Tungsten and arsenic substance flow analysis of a hydrometallurgical process for tungsten extracting from wolframite Tungsten

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Tungsten (2021) 3:348–360

https://doi.org/10.1007/s42864-021-00090-w ORIGINAL PAPER

Tungsten and arsenic substance flow analysis of a hydrometallurgical process for tungsten extracting from wolframite

Yuan‑Lin Chen¹ · Xue‑Yi Guo¹ · Qin‑Meng Wang¹ · Qing‑Hua Tian¹ · Shao‑Bo Huang² · Jin‑Xiang Zhang³

Received: 5 February 2021 / Revised: 24 February 2021 / Accepted: 3 March 2021 / Published online: 15 June 2021

Abstract

In this study, the metabolism of a hydrometallurgical process for tungsten extracting from wolframite was studied through substance flow analysis. The mass balance accounts, substance flow charts of tungsten and arsenic were established to evaluate the metabolism efficiency of the investigated system. The results showed that, the total tungsten resource efficiency of the system was 97.56%, and the tungsten recovery of unit process autoclaved alkali leaching, ion exchange, Mo removing, concentration and crystallization was 98.16%, 98.94%, 99.71%, 99.89%, respectively. Meanwhile, for extracting 1 ton of tungsten into the qualified ammonium paratungstate, 10.0414 kg of arsenic was carried into the system, with the generation of 7.2801 kg of arsenic in alkali leaching residue, 1.5067 kg of tungsten in arsenic waste residue, and 1.2312 kg of tungsten in Mo residue. Besides, 7.9 g of arsenic was discharged into nature environment with waste water, 15.5g of arsenic was entrained into the final APT. The distribution and transformation behaviors of arsenic during production were analyzed through phases change analysis, and some recommendations for improving the resource efficiency of tungsten and pollution control during production were also proposed based on the substance flow analysis in this study.

Keywords Tungsten · Arsenic · Substance flow analysis · Resource efficiency · Pollution control

1 Introduction

Tungsten is a kind of important strategic metal as it is widely used in many fields such as machinery manufacturing, chemical industry, aerospace industry and national defense industry [1]. With the continued exploitation of high-grade tungsten resource, the content of impurity elements in tungsten ores becomes higher and higher [2, 3]. For example, arsenic commonly occurs in wolframite concentrate, while it is strictly controlled in tungsten products [4]. Moreover, the emission of arsenic into nature environment during tungsten extraction process is a serious environmental issue [5, 6].

At present, scholars have done a great amount of researches on the efficient extraction of tungsten in a unit process, including tungsten minerals decomposition [7, 8], purifica- tion and transition of leaching solution [9, 10]. However, few researches focus on the metabolism of tungsten or other detrimental elements within a production process. Taking deep insight into the metabolism of tungsten and other key elements in a production system could provide a meaningful reference for process improvement and pollution control.

Substance flow analysis (SFA), a practical analytical tool, has been employed widely for studying the cycle and metabolism of a specific substance in a given system, tracking the released substances and then evaluating the effects of substance metabolism on economy and environment [11, 12].

In the past decades, SFA has been applied widely especially for analyzing the stock and consumption flows of metals, such as lead, zinc, copper, nickel, and other substances in a scope of local region [13, 14], country [15–19], or global [20, 21]. Recently, SFA was also applied as a supported tool in mineral resources management [22, 23], industry chain evaluation [24], waste management [25], environmental risk assessment and control [26, 27], and sustainable development assessment [28]. However, most of the previous studies

Tungsten

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* Xue-Yi Guo xyguo@csu.edu.cn

* Qin-Meng Wang qmwang@csu.edu.cn

1 School of Metallurgy and Environment, Central South University, Changsha 410083, China

2 Changsha Research Institute of Mining and Metallurgy Co. LTD, Changsha 410012, China

3 Ganzhou Nonferrous Metallurgy Research Institute, Ganzhou 341000, China

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mainly focused on the metals stock and potential waste generation in a system based on the statistics of production and consumption amount or analyzing the environmental impacts in the long life cycle of objective substance on a perspective of large system.

Applying SFA to trace the substance distribution and migrating in a production process could evaluate the metabolism efficiency of the process and reveal the path- ways through which pollutants are generated, which is ben- eficial to the resource management and pollution control of a factory. Yoshida et al. [29] studied the fate of total organic carbon, 32 elements and four groups of organic pollutants in a conventional wastewater treatment plant through SFA, and provided an assessment on the treatment efficiency and environmental impact of the plant. Bai et al. [12] applied SFA to calculate lead mass balance in each stage of a lead smelting process, and the indicators such as waste circula- tion rate and resource efficiency were used to evaluate the metabolism efficiency of the system, through which some recommendations on improving emission control and pollution prevention for the lead smelting factory were put for- ward. To our knowledge, there is no report of the SFA on the metallurgical processes or life cycle of tungsten in current.

