CN114644357A - Method for preparing ammonium metavanadate by sodium modification vanadium extraction combined alkali preparation and low-cost vanadium slag utilization - Google Patents

Abstract

The invention discloses a method for combined alkali production by sodium vanadium extraction and low-cost utilization of vanadium slag to prepare ammonium metavanadate. The method comprises the following steps: A. mixing and calcining vanadium slag and sodium carbonate; Obtain leaching residue and leaching solution, successively add calcium chloride and polyaluminum chloride to the leaching solution; C, add ammonium chloride to the qualified solution to carry out vanadium precipitation; D, add ferric chloride to remove vanadium in the vanadium precipitation tail liquid, then add chlorine Ferrous oxide and sodium hydroxide are used to remove chromium, and then ammonium bicarbonate is added to precipitate sodium bicarbonate; E, the filtrate is evaporated and crystallized to obtain ammonium chloride. The invention transforms the solution system from a complex multi-type anion system to a single anion system, thereby avoiding the collapse of the treatment system, and at the same time realizing the recycling of auxiliary materials and waste water when preparing ammonium metavanadate products with high efficiency, "three wastes""The emission is very low, which meets the national environmental protection requirements at this stage, and completely solves the environmental protection pressure currently faced by production enterprises.

Description

Sodium extraction and vanadium extraction combined with alkali production and low-cost method for preparing ammonium metavanadate by using vanadium slag

technical field

The invention relates to the technical field of vanadium extraction by hydrometallurgy, in particular to a method for combined alkali production by sodium vanadium extraction and low-cost utilization of vanadium slag to prepare ammonium metavanadate.

Background technique

The current world's mineral vanadium extraction technology mainly includes sodium vanadium extraction and calcium extraction vanadium. The sodium vanadium extraction process has many types of impurity elements in the ore and is also easy to sodium to form the corresponding sodium salt and dissolve in water. After a series of processes such as filtration and impurity removal, a precipitant is added to precipitate qualified metavanadic acid. Ammonium or ammonium polyvanadate, in a series of impurity removal and precipitation processes, correspondingly introduce different types of ions. In order to ensure the quality of the final product and product yield, it is generally added in excess, resulting in the precipitation of vanadium tail liquid. There are many kinds of ions, mainly sodium sulfate or sodium chloride. Generally speaking, the concentration of tail liquor after precipitating ammonium metavanadate is higher than that after precipitating ammonium polyvanadate. The main ions in tail liquor are sodium ion, ammonium ion, sulfate group, chloride ion and a small amount of vanadate group. and hexavalent chromium. At present, the wastewater of many vanadium products (hydrometallurgy) enterprises is treated by the process mode of pretreatment-membrane separation-evaporation and crystallization to obtain industrial sodium chloride and sodium sulfate, and sodium chloride is sometimes used as sodium ion during roasting. The supplements and pH adjusters of the roasting materials are returned to the front-end section for use, but the market use of sodium sulfate is relatively narrow, and it is difficult to find a market. The treatment method of some enterprises is to evaporate the mixed salt of sodium sulfate and sodium chloride directly after pretreatment, and its market application is more narrow. With the increasingly stringent national environmental protection requirements, the state has higher requirements for wastewater discharge standards for vanadium products (hydrometallurgy) enterprises, which strictly requires relevant enterprises to properly handle the wastewater and waste generated by the industry. However, the sodium sulfate obtained by evaporation in the existing treatment process has low added value, high processing cost and equipment requirements, high processing cost, high environmental protection pressure, and low product added value. The drawbacks are increasingly prominent.

In view of the current wastewater of vanadium hydrometallurgy, the Chinese Academy of Sciences has carried out relevant research on it, and proposed a process of concentration-soda-making ammonium sulfate. The solution disposal after ammonium sulfate filtration is to return to the previous metathesis reaction. Due to the presence of chloride ions in the solution, on the one hand, the purity of the finished product is affected, and on the other hand, the enrichment of chloride ions affects the reaction. Yield, but also a great impact on the corrosion of equipment. Therefore, the treatment method of this process will inevitably need to discharge concentrated brine due to the influence of impurities in the later stage of the cycle, and the disposal cost of such concentrated brine will be very high, which is also an inestimable increase in the treatment cost of this process.

