Cuicui Lü,Yongliang Wang,Peng Qian,Ya Liu,Guoyan Fu,Jian Ding,Shufeng Ye,*,Yuanfa Chen

1Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

2University of Chinese Academy of Sciences,Beijing 100049,China

Keywords:Copper tailings Ammonium humate Depression Response Surface Methodology

A B S T R A C T Copper tailings constitute a large proportion of mine wastes.Some of the copper tailings can be recycled to recover valuable minerals.In this paper,a copper tailing was studied through the chemical analysis method,X-ray diffraction and scanning electron microscope-energy dispersive spectrum.It turned out that chalcopyrite(Cu)and pyrite(S)were the main recoverable minerals in the tailing.In order to separate chalcopyrite from pyrite in low pulp pH,ammonium humate(AH)was singled out as the effective regulator.The depression mechanism of AH on the flotation of pyrite was proved by FTIR spectrum and XPS spectrum,demonstrating that there was a chemical adsorption between AH and pyrite.By Response Surface Methodology(RSM),the interaction between AH,pulp pH and iso-butyl ethionine (Z200)was discussed. Itwas illustrated that the optimal dosage of AH was 1678 g·t-1involving both the recovery of Cu and S.The point prediction by RSM and the closed-circuit flotation displayed that the qualified Cu concentrate and S concentrate could be obtained from the copper tailing.The study indicated that AH was a promising pyrite depressor in the low pulp pH from copper tailings.

1.Introduction

Copper tailings generated from engineering process of copper-containing mine constitute a large proportion of mine wastes, which were estimated to be more than 30million tons produced worldwide [1]. Copper tailings can serve as the source materials for the residual minerals, such as pyrite,chalcopyrite,and chalcocite[2–5].Besides,the discarding of copper tailings could bring severe environmental problems because the residual sulfide minerals may go through enormous oxidation to produce acid drainage[6–8].Thus,it is of great economic and environmental significance to recover the valuable minerals from copper tailings.

Froth flotation is an effective and low-cost method to collect valuable minerals from low grade ores[9].For the copper tailings,the objective of the flotation is to detach the copper minerals from other sulfide minerals(usually pyrite)and the gangue minerals.The main difficulty lies in the depression of pyrite,which conventionally proceeds in high pH conditions by using large amounts of lime(CaO).However,this conventional method may negatively prohibit the flotation of desired minerals.Thus,effective pyrite depressors utilized in low pulp pH are in urgent need.

Organic depressors have been put forward as a category of much more mild pyrite-depressors to replace the large amount of lime[10–15].Chen et al.used sodium humate to depress pyrite in the treatment of Dexing Copper Mine and achieved the sphalerite and pyrite concentrate with success[16]. Glycerine-xanthate (SGX)was designed and used for selective depression of pyrite in chalcopyrite flotation from Dahongshan Copper Mine.Electrokinetic and FTIR measurements revealed that SGX was pre-absorbed on pyrite,intensely depressing the adsorption of SBX on pyrite[10].Dextrin was utilized as a suitable depressant for pyrite and sphalerite in the first stage of flotation of Cu–Zn sulfide,which was superior to cyanide on biodegradability and nontoxicity[17].The combination of Ca(ClO)2with DT-2#induced the pyrite surface to form a hydrophilic film,greatly enhancing the hydrophilicity of pyrite in a lime-free condition,thus contributed to the separation of chalcopyrite from pyrite[18].

However,the main problem existed in the present organic depressors lies in their high prices.It is demanded to explore a low cost and effective pyrite depressor.From the above literatures,it is summarized that the organic depressions contain a hydrophilic functional group such as NH3+,OH-or COOH-.Ammonium humate(showed as AH)is a byproduct of peat treatment which contains a lot of hydrophilic groups with low price. In this paper, it was explored as the proper pyrite depressor in the recovery of copper mineral from the copper tailing. The depression mechanism and the interactions of AH with pulp pH and collector were also deliberated.