In this study, SFA was used as an analytical tool to inves- tigate the metabolism of a hydrometallurgical process for producing ammonium paratungstate (APT) from wolframite.

Tungsten and arsenic were selected as the objective substances, and then the mass balance accounts, substance flow charts of tungsten and arsenic in the production system were established. The metabolism efficiency of the system was evaluated by indicators including tungsten recovery of the main unit extraction processes, waste recycle ratio, resource efficiency, and the distributing and transforming behaviors of arsenic in production were also analyzed. This study focuses on the element metabolism efficiency from a microcosmic perspective, providing a reference for process improvement.

2 Calculation methodology

Compared with applying SFA on a large system with the scope of global or regional scale, applying SFA to a production process is more microcosmic and more specific.

The SFA model of a production process always includes the substance flow of a unit process and the whole production system which is composed of several unit processes [7].

2.1 Definition of substance flows in a unit process If we define one unit process in the production system as process j, for a continuous production system, the substance flow input to and output from process j are defined as follows:

(1) Input raw material flow, Aj.

(2) Input upstream product substance flow, P_{j − m}.

(3) Recycle substance flow from downstream process i, R_i,j. The substance flow which is recycled to upstream process k from process j is called R_j,k.

(4) Emission substance flow, E_j. It includes the by-products and pollutants which are discharged outside the production system from process j.

(5) Output product substance flow, Pj. The substance flow which is the by-product of process j and translated to process k for further treating is called P_j,k.

(6) If some products are stocked in warehouse temporarily during production, the stock substance flow, Sj, should be taken into consideration. In this study, no product is stored temporarily, and the stock substance flow Sj is not under calculation.

According to the conservation of mass, for the unit process j, the substance flows can be calculated as:

2.2 Substance flows of a whole system

If all the unit processes are combined, the whole system is formed, and the substance flows of the whole system are the sums of corresponding flows in all unit processes, as follows:

(1) Input raw material flow A is expressed as

(2) Recycle substance flow R is expressed as

(3) Emission substance flow E is expressed as

(4) Output product substance flow P is expressed as

Correspondingly, according to the conservation of mass, for the whole system, it can be expressed as

(1) A_j+P_j−m+P_i,j+R_i,j=P_j+P_j,k+R_j,k+E_j.

(2) A=

m

∑

j=1

A_j.

(3) R=

m

∑

j=1 m

∑

i=1

R_i,j.

(4) E=

m

∑

j=1

E_j.

(5) P=

m

∑

j=1

P_j.

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2.3 Mass balance calculation of a specific element To analyze the substance flow of a specific element, two parts of data are required. One part is the flow quantity M_j, which is the amount of each material which contains the objective element, and this part of data is always collected from the daily production reports of plant. The other part is the content C_j of objective element in each material, and this part of data is obtained by sampling and analyzing for each material. Then the flow quantity of an objective element mj

is calculated as

In this study, the mass balance calculation of unit processes and the whole system is based on one ton of tungsten output from the production system, and the flow ratio of the objective element in each material is expressed as fj

(t·t⁻¹, kg·t⁻¹)where mj is the quantity of the objective element in the substance flow j, and mm is the quantity of the objective element in final product output from the production system. The unit of f_j (t·t⁻¹ or kg·t⁻¹) means the quantity of an objective element in each substance flow for per ton of tungsten in APT.

2.4 Evaluation indicators of SFA

To evaluate the production efficiency of the process and its influence on environment, three indicators are proposed in this study as follows.

(1) Tungsten recovery of a unit tungsten extraction process, γ, and the proportion of tungsten in product flow to the total output flows of a unit process, %. For a unit process j, γj is calculated as

(2) Waste recycle ratio of the whole process, α, and the proportion of recycle substance flows in all the substance flows not included in the final product of the whole process, %:

(3) Resource efficiency, ε, and the proportion of objective element in final product to the total input raw material flows (wolframite concentrate in this study), %:

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m

∑

j=1

M_j,input=

m

∑

j=1

M_j,output.

(7) m_j=M_j×C_j j=1, 2,…, m.

(8) f_j=m_j/

m_m(

t⋅t⁻¹, kg⋅t⁻¹) ,

(9) 𝛾_j=P_j/(

P_j+P_j,k+R_j,k+E_j)

×100%.

(10) 𝛼=R∕(R+E) ×100%.

