Method for decomposing medium-/low-grade scheelite

Abstract

The present invention discloses a method for decomposing medium-/low-grade scheelite, specifically comprising steps of: grinding medium-/low-grade scheelite, decomposing in an autoclave by using sodium phosphate and activated magnesium fluoride as leaching agents, and treating by solid-liquid separation to obtain crude sodium tungstate solution and residue. In this way, the medium-/low-grade scheelite is decomposed. Magnesium chloride is added in a sodium fluoride solution to prepare activated magnesium fluoride as a leaching agent. The present invention has the advantage that the high-efficiency decomposition of medium-/low-grade scheelite can be realized with low consumption of leaching agents, and the leaching cost can be greatly reduced in comparison to the existing decomposition processes using sodium hydroxide and sodium carbonate. This process is short in route, simple in operation, readily available and reliable in production equipment, and easy for industrialization.

Claims

1. A method for decomposing scheelite, comprising steps of: step (1): grinding scheelite to below 325 meshes; step (2): preparing activated magnesium fluoride: adding magnesium chloride in an amount that is 1.1 to 1.3 times of a theoretical amount required to decompose scheelite in a sodium fluoride solution having a concentration of 30 to 40 g/L, and reacting at 60° C. to 80° C. under stirring to generate activated magnesium fluoride; and step (3): adding the ground scheelite in an autoclave, adding sodium phosphate, activated magnesium fluoride and water for hydrolysis under stirring, and treating by solid-liquid separation to obtain crude sodium tungstate leachate and residue containing calcium and magnesium.

2. The method according to claim 1, wherein the content of WO.sub.3 in the scheelite is 18 to 42 wt. %.

3. The method according to claim 1, wherein, in the step (2), a reaction time is 30 to 60 min, and a stirring speed is 200 to 350 r/min.

4. The method according to claim 1, wherein a consumption of the sodium phosphate is 1.5 to 2.5 times of the theoretical amount required to decompose scheelite, and a consumption of the activated magnesium fluoride is 1.2 to 2.5 times of the theoretical amount required to decompose scheelite.

5. The method according to claim 1, wherein in step (3) a reaction temperature is 160° C. to 220° C., a liquid-to-solid ratio in a reaction system is 3:1 to 5:1 ml/g, a reaction time is 1.5 h to 5 h, and a stirring speed is 200 to 400 r/min.

Description

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(1) The present invention provides a method for decomposing medium-/low-grade scheelite, including the following steps.

(2) Step 1: The medium-/low-grade scheelite is grinded by a vibrating ball mill, the grain size of the grinded ore being below 325 meshes.

(3) Step 2: Sodium fluoride solution having a concentration of 30 to 40 g/L is prepared and then added with magnesium chloride in an amount that is 1.1 to 1.3 times of the theoretical amount required to decompose scheelite; the mixture is reacted for 30 to 60 min at60° C. to 80° C. and at a stirring speed of 200 to 350 r/min; and, at the end of reaction, the mixture is treated by solid-liquid separation, where the residue is activated magnesium fluoride for further use as a leaching agent for the medium-/low-grade scheelite, and the filtrate is a mother liquor containing magnesium chloride.

(4) Step 3: The grinded medium-/low-grade scheelite is added in an autoclave, then added with sodium phosphate in an amount that is 1.5 to 2.5 times of the theoretical amount of sodium phosphate and activated magnesium fluoride in an amount that is 1.2 to 2.5 times of the theoretical amount of activated magnesium fluoride, and added with water at a liquid-to-solid ratio of 3:1 to 5:1 ml/g; the mixture is stirred and reacted for 1.5 h to 5 h at 160° C. to 220° C. and at a stirring speed of 200 to 400 r/min; and, the mixture is treated by solid-liquid separation to obtain leachate containing tungsten and residue containing calcium and magnesium.

(5) It can be known from the chemical equilibrium that, during the leaching of scheelite, if the solubility product of resulting products is smaller, the reaction equilibrium constant of the scheelite decomposition is larger. It is advantageous for efficient decomposition of scheelite. On this basis, the present invention proposes that the medium-/low-grade scheelite is collaboratively leached by sodium phosphate and activated magnesium fluoride to generate calcium fluophosphate with a very small solubility (Ksp=1×10.sup.−59). In this way, the efficient decomposition of scheelite is realized. In this process, the activated magnesium fluoride is prepared by reacting sodium fluoride solution with magnesium chloride. Compared with the ordinary magnesium fluoride reagent, the activated magnesium fluoride has the characteristics of high activity and high dissolution rate, so that the leaching of scheelite is accelerated to obtain crude sodium tungstate leachate and residue containing calcium and magnesium. In this way, the efficient decomposition of the medium-/low-grade scheelite is realized with low consumption of leaching agents.