Chinese patent CN110106346A discloses a method for preparing vanadium oxide by leaching and roasting clinker with ammonium carbonate. The steps are: 1. Mixing and roasting vanadium slag and calcium salt; 3. Add silicon remover to the leaching solution, then adjust pH to 7.8-8.5 by _CO The upper layer solution and the vanadium precipitation upper layer solution are directly used as leaching agent for recycling. In this patent, the pH of the solution after silicon removal is adjusted to 7.8-8.5 by introducing CO ₂ , which reduces the concentration of sodium ions in the solution and improves the rate of vanadium precipitation. Since there is no chromium removal process in this process, vanadium and chromium will not be produced. Filter cake and solid waste sodium sulfate realize low-cost and clean production of vanadium oxide from vanadium slag. However, what this patented technology adopts is the calcification vanadium extraction process. Compared with the sodium vanadium extraction, the calcification vanadium extraction has a large difference in the solubility of the roasted clinker, so the brine concentration of the calcification vanadium extraction is lower than that of the sodium vanadium extraction. Concentration, and because of this, the yield of the sodium vanadium extraction process is higher than that of the calcification vanadium extraction process. At the same time, when the vanadium precipitation upper layer liquid is recycled, the impurities such as chromium, molybdenum and phosphorus in the vanadium precipitation upper layer liquid are continuously enriched, which not only affects the purity of the finished product, but also affects the yield, which is not a preferred implementation. Program. In addition, this process has high requirements on the particle size of the material before leaching (high requirements for solid-liquid reaction), which has a great influence on the yield of vanadium.

SUMMARY OF THE INVENTION

The purpose of the invention of the present invention is to: in view of the above-mentioned problems, provide a kind of sodium vanadium extraction combined alkali production and low-cost method of utilizing vanadium slag to prepare ammonium metavanadate, to overcome the existing waste water treatment of the existing wet vanadium extraction process High cost, high difficulty, and low value of by-products.

The technical scheme adopted in the present invention is as follows: a method for sodium vanadium extraction and combined alkali production, comprising the following steps:

A, vanadium slag and sodium carbonate are mixed and roasted to obtain roasted clinker;

B, water leaching roasts the clinker to obtain leaching residue and leaching solution, successively adding calcium chloride and polyaluminum chloride to the leaching solution, to remove phosphorus and silicon in the leaching solution, to obtain dephosphorization slag, desiliconization slag and qualified solution;

C, in qualified solution, add ammonium chloride to carry out vanadium precipitation, filter to obtain ammonium metavanadate precipitation and precipitation vanadium tail liquor;

D, the vanadium precipitation tail liquid is adjusted pH to acidity with hydrochloric acid, then adds ferric chloride to remove vanadium, then adds ferrous chloride and sodium hydroxide to remove chromium, then adds ammonium bicarbonate to the solution to precipitate sodium bicarbonate, and filters to obtain Sodium bicarbonate precipitation and filtrate;

E. The filtrate is evaporated to form a supersaturated solution after adding hydrochloric acid to remove carbonate radicals, then cooling and separating out, and filtering to obtain ammonium chloride.

In the present invention, the vanadium slag and sodium carbonate are fully mixed in proportion, and the sodium calcination is carried out at 850-900 ° C. After the material is discharged, water is leached. After leaching and filtration, calcium chloride and polyaluminum chloride are successively used to remove the solution in the solution. Impurities of phosphorus and silicon are filtered to obtain a qualified solution, then ammonium chloride is added to precipitate vanadium, the precipitated ammonium metavanadate is purified and fused to obtain flakes of vanadium. Iron to remove vanadium, add ferrous chloride and sodium hydroxide to remove chromium, then add ammonium bicarbonate to precipitate sodium bicarbonate, filter, add hydrochloric acid to remove carbonate, evaporate to supersaturation, and cool to precipitate ammonium chloride crystals. When preparing the ammonium metavanadate product, the process of the invention adopts sodium extraction to ensure the yield of vanadium, and at the same time, the sodium bicarbonate and ammonium chloride with higher product value are prepared by using the vanadium precipitation tail liquid as the raw material for alkali production. To a certain extent, the cost of wastewater treatment is reduced, the value of by-products is increased, and the environmental protection pressure of production enterprises is relieved.