3 Description of the system boundary

In this study, a hydrometallurgical process for producing APT from wolframite is chosen as system boundary. The process is now in operation in a tungsten products production enterprise, with a production scale of 5000 tons APT per year, Jiangxi province, China. And this kind of process accounts for a large proportion of APT production capacity in China. The simplified flowsheet of objective process is shown in Fig. 1.

The preprocessing for wolframite concentrate is first conducted through ball milling, the slurry after grinding is transported into autoclaved alkali leaching process for tungsten minerals decomposition with NaOH solution, alkali leaching residue and raw sodium tungstate solution are sepa- rated from slurry. The alkali leaching residue is considered as hazardous waste according to National Hazardous Waste Category (2016) of China, and is always stockpiled in spe- cialized warehouse. The raw sodium tungstate solution is diluted to a proper concentration for ion exchange process, the tungstate in prepared solution is extracted through ion exchange and is transformed into ammonium tungstate after stripping from tungsten loaded resin, and then raw ammonium tungstate solution is obtained. The waste solution from ion exchange process is further treated for recovering of residual tungsten and removal of arsenic before discharging, and the tungsten containing solution is recycled. Normally, there is a certain portion of Mo in the wolframite concentrate and is extracted into tungstate solution with tungsten due to their extremely similar chemical properties. For the purpose of reducing the effect of Mo on the quality of subsequent tungsten products, Mo removing is conducted on raw ammonium tungstate solution through sulfide precipitation, and Mo residue is generated. Through concentration and crystallization of the purified ammonium tungstate solution, wet product of APT is obtained, and the crystallizing mother solution are recycled to tungsten stripping process which follows ion exchange process. After drying and screening of wet APT, qualified product of APT is produced, the dust generated from screening is collected and recycled to alkali leaching process.

4 Data collecting, sampling and analysis

Tungsten and arsenic were selected as the objective substances in this study. The quantity data of input and output material flows were collected from the production reports of objective plant, and the measured time

(11) 𝜀=P∕A×100% = (A−E)∕A×100%.

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scale was 1 month. To determine the tungsten and arsenic content in each material flow, all the flows in the forms of solid and solution were sampled and analyzed every production day in the measured time, and the content of tungsten and arsenic were averaged for mass balance calculation.

Some solid samples were dried at 50 °C in a vacuum oven for 12 h, and then analyzed by X-ray diffraction (XRD, TTR III, with Cu Kα radiation, Rigaku, Japan) for phase compositions analysis. The operating conditions is with 2θ ranging from 10° to 80°, step size of 0.02 and scanning speed of 2°·min⁻¹.

The elemental compositions and chemical phases of tungsten and arsenic were determined by Changsha Research Institute of Mining and Metallurgy Co., Ltd in Changsha, China.

5 Results and discussion

5.1 Substance flow analysis of tungsten 5.1.1 Mass balance calculation

Based on the flows quantities and analysis data, the mass balance account of tungsten in all unit processes of the production system is built upon 1 ton of tungsten extracted into the qualified APT. The input and output flow ratios of tungsten in each substance flow are shown in Table 1.

5.1.2 Substance flow chart of tungsten

The substance flow chart of tungsten, shown in Fig. 2, is plotted based upon tungsten substance flow data and the

Fig. 1 Simplified flowsheet of the metallurgical process for ATP production from wolframite

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tungsten production process. The production system includes 9 unit processes and there are 17 strands of substance flows within the whole system, and each of the substance flows has been identified with name and flow code as shown in Table 1.

5.1.3 Evaluation of the tungsten metallurgical process The evaluation results of the production system with the defined indicators (Sect. 2.4) are presented in Table 2.

The tungsten recovery of the main tungsten extraction processes including autoclaved alkali leaching, ion exchange, Mo removing, concentration and crystallization are evaluated. As can be seen, the tungsten recovery of these

processes is 98.16%, 98.94%, 99.71%, and 99.89%, respectively. The high tungsten extracting efficiency of these unit processes contributes to a high resource efficiency of the whole system, 97.56%. The waste recycle ratio of the production system is 24.70%. In other words, 24.70% of the tungsten loss in rejected products during production is recycled to the system. The details of the defined substance flows are discussed in the following sections.