2.Materials and Methods

2.1.Characterization analytical tests

The copper tailing in this study was obtained from Liaoning province,China.20 g of the sample was ground to less than 74 μm and dried at 90°C for 24 h.The sample was prepared for the analysis of chemical composition by an Atomic Absorption Spectrophotometer(AAS,WFX-130A,Beijing,China).The mineralogical determination of the feed sample was detected by X-ray Diffraction(XRD,Smartlab-201307,Rigaku Corporation,Japan).Another 4 g of the sample was embedded with resin and curing agent for 12 h until it was well-prepared to be polished on a SiC polishing pad of 200mesh,800mesh,1200mesh and finally 2000 mesh for the Scanning Electron Microscope test(SEM,JSM-7001F,JEOL,Japan,operating at 15 kV,assembling with an Energy Dispersive Spectrometer,EDS).The rest of untreated sample was divided into 500 g in seal sample bags for each flotation experiment.

Fourier Infrared Spectrum(FT-IR,Spectrum GX,PE,USA)was used to explore the mechanism of pyrite depressing.The mixture of AH and pyrite was done by leaching pure pyrite in the AH solution of 10%(mass fraction,the following was the same)for 5 min,10 min and 30 min,separately.The solutions were treated in an ultrasonic device to eliminate the residual reagents.Subsequently the solutions were filtered and dried.Finally,the AH and the treated samples were mixed with KBr uniformly for the FT-IR detection.

X-ray Photoelectron Spectroscopy(XPS,ESCALAB250Xi,Thermo Fisher Scientific,USA)was done to study the surface element change with different treatment times(5 min,10 min and 30 min)between pure pyrite and AH solution.The solutions were treated with the same process as that in FTIR.The source type for XPS detection was Al Kαand energy step size was fixed at 0.05 eV in this study.

2.2.Flotation reagents and flow sheet

The flotation experiments were conducted with iso-butyl ethionine(Z200),and sodium n-butyl xanthate(SNBX)as collectors,which were acquired from Jiangxi Copper Corporation.These collectors were all in industrial grade.Calcium oxide(CaO)was purchased from Sinopharm Chemical Reagent Beijing Co.,Ltd.,and AH was purchased from Pingxiang Humic Acid Co.,Ltd.Terpenic oil with monohydric alcohol content＞40%was also acquired from Jiangxi Copper Corporation.

The flotation flow sheet was shown in Fig.1.500 g of the sample was put in to the ball mill(Wuhan Rock&Grind Equipment Manufacture Co.,Ltd.,Hubei,China)for different grinding times to get the optimal grinding fineness.Then the ground sample was transferred into a 1.5 L XFD series single flotation cell(Jilin Exploring Machinery Plant,Jilin,China)and pulped to 30 wt%using city water,which was agitated at 1220 rpm for 1 min before any reagents were added.Subsequently,CaO,AH,Z200 and terpenic oil were added into the pulp in sequence.Thereafter,the pulp was aerated for 5 min and the Cu flotation was carried out.After Cu concentrate was recovered,the Cutailing served as the S feed samples.The pulp was adjusted with sulfuric acid,followed by SNBX and terpenic oil. Then the pulp was aerated for 5min and the S flotation was carried out. Froth height was maintained at the same level by adding water periodically throughout the tests.The flotation concentrates and tailings were filtered,dried,and weighed.The obtained concentrates and tailings were analyzed by chemical analytical method,shown as the recovery of Cu and S.

3.Results and Discussion

3.1.Characterization of the copper tailing

Fig.1.The sketchy flotation flowsheet of the mine tailing.

Table 1AAS result of the copper tailing

Fig.2.XRD of the copper tailing.

The chemical composition analysis by AAS(see Table 1)and XRD(see Fig.2)showed that S and Fe were the major elements in the feed sample,which constituted pyrite,while Si,Al,and Mg constituted quartz and other silicate gangue.The valuable metal minerals were mainly pyrite and slight copper minerals with a Cu grade of 0.57%.Thus the goal of this study was to recover Cu and S (representing copper minerals and pyrite,the below was the same).