Further, before the roasting, the vanadium slag is crushed to pass a 100-mesh sieve.

Furthermore, in A, the calcination temperature is 850-900 degreeC.

Further, in B, the water immersion time is 1-3h. In order to improve the leaching rate, the water immersion is carried out by countercurrent washing (multi-stage washing) to ensure that the converted soluble vanadium enters the solution as much as possible.

Further, in step D, the ferric vanadate obtained by adding ferric chloride to remove vanadium is recycled as vanadium slag.

Further, in step D, after adding ferrous chloride, the solution is adjusted to neutrality with sodium hydroxide, and then left to stand and filter to remove chromium.

Further, in step D, ammonium bicarbonate is added in a ratio of 1:1.1-1.3 molar ratio of sodium ion to bicarbonate ion.

The present invention also includes a low-cost method for preparing ammonium metavanadate by utilizing vanadium slag, comprising the following steps:

S1. The vanadium slag is crushed and ground, mixed with sodium carbonate or/and sodium bicarbonate, and roasted at 850-900 ° C for 1-3 hours to obtain roasted clinker;

S2, water immersion roasting clinker, water immersion time is 1-3h, obtain leaching slag and leaching solution, add calcium chloride and polyaluminum chloride successively to leaching solution, in order to remove phosphorus and silicon in leaching solution, obtain dephosphorization slag, Desiliconization slag and qualified liquid;

S3, add ammonium chloride to the qualified solution to carry out vanadium precipitation, filter to obtain ammonium metavanadate precipitation and precipitation vanadium tail liquor, ammonium metavanadate obtains vanadium pentoxide finished product after purification and roasting, and the precipitation vanadium tail liquor is used as raw material for making alkali deal with;

S4, the vanadium precipitation tail liquor is adjusted pH to acidity with hydrochloric acid, then adds ferric chloride to remove vanadium, then successively adds ferrous chloride and sodium hydroxide to the solution to remove chromium;

S5, evaporate and concentrate the solution obtained in step S4, then add ammonium bicarbonate according to the molar ratio of sodium ions to bicarbonate ions of 1:1.1-1.3, and react for a certain period of time to obtain sodium bicarbonate precipitation and filtrate by filtration, sodium bicarbonate The precipitation is directly used as the raw material for sodium vanadium extraction;

S6, the filtrate is evaporated to form a supersaturated solution after adding hydrochloric acid to remove carbonate radicals, then cooled and separated out, filtered to obtain ammonium chloride, and the ammonium chloride is directly returned to S2 for use as an auxiliary material, and the filtered liquid is used as a water source in the water leaching step.

In the present invention, in the sodium roasting stage, the sodium carbonate raw material can be obtained from the product sodium bicarbonate obtained by the combined alkali production process, thereby realizing the recycling of the alkali material, and at the same time, the ammonium chloride product obtained by the combined alkali production process can be used as vanadium precipitation The use of raw materials in this stage not only realizes the recycling of chloride salts, but also realizes the recycling of part of ammonium salts (about 50% ammonium salts) by means of ammonium chloride products, which greatly reduces the emission of ammonia gas. The wastewater generated in the ammonium chloride stage can be directly used as the water source in the sodium vanadium extraction stage, thus avoiding the discharge of wastewater. Further, the present invention uses ammonium bicarbonate to prepare sodium bicarbonate, and does not use carbonate to directly prepare sodium carbonate, the reason is that the solubility of sodium bicarbonate in water is significantly lower than the solubility of sodium carbonate in water, and ammonium bicarbonate is used to The preparation of sodium bicarbonate can obtain more sodium bicarbonate products, and can also reduce the salt concentration in wastewater.

Further, the vanadium slag is a vanadium slag obtained by smelting vanadium titanomagnetite, or a waste petroleum catalyst containing vanadium.