(1) Input substance flows of tungsten, A

As shown in Table 1, the total input substance flow is 1.0250 t·t⁻¹, indicating that for extracting 1 ton of tungsten into the final product (qualified APT), 1.0250 tons of pure tungsten contained in wolframite concentrate

Table 1 Input and output flow ratios of tungsten in each substance flow of the production system

No. Process unit Input Output

Substance flow Flow ratio

(t·t⁻¹) Substance flow Flow ratio

(t·t⁻¹)

1. Ball-milling A₁ Wolframite concentrate 1.0250 P₁ Slurry 1.0250

Total input 1.0250 Total output 1.0250

2. Autoclaved alkali leaching

P₁ Slurry 1.0250 E₂ Alkali leaching residue 0.0189

R_9,2 Recycled dust 0.0014 P₂ Raw sodium tungstate

solution 1.0075

3. Ion exchange P₂ Raw sodium tungstate solution 1.0075 P₃ Tungsten-loaded resin 1.0021

R_4,3 Recycled solution 0.0053 P_3,4 Waste solution 0.0107

4. Waste solution

treatment P_3,4 Waste solution 0.0107 R_4,3 Recycled solution 0.0053

E_4-1 Arsenic waste residue 0.0038 E_4-2 Discharged waste water 0.0016

5. Tungsten strip-

ping P₃ Tungsten-loaded solution 1.0021 P₅ Raw ammonium tungstate

solution 1.0036

R_7,5 Crystallizing mother solution 0.0015

6. Molybdenum

removing P₅ Raw ammonium tungstate

solution 1.0036 E₆ Molybdenum residue 0.0007

P₆ Purified ammonium tung-

state solution 1.0029

7. Concentration and crystallization

P₆ Purified ammonium tungstate

solution 1.0029 P₇ Wet product of APT 1.0014

R_7,5 Crystallizing mother solu-

tion 0.0015

8. Drying and

screening P₇ Wet product of APT 1.0014 P₈ Qualified APT 1.0000

P_8,9 Dust 0.0014

9. Dust collection P_8,9 Dust 0.0014 R_9,2 Recycled dust 0.0014

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is consumed. Table 3 shows the contents of main elements in wolframite concentrate. The tungsten content is 42.10 wt% (the content of WO₃ is 53.08 wt%)

—2.4347 tons of wolframite concentrate is put into this production system.

(2) Recycle substance flows of tungsten, R

There are three recycle substance flows in the production system: R_4,3, recycled solution from waste solution treatment process, (5.3 kg·t⁻¹); R_7,5, crystallizing mother solution, (1.5 kg·t⁻¹); and R_9,2, recycled dust, (1.4 kg·t⁻¹).

As a whole, the recycle substance flow is (8.2 kg·t⁻¹), accounts for 24.70% of the tungsten

loss in rejected products which consist of recycle and emission substance flows. The high tungsten extracting efficiency of the unit processes is a main reason for the low recycle ratio. Among the recycle substance flows, recycled solution from the waste solution treatment process accounts for the largest portion, 64.63%.

(3) Emission substance flows of tungsten, E

The emission substance flows consist of the alkali leaching residue (18.9 kg·t⁻¹), arsenic waste residue (3.8 kg·t⁻¹), discharged waste water (1.6 kg·t⁻¹), and Mo residue (0.7 kg·t⁻¹). These flows are discharged outside the production system for further treatment, but not into the nature environment, except for the treated waste water that meets emissions standard.

The alkali leaching residue, accounting for 75.60% of all the emission substance flows, is the main emission substance. According to the elemental composition of alkali leaching residue (Table 4), the tungsten content is 1.82 wt%. The result indicates that, for extracting 1 ton of tungsten into qualified APT, 1.0385 tons of residue

Fig. 2 Tungsten substance flow chart of the investigated production system (t·t⁻¹)

Table 2 Evaluation results of the tungsten metallurgical system

No. Evaluation indicator Unit Result

1 Input raw material flow for per ton of copper, A t·t⁻¹ 1.0250

2 Cycle substance flow for per ton of copper, R t·t⁻¹ 0.0082

3 Emission substance flow for per ton of copper, E t·t⁻¹ 0.0250

4 Tungsten recovery of autoclaved alkali leaching process, γ₂ % 98.16

5 Tungsten recovery of ion exchange process, γ₄ % 98.94

6 Tungsten recovery of Mo removing process, γ₇ % 99.71

7 Tungsten recovery of concentration and crystallization process, γ₈ % 99.89

8 Resource efficiency, ε % 97.56

Table 3 Contents of the main elements in wolframite concentrate

Element W Fe Mn Ca As

Content/wt% 42.10 7.60 6.88 1.28 0.41

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is generated from autoclaved alkali leaching process.

Table 5 shows the chemical phase analysis of tungsten in wolframite concentrate and alkali leaching residue.

The result shows that, 90.83 wt% of residual tungsten in leaching residue occurs as scheelite, and the leaching efficiency of scheelite occurring in concentrate should be improved for a higher resource efficiency. Moreover, the contents of some valuable metals, such as W, Sn, and Nb are higher than that of raw ore, and the resource value of the residue is fairly high.

