2 +H 2 C 2 O 4 →CaC 2 O 4 ↓+2H 2 O
The wastewater after neutralization treatment is in a clear state, the acidity is reduced to pH=7-8, and no harmful substances are contained, which meets the discharge standard [1] .

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