To sum up, due to the adoption of the above-mentioned technical solutions, the beneficial effects of the present invention are:

1. In the present invention, the auxiliary material containing chloride ions is used to replace the auxiliary material containing sulfate ions, so that the solution system is changed from a complex variety of anions to a single anion system, thereby avoiding the later cycle use, due to the richness of some ions. System crash caused by the set;

2. The present invention converts the existing precipitation ammonium polyvanadate process into a precipitation ammonium metavanadate process, which can make the vanadium concentration in the solution higher, help to improve the vanadium precipitation efficiency, and the increase of the salt concentration in the waste water also has It helps to improve the efficiency of the subsequent combined alkali-making process; at the same time, during the vanadium precipitation, it is no longer necessary to heat and adjust the pH to an acidic condition, which saves energy consumption and auxiliary material costs, and reduces production costs;

3. The present invention realizes the recycling of auxiliary materials and waste water when preparing ammonium metavanadate products with high efficiency, and the discharge amount of "three wastes" is very low (solid waste only has leaching slag, dephosphorization slag and desiliconization slag, which is almost realized. The waste water is fully circulated, and the amount of waste gas generated is extremely low, which can be ignored), which meets the current national environmental protection requirements and completely solves the environmental protection pressure currently faced by production enterprises;

4. In the method of the present invention, the conversion rate of vanadium is greater than 90%, the total yield of vanadium reaches more than 85%, and the obtained ammonium metavanadate is roasted to obtain a vanadium pentoxide (vanadium sheet) product, and the vanadium pentoxide product is obtained after roasting. Inspection, its quality indicators meet the quality requirements of 98% of the grades in YB/T5304-2017.

Description of drawings

Fig. 1 is a kind of low-cost method schematic diagram of the present invention utilizing vanadium slag to prepare ammonium metavanadate;

Fig. 2 is the vanadium pentoxide product figure obtained after the ammonium metavanadate roasting prepared by the method of the present invention;

Fig. 3 is the product figure of sodium ammonium bicarbonate prepared by the sodium extraction vanadium extraction combined alkali production method of the present invention.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings.

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, but not to limit the present invention.

As shown in Figure 1, a low-cost method for utilizing vanadium slag to prepare ammonium metavanadate comprises the following steps:

S1. The vanadium slag is crushed and ground, mixed with sodium carbonate or/and sodium bicarbonate, and roasted at 850-900 ° C for 1-3 hours to obtain roasted clinker;

S2, water immersion roasting clinker, water immersion time is 1-3h, obtain leaching slag and leaching solution, add calcium chloride and polyaluminum chloride successively to leaching solution, in order to remove phosphorus and silicon in leaching solution, obtain dephosphorization slag, Desiliconization slag and qualified liquid;

S3, add ammonium chloride to the qualified solution to carry out vanadium precipitation, filter to obtain ammonium metavanadate precipitation and precipitation vanadium tail liquor, ammonium metavanadate obtains vanadium pentoxide finished product after purification and roasting, and the precipitation vanadium tail liquor is used as raw material for making alkali deal with;

S4, the vanadium precipitation tail liquor is adjusted pH to acidity with hydrochloric acid, then adds ferric chloride to remove vanadium, obtains iron vanadate, iron molybdate, and iron vanadate can be used as vanadium slag recycling, then adds oxychloride successively to the solution Iron is used to reduce chromium, then pH is adjusted to neutrality with sodium hydroxide to precipitate chromium sludge, which is filtered;

S5, evaporate and concentrate the solution filtered in step S4, then add ammonium bicarbonate according to the molar ratio of sodium ion to bicarbonate ion of 1:1.1-1.3, after metathesis reaction for a certain period of time, filter to obtain sodium bicarbonate precipitate and filtrate, carbonic acid Sodium hydrogen precipitation is directly used as the raw material for sodium vanadium extraction;

S6, the filtrate is evaporated to form a supersaturated solution after adding hydrochloric acid to remove carbonate, then cooling and precipitation, filtering to obtain ammonium chloride, ammonium chloride is directly used as a vanadium precipitation auxiliary material, and the filtered liquid and condensed water are used as the water source of the water immersion step.