However, the residue also contains some toxic elements (mainly As and Pb), and is classified as hazardous waste according to National Hazardous Waste Category (2016) of China. Therefore, the alkali leaching residue possesses dual properties of resource value and environmental pollution risk. Clean extraction of valuable metals from the residue and the safe disposal is in great necessary, which will also be presented in our later works.

The tungsten loss in arsenic waste residue and discharged waste water accounts for 50.47% of the tungsten in waste solution from ion exchange, in other words, only 49.53% of tungsten in waste solution is recovered. The low concentration of tungsten in waste solution after ion exchange is the main reason for the difficulty in recovering. Thus, improving the selectivity extraction for tungsten in ion exchange and waste solution treatment process is the key measure to reduce the loss of tungsten.

Mo residue accounts for a small portion of emission flows, 5.60%. The molybdenum content of this resi-

due is high, and therefore it is used as raw material for molybdenum metallurgy.

5.2 Substance flow analysis of arsenic

As arsenic is considered as a hazardous element entrained into the system rather than valuable element, the evaluation indicators defined in Sect. 2.4 is not conducted for arsenic substance flows. However, for the purpose of giving a deep insight into the arsenic migrating and translating during production, the distribution and transformation behaviors of arsenic in the main tungsten extraction and impurity removing processes (autoclaved alkali leaching, ion exchange, waste solution treatment, Mo removing, concentration and crystallization) are analyzed based on arsenic substance flow analysis and the phase analysis of some products.

5.2.1 Mass balance calculation

The mass balance account of arsenic in all unit processes of the system is also built upon extracting 1 ton of tungsten output from the system. The flow ratio of each substance flow is shown in Table 6.

5.2.2 Substance flow chart of arsenic

The substance flow chart of arsenic based on substance flow data and the production process is shown in Fig. 3. All the arsenic substance flows are corresponding to the tungsten substance flows in Fig. 2. The total quantity of each arsenic substance flow in the system is calculated as shown in Table 7, the details of each substance flow are discussed in the following sections.

(1) Input substance flows of arsenic

The total input substance flow of arsenic is 10.0414 kg·t⁻¹ tungsten, which indicates that extracting 1 ton of tungsten into the qualified APT (10.0414 kg) of arsenic is carried into the production system. All the input arsenic is carried by wolframite concentrate, and the arsenic content of wolframite concentrate is 0.41 wt% (Table 3), which belongs to the kind of high arse-

Table 4 Main elemental compositions of alkali leaching residue

Element W Mn Fe Sn Nb Bi

Content/wt% 1.82 12.48 16.12 0.74 0.37 0.54

Element Mo As Pb Al Ca Si

Content/wt% 0.038 0.71 0.32 1.16 12.43 5.14

Table 5 Chemical phase analysis of tungsten in wolframite concentrate and alkali leaching residue

Phase Mass fraction/wt%

Wolframite concentrate Alkali leaching residue

Wolframite 76.87 3.49

Scheelite 21.78 90.83

Tungstite 1.36 5.68

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nic wolframite concentrate. This result also reveals that, with the tungsten resources being complex globally, the amount of arsenic input to tungsten production system increases so that clarifying the evolution behavior of arsenic in tungsten extracting process is very necessary.

(2) Recycle substance flows of arsenic

Arsenic is recycled to the system with three flows:

R_4,3, recycled solution; R_7,5, crystallizing mother solution; and R_9,2, recycled dust. The total recycle arsenic substance flow is 51.2 g·t⁻¹ tungsten, accounting for a very small portion (0.51%) of the arsenic distributes in rejected products (including alkali leaching residue 7.2801 kg·t⁻¹), waste solution 1.5211 kg·t⁻¹, Mo residue 1.2312 kg·t⁻¹, crystallizing mother solution

44.6 g·t⁻¹, and a small amout of recycled dust 0.1 g·t⁻¹). The low recycle ratio of arsenic attributes to the high efficiency of impurity removing processes and helps to avoid the accumulation of arsenic in the production system and improve the quality of APT.

(3) Emission substance flows of arsenic

Most of arsenic carried into the production system is discharged outside the system in the forms of alkali leaching residue E₂ (7.2801 kg·t⁻¹ tungsten), arsenic waste residue E_4-1 (1.5067 kg·t⁻¹), and Mo residue E₆ (1.2312 kg·t⁻¹).

Besides, a small amount of arsenic is in waste water (7.9 g·t⁻¹). These flows are discharged outside the production system for further treatment through other processes which are not included in the discussed production system.