Various mineral species in the feed sample can be qualified by SEM–EDS analysis,as shown in Fig.3.Different brightness signified different mineral phases.The bright mineral phase a contained elements of S and Fe,as shown in the spot scanning of a.The atomic ratio of S:Fe was approximately equal to 2:1,which was similar to the composition of pyrite,suggesting the existence of pyrite.In the spot scanning of a,F was also detected with an atomic amount of 5.39%.It was probably because of instrument error,as no element F was detected in the feed sample by AAS.Phase b contained mainly S,Fe and Cu,as shown in the spot scanning of b.The atomic ratio of S:Fe:Cu was approximately equal to 2:1:1,which suggested the existence of CuFeS2.Phase c contained mainly O,Ca,Mg,Si,and Fe,as shown in spot scanning of c.Element O and Si took up a large proportion of 80.02%in total,suggesting the existence of various silicates.Phase d mainly contained O and Si with an atomic ratio of 2:1,suggesting the existence of quartz.It was further proved that SEM–EDS results were in line with XRD results.The presence of a variety of mineral phases such as pyrite,chalcopyrite,augite,clinochlore,amphibole,and quartz could be concluded.Chalcopyrite was embedded in pyrite and silicate,rather than formed a solid-solution which was structurally bound.This was very beneficial to the flotation.By grinding,chalcopyrite can be separated from pyrite and silicate theoretically[19].

3.2.Influence of grinding fineness on the flotation

The grinding fineness(defined as mass fraction of particles percentage below 74 μm)was influential in the liberation degree of chalcopyrite from pyrite,which affected the Cu recovery enormously.The copper tailing was a coarse grinding tailing with 64%particle size less than 74 μm.Regrinding was needed to seek out the optimal grinding fineness[20].As shown in Fig.4,when the grinding fineness was 75%,the recovery of Cu was only 66.50%.With the increase of the grinding fineness,the flotation recovery of Cu also increased.However,while the grinding fineness was larger than 85%,there was only a slight increase in the Cu recovery,indicating that the Cu mineral was almost completely liberated from other minerals[21].Besides,when the sample was ground too fine,the fine particles were prone to absorb flotation reagents without selectivity,thus increasing the recovery of other minerals in Cu concentrate,which was undesirable[22,23].The results showed that the Cu recovery was closely connected with the sample grinding fineness.The grinding fineness of 85%was thought to be the most proper grinding condition.

Fig.3.SEM–EDS of the copper tailing.

Fig.4.Influence of grinding fineness on the recovery of Cu and S.

3.3.The response surface central composite analysis

3.3.1.Model fitting

The effects of three-level factors(pulp pH,AH dosage,and Z200dosage)were estimated through the Central Composite Design(CCD)and Response Surface Methodology(RSM).The experimental results of their interactions on Cu recovery and S recovery were shown in Table 2.The statistical analysis was carried out on the three factors by Design Expert8.0.As a consequence,the predicted response of Cu recovery and S recovery could be expressed by a second order polynomial model interms of actual factors by applying analysis of variance(ANOVA)to the regression model,as shown in Eqs.(1)–(2).

Table 2Central composite design experimental results

Fitting the experimental results to a quadratic polynomial model,ANOVA indicated a determination coefficient(R2)for Cu recovery of 0.9902 and for S recovery of 0.9528.The predictive R2(Pred R2)values were both in reasonable agreement with the adjusted R2(Adj R2)for Cu recovery(0.9100 vs.0.9813)and S recovery(0.8873 vs.0.9104),as shown in Fig.5.What's more,the F values and p values were sufficient to illustrate that the models were well adapted to the recovery response [24–26]. Therefore, the above models can be used to navigate the design space.

3.3.2.Influence of variables on the flotation

The 3D response surface plots obtained from Design Expert 8.0 indicated the effects of independent variables and their mutual influence on the Cu recovery and S recovery.Fig.6 showed the 3D response surface of pulp pH and AH dosage on Cu recovery and S recovery while Z200 dosage was fixed at 48 g·t-1.From Fig.6(a),it was revealed that both Cu recovery and S recovery decreased slightly with the increase of pulp pH,suggesting that high alkalinity of pulp was unbeneficial to the flotation of Cu and S.Several explanations for the depression included the hydrophilic compounds,such as iron hydroxyl,were produced on the minerals surface which hindered the adsorption of collectors[27].Another view suggested that OH-could exchange with xanthate anion,leading to the failure of such collectors[28].Hu et al.recommended that the depression of pyrite by CaO could also be ascribed to the CaSO4species on the pyrite surface[29].The high alkaline environment not only reduced the desirable Cu recovery,but also deteriorated the recycle of pyrite from the Cu tailing.Therefore,it was essential to carry on the flotation in a low pulp pH.