In order to better implement the present invention, specific examples are listed below:

Example 1

In this example, vanadium slag smelted from vanadium titanomagnetite (in mass percentage, the main components of vanadium slag include: CaO: 1.5-2.5%, SiO ₂ : 16-18%, V ₂ O ₅ : 16-18%, TFe: 30-35%, P: 0.057%) as raw materials, the above-mentioned process is used to conduct the test, including the following steps:

S1. After crushing and grinding the vanadium slag and passing through a 100-mesh sieve, the iron filings are magnetically separated and mixed with sodium carbonate. The mixing mass ratio is 10:3. After roasting at 850°C for 1.5 hours, take out to obtain roasted clinker;

S2, take roasting clinker, according to the ratio of solid-to-liquid ratio of 1:2, leaching with water to the vanadium content in the leaching residue as low as possible, if necessary, use countercurrent washing to improve the washing efficiency and increase the concentration of vanadium in the solution, filter to obtain the filtrate spare;

S3. Add calcium chloride and polyaluminum chloride to the filtrate in turn to remove phosphorus and silicon. The addition amount of calcium chloride is 2-3 times (molar ratio) of impurity phosphorus, and the addition amount of polyaluminum chloride is 1.5 times that of impurity silicon. times (molar ratio), filtered for use;

S4, in the filtrate of step S3, add ammonium chloride, precipitate out ammonium metavanadate, filter, obtain ammonium metavanadate finished product, remaining filtrate is raw material for making alkali, and the add-on of ammonium chloride is 3 times (molar) of vanadium equivalent Compare);

S5, adjust the pH of the filtrate of step S4 to acidity with hydrochloric acid, then add ferric chloride to remove vanadium, and the addition of ferric chloride is 1/3 (molar equivalent) of vanadium equivalent in the tail liquor; the ferric vanadate precipitation obtained Return to the aforementioned sodium roasting, add ferrous chloride to the filtrate (the amount added is 30-40 times the equivalent of chromium, the lower the chromium content, the higher the equivalent), after reducing the chromium, adjust the pH to neutrality with sodium hydroxide, filter Remove chromium, the filtrate is ready for use;

S6, the filtrate of evaporation concentration step S5 is nearly saturated, then adds ammonium bicarbonate by sodium ion concentration molar ratio 1:1.1-1.3, reacts after half an hour to filter to obtain sodium bicarbonate, and the filtrate is for subsequent use;

S7, in the filtrate of step S6, add hydrochloric acid to adjust pH after decarbonization, evaporate to supersaturation, cool and separate out to obtain ammonium chloride crystal;

S8, repeat the above steps, use the obtained sodium bicarbonate and ammonium chloride as auxiliary materials to carry out sodium extraction of vanadium, after the obtained ammonium metavanadate is roasted into vanadium pentoxide (vanadium sheet), after inspection, it meets the national standard.

In the above experimental process, the conversion rate of vanadium roasting is >90%, which is basically consistent with the current process, and the total yield of vanadium can reach more than 85%, which is slightly higher than the current process. Because the dephosphorization slag and desiliconization slag are returned to the leaching system and mixed with the leaching slag At the same time, for each 1 ton of vanadium slag treated, the amount of leaching slag (20-25% water content) is about 1 ton, the yield of sodium is about 50%, and the yield of ammonia nitrogen is about 67% (single cycle). This process saves the cost of steam, and the cost of subsequent evaporation and crystallization can also be covered by the purchase cost of auxiliary materials. The obtained vanadium pentoxide (as shown in Figure 2) is melted and inspected, and its quality index conforms to YB/T5304-2017 The quality requirements of 98% of the grades. Since the dephosphorization slag and the desiliconization slag are returned to the leaching system and mixed with the leaching slag, the amount of leaching slag (with water content of 20-25%) is about 1 ton per ton of vanadium slag treated, and the yield of sodium is about 50%. The yield was about 67% (single cycle).