Table 6 Input and output flow ratios of arsenic in each substance flow of the production system

No. Process unit Input Output

Substance flow Flow ratio

(kg·t⁻¹) Substance flow Flow ratio

(kg·t⁻¹)

1. Ball-milling A₁ Wolframite concentrate 10.0414 P₁ Slurry 10.0414

2. Autoclaved alkali leaching

P₁ Slurry 10.0414 E₂ Alkali leaching residue 7.2801

R_9,2 Recycled dust 0.0001 P₂ Raw sodium tungstate

solution 2.7614

3. Ion exchange P₂ Raw sodium tungstate

solution 2.7614 P₃ Tungsten-loaded resin 1.2468

R_4,3 Recycled solution 0.0065 P_3,4 Waste solution 1.5211

4. Waste solution

treatment P_3,4 Waste solution 1.5211 R_4,3 Recycled solution 0.0065

E_4-1 Arsenic waste residue 1.5067 E_4-2 Discharged waste water 0.0079

Total input 1.5211 Total input 1.5211

5. Tungsten strip-

ping P₃ Tungsten-loaded resin 1.2468 P₅ Raw ammonium tung-

state solution 1.2914 R_7,5 Crystallizing mother

solution 0.0446

6. Molybdenum

removing P₅ Raw ammonium tung-

state solution 1.2914 E₆ Molybdenum residue 1.2312

P₆ Purified ammonium

tungstate solution 0.0602

7. Concentration and crystallization

P₆ Purified ammonium

tungstate solution 0.0602 P₇ Wet product of APT 0.0156

R_7,5 Crystallizing mother

solution 0.0446

8. Drying and

screening P₇ Wet product of APT 0.0156 P₈ Qualified APT 0.0155

P_8,9 Dust 0.0001

9. Dust collection P_8,9 Dust 0.0001 R_9,2 Recycled dust 0.0001

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Among these emissions, alkali leaching residue accounts for 72.61%, and the arsenic content in the residue is as high as 0.71 wt% (Table 4), and Pb (0.32 wt%) is also contained. As the residue is specified as hazardous waste, it can only be stored in special warehouse.

After decades of production, the stock of this residue is in a very large amount. What is worse, the slow releas- ing of the toxic elements is a huge threat to the environment and therefore the safe disposal of this residue is in great necessity.

The arsenic waste residue from waste solution treatment, which is also specified as hazardous waste, accounts for 15.03% of the emission substance flows.

The stabilization level of arsenic in this residue should be assessed and some safe disposal measures should be carried out for the purpose of minimizing the risk of environmental pollution.

Some arsenic is deposited with Mo in the form of Mo residue (12.28% of the emission substance of arsenic), and is transported to the Mo metallurgy process. The

amount of arsenic discharged into nature environment with waste water is very small, 0.08%. However, the discharged arsenic with wastewater is also a potential threat to environmental organisms in consideration of biological accumulation. Recycling the wastewater to the leaching process as much as possible is a reasonable way.

5.3 The distribution and transformation behaviors of arsenic

The distribution of arsenic in the main tungsten extraction and impurity removing processes is shown in Table 8. The results indicate that, in the autoclaved alkali leaching process, 72.50% of the arsenic in wolframite concentrate stays in the alkali leaching residue, and the rest is extracted into raw sodium tungstate solution.

Slow scanning XRD was applied to analyzed the tungsten and arsenic phases in wolframite concentrate and alkali leaching residue, the results were presented in Fig. 4. The main occurring forms of arsenic in the concentrate (Fig. 4a) are pararealgar (As₄S₄), arsenic sulfide (As₂S₃), arseno- pyrite (FeAsS), lollingite (FAs₂), iron arsenate (FeAsO₄), and arsenic oxide (As₂O₅). While the main occurring forms of arsenic in the leaching residue (Fig. 4b) are arsenic sulfide (As₂S₃), iron arsenate (FeAsO₄), ferrous arsenate (Fe₃(AsO₄)₂), and lollingite (FeAs₂). The chemical phase analysis for arsenic was also carried out, and the results were shown in Table 9. For the concentrate, the mass fraction of arsenic oxides, arsenic sulfides, arsenate, and insoluble arsenic compounds is 7.55 wt%, 8.21 wt%, 3.08 wt%, and 81.16 wt%, respectively. And for the leaching residue, the

Fig. 3 Arsenic substance flow chart of the investigated production system (kg·t⁻¹)

Table 7 The total quantity of arsenic in each substance flow of the production system

No. Substance flow Unit Result

1 Input raw material flow for per ton of

tungsten, A kg·t⁻¹ 10.0414

2 Cycle substance flows for per ton of tung-

sten, R kg·t⁻¹ 0.0512

3 Emission substance flows for per ton of

tungsten, E kg·t⁻¹ 10.0259

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mass fraction of arsenic oxides, arsenic sulfides, arsenate, and insoluble arsenic compounds is 1.46 wt%, 8.11 wt%,

41.22 wt%, and 49.21 wt%, respectively. The insoluble arsenic compounds in chemical phase analysis may include arse- nopyrite (FeAsS) and lollingite (FeAs₂). Compared to wolframite concentrate, the mass fraction of arsenate in residue increases significantly, while the mass fraction of arsenic oxides and insoluble arsenic compounds decrease sharply and the mass fraction of arsenic sulfides decrease slightly (considering that 27.50% of arsenic have been leached).