As respect to AH,the dosage of AH gave a major push to the recovery of Cu,as shown in Figs.6 and 7.When the flotation pulp was treated with AH from 0 g·t-1to 1000 g·t-1,the recovery of Cu was obviously increased from 71%to 84%,but it decreased subsequently.The recovery of S was depressed to below 6%with the increase of AH dosage from 0 g·t-1to 2000 g·t-1.Taking both of the two metal minerals into consideration,we determined the optimal addition of AH to be 1678 g·t-1which was advantageous to the separation of Cu and S.

Z200 was generally used as a selective collector for chalcopyrite flotation,particularly against gangue iron sulfides [30]. In the separation of chalcopyrite from pyrite,Z200 was chosen as the distinctive collector.The recoveries of Cu and S as a function of Z200 dosage were shown in Figs.7 and 8.It was noticeable that the recovery of Cu increased greatly with Z200 dosage increasing from 0 g·t-1to 48 g·t-1and subsequently the Cu recovery had a slight change with the dosage of Z200 continually increasing to 96 g·t-1.The S recovery increased slowly with the increase of Z200 dosage from 0 to 48 g·t-1.When the dosage of Z200 was increased to 96 g·t-1,the S recovery increased greatly to 11%,which was a disadvantageous sign for the separation.Thus,it was suggested that the optimal dosage for Z200 as a collector was 48 g·t-1.

Fig.5.Comparison between actual values and predicted values.(a)Cu recovery;(b)S recovery.

Fig.6.Interaction influence of pulp pH and AH dosage.(a)Cu recovery;(b)S recovery.

Fig.7.Interaction influence of Z200 dosage and AH dosage.(a)Cu recovery;(b)S recovery.

Fig.8.Interaction influence of pulp pH and AH dosage.(a)Cu recovery;(b)S recovery.

From the contours of each figure,we can infer that the three factors(pulp pH,AH dosage and Z200 dosage)influenced each other as to the Cu recovery and S recovery.The optimum conditions for Cu–S separation were predicted by “point prediction”tool in Design Expert 8.0.The responses of Cu recovery and S recovery were 83.11%and 5.32%,respectively,with AH dosage of 1678 g·t-1,Z200 dosage of 48 g·t-1,and pulp pH of 10.29.

3.4.Depression mechanism of AH on the pyrite flotation

Fig.9 showed the FTIR spectrum of AH and pure pyrite treated in AH solution for different times.For the AH,major absorption peaks appeared at 915 cm-1,1021.15 cm-1,1083.84 cm-1,1371.07 cm-1,1600.24 cm-1,and 3678.13 cm-1,which belonged to the bending vibration of NH3,CH2twisting vibration,CH2OH stretching vibration,ring stretching of AH,C--OH deformations and H-bonded OH groups,respectively[31,32].When the pure pyrite was treated by AH,the main absorption bands were nearly the same as that of the sole AH.With the treatment time more than 5 min,the intensity of CH2OH stretching vibration and C--OH deformation became much obvious,demonstrating added AH molecule appeared on the surface of pyrite.Besides,it has been proved that AH was more prone to be adsorbed with Fe to form stable polymeric associates as compared to Cu,indicating that AH had a stronger depression on pyrite than on chalcopyrite[33].

Fig.9.FTIR spectrum of AH with treated pyrite.

Fig.10displayed the entire XPS spectrum of AH and treated pyrite in AH solution.For AH,there were obvious C1s peak and O1s peak at 284.80 eV and 532.28 eV,which belonged to CH2or CH3groups and C--OH groups,respectively.However,there was only a very weak peak for N1s(at 399.51 eV)of NH3groups,which may be due to its solubility in water.For the pyrite treated in AH solution for 5 min,there were obvious S 2p peak,S 2s peak and Fe 2p peak at 162 eV,226 eV and 707 eV,respectively.With the increase of the treatment time,the peaks of S and Fe became smaller and smaller,while the peaks of C and O became larger and larger on the contrary,proving that AH was indeed adsorbed on the surface of pyrite and a prolonged time was required to reach the adsorption equilibrium[34,35].Combining with FTIR spectrum,it could be concluded that there was a chemical adsorption of hydrophilic groups from AH molecule on the surface of pyrite,which resulted in the depression of pyrite during the flotation process.