(6) It is to be noted that, unlike the decomposition of scheelite by using sodium fluoride, magnesium fluoride will be slowly dissolved during the leaching process due to its low solubility and then reacted with sodium phosphate and calcium tungstate to generate calcium fluophosphate. Accordingly, excessive fluorine in the leachate is avoided, and the burden for the subsequent wastewater treatment is reduced. This process can overcome the disadvantages of high consumption of leaching agents, high production cost and low decomposition rate during the treatment of the medium-/low-grade scheelite by conventional processes of autoclaving using soda and decomposing using sodium hydroxide.

(7) This process will be described below by embodiments. Although the technical solutions of the present invention are to be specifically described, the embodiments to be described are some but not all of the embodiments of the present invention.

(8) Comparison Embodiment 1

(9) 120 g of the grinded (medium-grade) scheelite having a content of WO.sub.3 of 42.3% was added in an autoclave, and then added with sodium phosphate in an amount that is 1.5 times of the theoretical amount required to decompose scheelite and ordinary magnesium fluoride in an amount that is 1.5 times of the theoretical amount required to decompose scheelite, where the liquid-to-solid ratio during the reaction was 5:1 mg/l. When the reaction temperature was up to 180° C., the system was reacted at a stirring speed of 250 r/min, maintained at this temperature for 2.5 h, and filtrated to obtain residue containing calcium and magnesium and crude sodium tungstate leachate, where the tungsten leaching rate was 94.3%.

(10) Embodiment 1

(11) Step 1: The medium-grade scheelite (having a content of WO.sub.3 of 42.3%) was grinded by a vibrating ball mill, the grain size of the grinded ore being below 325 meshes.

(12) Step 2: Sodium fluoride solution having a concentration of 40 g/L was prepared and then added with magnesium chloride in an amount that is 1.1 times of the theoretical amount required to decompose scheelite; and, the mixture was reacted for 30 min at 80° C. and at a stirring speed of 200 r/min and then filtrated to obtain activated magnesium fluoride precipitate for use as a leaching agent for the medium-/low-grade scheelite.

(13) Step 3: 120 g of the grinded scheelite was added in an autoclave, and then added with sodium phosphate in an amount that is 1.5 times of the theoretical amount required to decompose scheelite and activated magnesium fluoride in an amount that is 1.5 times of the theoretical amount required to decompose scheelite, where the liquid-to-solid ratio during the reaction was 5:1 mg/l. When the reaction temperature was up to 180° C., the system was reacted at a stirring speed of 250 r/min, maintained at this temperature for 2.5 h, and filtrated to obtain residue containing calcium and magnesium and crude sodium tungstate leachate, where the tungsten leaching rate was 98.6%.

Embodiment 2

(14) Step 1: The medium-grade scheelite (having a content of WO.sub.3 of 40.6%) was grinded by a vibrating ball mill, the grain size of the grinded ore being below 325 meshes.

(15) Step 2: Sodium fluoride solution having a concentration of 35 g/L was prepared and then added with magnesium chloride in an amount that is 1.2 times of the theoretical amount required to decompose scheelite; and, the mixture was reacted for 60 min at 60° C. and at a stirring speed of 250 r/min and then filtrated to obtain activated magnesium fluoride precipitate for use as a leaching agent for the medium-/low-grade scheelite.

(16) Step 3: 150 g of the grinded scheelite was added in an autoclave, and then added with sodium phosphate in an amount that is 1.5 times of the theoretical amount required to decompose scheelite and activated magnesium fluoride in an amount that is 1.6 times of the theoretical amount required to decompose scheelite, where the liquid-to-solid ratio during the reaction was 5:1 mg/l. When the reaction temperature was up to 160° C., the system was reacted at a stirring speed of 300 r/min, maintained at this temperature for 4 h, and filtrated to obtain residue containing calcium and magnesium and crude sodium tungstate leachate, where the tungsten leaching rate was 96.2%.

Embodiment 3

(17) Step 1: The medium-grade scheelite (having a content of WO.sub.3 of 37.6%) was grinded by a vibrating ball mill, the grain size of the grinded ore being below 325 meshes.

(18) Step 2: Sodium fluoride solution having a concentration of 30 g/L was prepared and then added with magnesium chloride in an amount that is 1.3 times of the theoretical amount required to decompose scheelite; and, the mixture was reacted for 50 min at 70° C. and at a stirring speed of 300 r/min and then filtrated to obtain activated magnesium fluoride precipitate for use as a leaching agent for the medium-grade scheelite.