During the leaching process with strong alkalinity, arsenic oxides and arsenate will be decomposed as follows:

Considering the presence of dissolved oxygen in the solution at the initial leaching stage, the following reaction may also occur:

As a result, a part of arsenic is dissolved into raw sodium tungstate solution.

Due to the lack of oxygen and that some of arsenic minerals may be wrapped by other minerals, the reactions (12–16) may not proceed completely. In addition, the AsO₄³⁻ in solution may be deposited back into residue as follows (which may result in the increase of the mass fraction of arsenate in the leaching residue):

2FeAsO₄+ 6NaOH = 2Na₃AsO₄+ Fe₂O₃+ 3H₂O,(12) (13) As₂O₅+ 6NaOH = 2Na₃AsO₄+ 3H₂O.

As₂S₃+ 7O₂+ 12NaOH = 2Na₃AsO₄+ 3Na₂SO₄+ 6H(14)₂O, As₄S₄+ 11O₂+20NaOH = 4Na₃AsO₄+ 4Na₂SO₄+10H(15)₂O,

(16) 2FeAsS + 7O₂+10NaOH = Fe₂O₃+ 2Na₃AsO₄

+ 2Na₂SO₄+ 5H₂O.

(17) Fe²⁺+ AsO³⁻₄ = Fe₃(

AsO₄)

2,

Table 8 Distribution of arsenic in the main tungsten extracting and impurity removing processes of the system

No. Process Product Flow ratio/

(kg·t⁻¹) Distribu- tion ratio/

wt%

1 Autoclaved alkali leaching Alkali leaching residue 7.2801 72.50 Raw sodium tungstate solution 2.7614 27.50

2 Ion exchange Tungsten-loaded resin 1.2468 45.04

Waste solution 1.5211 54.96

3 Waste solution treatment Arsenic waste residue 1.5067 99.05

Discharged waste water 0.0079 0.52

Recycled solution 0.0065 0.43

4 Mo Removing Molybdenum residue 1.2312 95.35

Purified ammonium tungstate solution 0.0602 4.65 5 Concentration and crystallization Wet product of APT 0.0156 25.91 Crystallizing mother solution 0.0446 74.09

Fig. 4 XRD pattern of a wolframite concentrate and b alkali leaching residue

Table 9 Chemical phase analysis of the arsenic in wolframite concentrate and alkali leaching residue

Phase Mass fraction/wt%

Wolframite concentrate Alkali leaching residue

Arsenic oxides 7.55 1.46

Arsenic sulfides 8.21 8.11

Arsenate 3.08 41.22

Insoluble arsenic com-

pounds 81.16 49.21

Total 100.00 100.00

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In the ion exchange process, 45.04% of the arsenic in sodium tungstate solution is entrained into tungsten-loaded resin, which leads to the dispersion of arsenic in molybdenum residue, and then increases the difficulty of arsenic treatment in subsequent process and also increase environmental pollution risk. Thus, improving the selectivity extraction of tungsten in ion exchange is necessary for reducing the entrainment of arsenic. The arsenic stays in waste solution after ion exchange is transported to waste solution treatment process, in which most of arsenic (99.05%) is removed into arsenic waste residue, 0.52% of arsenic is discharged into nature environment with water, and 0.43% of arsenic is recycled to the production system with recovered tungsten.

In Mo removing process, 95.35% of arsenic in raw ammonium tungstate solution is removed into Mo residue through sulfide precipitation. Figure 5 shows the XRD pattern of molybdenum residue. The detected molybdenum phases incluing molybdenite (MoS₂) and copper molybdenum sulfide (CuMo₂S₃). The detected arsenic phases including arsenic sulfide (As₂S₃), enargite (Cu₃AsS₄), and sulfur (S). In this process, Mo has been proved to be precipitated through the following reactions [30, 31]:

In addition to the reactions (19) and (20), as MoS₂ and S are also detected, another reaction may occur:

(18) Ca²⁺+ AsO³⁻₄ = Ca₃(

AsO₄)

2.

(19) MoO²⁻₄ + 4S²⁻+4H₂O ↔ MoS²⁻₄ + 8OH⁻,

(20) MoS²⁻₄ + CuS→Cu_xMo_yS_z+ S²⁻.

(21) MoO²⁻₄ + 3S²⁻+ 4H₂O → MoS₂+ S + 8OH⁻.