Fig.10.XPS spectrum of AH and treated pyrite.(a:AH;b:pyrite treated in AH solution for 5min;c:pyrite treated in AH solution for10min;d:pyrite treated in AH solution for 30min.)

3.5.Flotation of pyrite

After the recovery of Cu,the Cu tailing was mainly composed of pyrite as the valuable mineral.However,pyrite was depressed in the Cu flotation by CaO and AH.In order to activate pyrite,sulfuric acid was added to the pulp to adjust the pulp pH to 5–6 which displayed the best pyrite recovery and pyrite grade,as shown in Fig.11(a).At this pH range, the depressed pyrite can be activated to expose the fresh crystal face that was potential to react with the pyrite collectors[36,37].SNBX was believed to be an effective collector for sulfide minerals[38].After the activation of pyrite, the SNBX was adsorbed to the surface of pyrite to form a ferrous xanthate,which enhanced the hydrophobicity of pyrite[39].The influence of the dosage of SNBX was shown in Fig.11(b),which illustrated that the dosage of SNBX should be higher than 300 g·t-1.

3.6.The closed-circuit process of Cu–S flotation

Based on the optimal conditions(grinding fineness of 85%,pulp pH of 10.29,AH dosage of 1678 g·t-1,Z200 dosage of 48 g·t-1,terpenic oil of 20 g·t-1),the Cu–S separation flotation was done with one roughing–two concentrating–two scavenging flowsheet to gain the Cu concentrate and Cu flotation tailing.The Cu flotation tailing was treated with sulfate acid,followed by SNBX of 320 g·t-1and terpenic oil of 40 g·t-1.The S flotation was completed with one roughing–two concentrating–two scavenging flowsheet.The whole flowsheet was shown in Fig.12.The closed-circuit process result was displayed in Table 3.

Table 3Result of the closed-circuit flotation

Table 3 indicated that Cu was mainly recovered in the Cu concentrate with a recovery of 84.32%and grade of 19.92%.About 5.40%of Cu was wasted in the final tailing with a grade of 0.06%,which was below the recovery level.S was mainly recovered in the S concentrate,with a grade of 47.00%and recovery of 83.42%.The closed-circuit flotation results were in accordance with the results from RSM,showing that Cu and S were successfully recovered from the copper tailing.

Fig.11.Influence of pulp pH(a)and SNBX dosage(b)on the recovery of pyrite.

Fig.12.Closed-circuit of Cu–S flotation flowsheet.

The flotation reagent cost was estimated on the basis of the above experimental conditions,which was shown in Table 4.The total price of the reagents was about 15.64 CNY·t-1for the feed copper tailing,which was acceptable to the flotation operation[40].Besides,the flotation of Cu concentrate and S concentrate from the mine tailing waste was environmentally friendly,and high quality products can be obtained.In summary, AH could be recommended as an excellent adjustor in the treatment of copper tailing.

Table 4Estimation of the flotation reagents cost in the experimental process

4.Conclusions

In this work,the comprehensive flotation tests were conducted on the copper tailing.The use of AH made it possible to separate chalcopyrite from pyrite in low pulp pH of only 10.29. The three influence factors (pulp pH,AH dosage and Z200 dosage) on the recovery of Cu and S were discussed by the Response Surface Methodology(RSM),which displayed an obvious interaction between the three factors.Besides,the RSM suggested the optimal flotation conditions with the pulp pH of 10.29,AH dosage of 1678 g·t-1and Z00 dosage of 48 g·t-1,under which a qualified Cu concentrate with a Cu grade of 19.92%was obtained.What's more,a qualified S concentrate with S grade of 47.00%can be acquired from the Cu flotation tailing.The FTIR spectrum and XPS spectrum showed that AH was chemically adsorbed on the surface of pyrite,leading to the hydrophilia of pyrite,which guaranteed the feasibility of separating chalcopyrite from pyrite.Thus,the comprehensive recycle of valuable minerals from the copper tailing was fulfilled successfully.