Example 2

In this example, the waste vanadium-containing petroleum catalyst (solid, in mass percentage), the main components of the waste vanadium-containing petroleum catalyst include: moisture 5-10%, waste oil 10-15%, vanadium 5-7%, alumina 30% , cobalt＜2%, nickel＜2%) as vanadium slag raw material, adopt the above-mentioned process to carry out the test, including the following steps:

S1, the waste and old vanadium-containing petroleum catalyst is crushed and ground and mixed with sodium carbonate after passing through a 100-mesh sieve, and the mixing mass ratio is 100:35, and after 1.5 hours of roasting at 850 °C, take out to obtain roasted clinker;

S2. Take the roasted clinker, according to the ratio of mass-solid-to-liquid ratio of 1:2, leaching with water until the vanadium content in the leaching slag is as low as possible, and if necessary, use countercurrent washing to improve the washing efficiency and increase the vanadium concentration in the solution. Filtrate backup;

S3. Add calcium chloride, polyaluminum chloride to remove phosphorus and silicon in turn into the filtrate. The amount of calcium chloride added is 2-3 times (molar ratio) of impurity phosphorus, and the amount of calcium chloride The amount of addition is 1.5 times (molar ratio) of impurity silicon, and it is filtered for subsequent use;

S4, in the filtrate of step S3, add ammonium chloride, precipitate out ammonium metavanadate, filter, obtain ammonium metavanadate finished product, and the remaining filtrate is raw material for making alkali, 3 times (mol ratio) of vanadium equivalent;

S5, adjust the pH of the filtrate of step S4 to acidity with hydrochloric acid, then add ferric chloride to remove vanadium, and the addition of ferric chloride is 1/3 (molar equivalent) of vanadium equivalent in the tail liquor; the ferric vanadate precipitation obtained Return to the aforementioned sodium roasting, add ferrous chloride to the filtrate (the amount added is 30-40 times the equivalent of chromium, the lower the chromium content, the higher the equivalent), after reducing the chromium, adjust the pH to neutrality with sodium hydroxide, filter Remove chromium, the filtrate is ready for use;

S6, the filtrate of evaporation concentration step S5 is nearly saturated, then adds ammonium bicarbonate by sodium ion concentration molar ratio 1:1.1-1.3, reacts after half an hour to filter to obtain sodium bicarbonate, and the filtrate is for subsequent use;

S7, in the filtrate of step S6, add hydrochloric acid to adjust pH after decarbonization, evaporate to supersaturation, cool and separate out to obtain ammonium chloride crystal;

S8, repeat the above steps, use the obtained sodium bicarbonate and ammonium chloride as auxiliary materials to carry out sodium extraction of vanadium, after the obtained ammonium metavanadate is roasted into vanadium pentoxide (vanadium sheet), after inspection, it meets the national standard.

In the above-mentioned experimental process, the roasting conversion rate of vanadium can reach 92%, which is higher than the current conversion rate of 88%, and the total yield can reach more than 90%, which is also higher than the yield level of 85% in the current process. Phosphorus slag and desiliconization slag are returned to the leaching system and mixed with leaching slag. Therefore, for each 1 ton of vanadium slag treated, the leaching slag content (20-25% water content) is about 1 ton, the yield of sodium is about 50%, and the yield of ammonia nitrogen is about 50%. About 67% (single cycle). This process completely realizes the recycling of sodium ions and water and the recycling of part of ammonium (50%) in the sodium vanadium extraction process, reduces the production cost, reduces the environmental pollution while improving the production efficiency, and the obtained five After the vanadium oxide is melted, the quality index meets the quality requirements of 98% of the grades in YB/T5304-2017.

The above descriptions are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention shall be included in the protection of the present invention. within the range.