(19) Step 3: 150 g of the grinded medium-grade scheelite was added in an autoclave, and then added with sodium phosphate in an amount that is 1.7 times of the theoretical amount required to decompose scheelite and activated magnesium fluoride in an amount that is 1.8 times of the theoretical amount required to decompose scheelite, where the liquid-to-solid ratio during the reaction was 4:1 mg/l. When the reaction temperature was up to 220° C., the system was reacted at a stirring speed of 350 r/min, maintained at this temperature for 2.0 h, and filtrated to obtain residue containing calcium and magnesium and crude sodium tungstate leachate, where the tungsten leaching rate was 99.1%.

Embodiment 4

(20) Step 1: The low-grade scheelite (having a content of WO.sub.3 of 24.4%) was grinded by a vibrating ball mill, the grain size of the grinded ore being below 325 meshes.

(21) Step 2: Sodium fluoride solution having a concentration of 38 g/L was prepared and then added with magnesium chloride in an amount that is 1.1 times of the theoretical amount required to decompose scheelite; and, the mixture was reacted for 40 min at 80° C. and at a stirring speed of 250 r/min and then filtrated to obtain activated magnesium fluoride precipitate for use as a leaching agent for the low-grade scheelite.

(22) Step 3: 180 g of the grinded low-grade scheelite was added in an autoclave, and then added with sodium phosphate in an amount that is 1.5 times of the theoretical amount required to decompose scheelite and activated magnesium fluoride in an amount that is 1.3 times of the theoretical amount required to decompose scheelite, where the liquid-to-solid ratio during the reaction was 5:1 mg/l. When the reaction temperature was up to 170° C., the system was reacted at a stirring speed of 200 r/min, maintained at this temperature for 3.5 h, and filtrated to obtain residue containing calcium and magnesium and crude sodium tungstate leachate, where the tungsten leaching rate was 96.8%.

Embodiment 5

(23) Step 1: The low-grade scheelite (having a content of WO.sub.3 of 18.4%) was grinded by a vibrating ball mill, the grain size of the grinded ore being below 325 meshes.

(24) Step 2: Sodium fluoride solution having a concentration of 35 g/L was prepared and then added with magnesium chloride in an amount that is 1.2 times of the theoretical amount required to decompose scheelite; and, the mixture was reacted for 50 min at 70° C. and at a stirring speed of 220 r/min and then filtrated to obtain activated magnesium fluoride precipitate for use as a leaching agent for the low-grade scheelite.

(25) Step 3: 160 g of the grinded low-grade scheelite was added in an autoclave, and then added with sodium phosphate in an amount that is 2.5 times of the theoretical amount required to decompose scheelite and activated magnesium fluoride in an amount that is 2.5 times of the theoretical amount required to decompose scheelite, where the liquid-to-solid ratio during the reaction was 3:1 mg/l. When the reaction temperature was up to 200° C., the system was reacted at a stirring speed of 400 r/min, maintained at this temperature for 2.5 h, and filtrated to obtain residue containing calcium and magnesium and crude sodium tungstate leachate, where the tungsten leaching rate was 98.8%.

Embodiment 6

(26) Step 1: The low-grade scheelite (having a content of WO.sub.3 of 21.1%) was grinded by a vibrating ball mill, the grain size of the grinded ore being below 325 meshes.

(27) Step 2: Sodium fluoride solution having a concentration of 32 g/L was prepared and then added with magnesium chloride in an amount that is 1.2 times of the theoretical amount required to decompose scheelite; and, the mixture was reacted for 40 min at 80° C. and at a stirring speed of 200 r/min and then filtrated to obtain activated magnesium fluoride precipitate for use as a leaching agent for the low-grade scheelite.

(28) Step 3: 140 g of the grinded low-grade scheelite was added in an autoclave, and then added with sodium phosphate in an amount that is 1.6 times of the theoretical amount required to decompose scheelite and activated magnesium fluoride in an amount that is 1.3 times of the theoretical amount required to decompose scheelite, where the liquid-to-solid ratio during the reaction was 4:1 mg/l. When the reaction temperature was up to 180° C., the system was reacted at a stirring speed of 250 r/min, maintained at this temperature for 5 h, and filtrated to obtain residue containing calcium and magnesium and crude sodium tungstate leachate, where the tungsten leaching rate was 96.5%.

(29) It can be known from the data in the embodiments that, compared with the use of ordinary magnesium fluoride as a leaching agent, the activated magnesium fluoride and sodium phosphate can better decompose the medium-/low-grade scheelite.By properly increasing the amount of sodium phosphate and the activated magnesium fluoride and increasing the reaction temperature, it is advantageous for decomposition of scheelite. Compared with the existing processes for decomposing the medium-/low-grade scheelite by using sodium hydroxide and sodium carbonate, this process can significantly reduce the consumption of leaching agents and reduce the production cost.

(30) The foregoing embodiments are merely for describing the present invention, rather than limiting the present invention.