The peaks of molybdenum phases (including MoS₂ and CuMo₂S₃) in XRD pattern are fairly weak, which may be due to the low crystallinity of molybdenum phases.

According to the arsenic phases (As₂S₃ and Cu₃AsS₄) in Mo residue, it is speculated that, the AsO₄²⁻ in raw ammonium tungstate solution may be precipitated as follows:

74.09% of arsenic in purified ammonium tungstate solution stays in mother solution during the concentration and crystallization process, and the rest small part is entrained into wet product of APT. The arsenic content in final APT product is 0.001 wt%, and the product meets the APT-0 standard specified in GB/T 10116-2007 of China.

6 Conclusion and recommendations

(1) For the substance flow of tungsten, our results show that, the resource efficiency of tungsten for the discussed production system is 97.56%, the tungsten recovery of autoclaved alkali leaching, ion exchange, Mo removing, concentration and crystallization is 98.16%, 98.94%, 99.71%, 99.89%, respectively.

For extracting 1 ton of tungsten into the qualified APT, 2.4347 tons of wolframite concentrate containing 42.10 wt% W is consumed, 8.2 kg of extra tungsten recycles repeatedly within the system. Meanwhile, 18.9 kg of tungsten in alkali leaching residue, 3.8 kg of tungsten in arsenic waste residue, and 0.7 kg of tungsten in Mo residue are generated, and 1.6 kg of tungsten is discharged into nature environment with waste water.

(2) For the substance flow of arsenic, our results further reveal that, for extracting 1 ton of tungsten, 10.0414 kg of arsenic is carried into the system. 7.2801 kg of arsenic in alkali leaching residue, 1.5067 kg of arsenic in arsenic waste residue, and 1.2312 kg of arsenic in Mo residue are generated, and 7.9 g of arsenic is discharged into nature environment with waste water.

In autoclaved alkali leaching process, 27.50% of the arsenic is extracted into raw sodium tungstate solution, and most of the arsenic stays in the alkali leaching residue. In ion exchange process, 45.04% of the arsenic in sodium tungstate solution is entrained into tungsten- loaded resin; for the arsenic stays in waste solution after ion exchange, 99.05% of that is removed into arsenic waste residue by further treatment, 0.52% of that is dis-

(22) AsO³⁻₄ + 4S²⁻+4H₂O ↔ AsS³⁻₄ + 8OH⁻,

(23) AsS³⁻₄ + 3Cu⁺→ Cu₃AsS₄,

(24) 2AsO³⁻₄ + 5S²⁻+8H₂O → As₂S₃+ 2S + 16OH⁻.

Fig. 5 XRD pattern of molybdenum residue

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charged into nature environment with waste water, and 0.43% of that is recycled to the system with recovered tungsten. In Mo removing process, 95.35% of arsenic is removed into Mo residue through sulfide precipitation process, and only 4.65% of arsenic stays in purified ammonium tungstate solution. Finally, 25.91% of arsenic in purified ammonium tungstate solution is entrained into wet product of APT.

(3) Based on the substance flow analysis of the production system, some recommendations for production improvement can be obtained:

The leaching efficiency of scheelite occurring in concentrate should be improved during autoclaved alkali leaching process to improve the resource efficiency of tungsten.

Alkali leaching residue is the main emission of the system which accounts for 75.60% of all the tungsten emissions and 72.61% of the arsenic emissions. On one hand, the resource value of this residue is fairly high for the contents of some valuable metals, such as W, Sn, and Nb. On the other hand, the residue contains some toxic elements (mainly As and Pb), and is classified as hazardous waste. Therefore, the safe disposal of this residue is in great necessity. The utilization and harm- less disposal of the alkali leaching residue will also be investigated in our later works.

In the ion exchange process, 45.04% of the arsenic in sodium tungstate solution is entrained into tungsten-loaded resin. The result leads to the dispersion of arsenic in molybdenum residue, which means the difficulty of arsenic treatment in subsequent process is increased and a higher environmental pollution risk. In addition, as the content of arsenic is strictly controlled in tungsten products, the selectivity extraction of tungsten in ion exchange should be improved.

The stabilization level of arsenic waste residue from waste solution treatment should be assessed and some safe disposal measures should be carried out for the purpose of minimizing the risk of environmental pollution. In addition, the discharged arsenic with wastewater is a potential threat to environmental organisms despite its small amount in consideration of biological accumulation, and recycling the waste water to the leaching process as much as possible is a reasonable way.

Acknowledgements This paper was financially supported by the National Key R&D Program of China (Grant No. 2019YFC1907400) and the National Natural Science Foundation of China (Grant Nos.

51904351 and 51620105013).

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