Claims (9)

1. A method for preparing alkali by combining sodium modification and vanadium extraction is characterized by comprising the following steps:

A. uniformly mixing vanadium slag with sodium carbonate or/and sodium bicarbonate, and roasting to obtain roasted clinker;

B. leaching the roasted clinker with water to obtain leaching slag and leaching liquid, and sequentially adding calcium chloride and polyaluminium chloride into the leaching liquid to remove phosphorus and silicon in the leaching liquid to obtain dephosphorized slag, desilicated slag and qualified liquid;

C. adding ammonium chloride into the qualified solution to precipitate vanadium, and filtering to obtain ammonium metavanadate precipitate and vanadium precipitation tail solution;

D. regulating the pH of the vanadium precipitation tail liquid to acidity by hydrochloric acid, then adding ferric chloride to remove vanadium, then adding ferrous chloride and sodium hydroxide to remove chromium, then adding ammonium bicarbonate into the solution to precipitate sodium bicarbonate, and filtering to obtain sodium bicarbonate precipitate and filtrate;

E. and adding hydrochloric acid into the filtrate to remove carbonate, evaporating to form supersaturated solution, cooling to separate out, and filtering to obtain ammonium chloride.

2. The method for preparing alkali by combining sodium modification and vanadium extraction as claimed in claim 1, wherein before roasting, the vanadium slag is crushed to pass through a 100-mesh sieve.

3. The method for preparing alkali by sodium-modified vanadium extraction in combination as claimed in claim 2, wherein in A, the roasting temperature is 850-.

4. The method for preparing alkali by sodium-modified vanadium extraction in combination as claimed in claim 1, wherein in B, the water leaching time is 1-3 h, and the water leaching is carried out by adopting a counter-current washing mode.

5. The method for preparing alkali by sodium modification and vanadium extraction in combination as claimed in claim 1, wherein in step D, iron vanadate obtained by removing vanadium by adding iron chloride is recycled as vanadium slag.

6. The method for preparing alkali by sodium-modified vanadium extraction in combination as claimed in claim 1, wherein in the step D, after ferrous chloride is added, the solution is adjusted to be neutral by sodium hydroxide, and then the solution is kept still to remove chromium by filtration.

7. The method for preparing alkali by combining sodium-modified vanadium extraction and according to any one of claims 1 to 6, wherein in the step D, the ratio of sodium ions to bicarbonate ions is 1: 1.1-1.3 of ammonium bicarbonate.

8. A method for preparing ammonium metavanadate from vanadium slag at low cost is characterized by comprising the following steps:

s1, crushing and grinding the vanadium slag, mixing the crushed and ground vanadium slag with sodium carbonate or/and sodium bicarbonate, and roasting for 1-3 h at the temperature of 850-;

s2, leaching the roasted clinker by water for 1-3 h to obtain leaching slag and leaching liquid, and adding calcium chloride and polyaluminium chloride into the leaching liquid in sequence to remove phosphorus and silicon in the leaching liquid to obtain dephosphorized slag, desilicated slag and qualified liquid;

s3, adding ammonium chloride into the qualified liquid to carry out vanadium precipitation, filtering to obtain ammonium metavanadate precipitate and a vanadium precipitation tail liquid, purifying and roasting the ammonium metavanadate to obtain a vanadium pentoxide finished product, and treating the vanadium precipitation tail liquid as an alkali preparation raw material;

s4, adjusting the pH value of the vanadium precipitation tail liquid to acidity by hydrochloric acid, then adding ferric chloride to remove vanadium, and then adding ferrous chloride and sodium hydroxide into the solution in sequence to remove chromium;

s5, evaporating and concentrating the solution obtained in the step S4, and then mixing the solution according to the molar ratio of sodium ions to bicarbonate ions of 1: 1.1-1.3, adding ammonium bicarbonate, reacting for a certain time, filtering to obtain sodium bicarbonate precipitate and filtrate, wherein the sodium bicarbonate precipitate is directly used as a sodium-modified vanadium extraction raw material;

and S6, adding hydrochloric acid into the filtrate to remove carbonate, evaporating to form supersaturated solution, cooling to separate out, filtering to obtain ammonium chloride, directly returning the ammonium chloride to S2 to be used as an auxiliary material, and using the filtered liquid as a water source in the water leaching step.

9. The method for preparing ammonium metavanadate from vanadium slag at low cost according to claim 8, wherein the vanadium slag is vanadium slag obtained by smelting vanadium titanomagnetite or is a vanadium-containing waste petroleum catalyst.

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