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深锥浓密技术是全尾砂膏体充填的关键技术之一,应用深锥浓密机可以获得高浓度的底流料浆与澄清的溢流水,对于尾矿绿色处置具有重要意义[1-2]。目前,针对深锥浓密机的底流浓度、溢流水浊度和处理能力(或者沉降速率)等方面开展了大量的静态或者动态浓密实验与模拟研究,探究了尾砂性质、深锥浓密机结构和工艺参数对全尾砂浓密效果的影响[3-6]。其中,带有导水杆的耙架结构是获得高浓度底流的关键[7-8]。但是,因为深锥浓密机内底部料浆浓度高,从而导致屈服应力高,容易导致耙架扭矩过载而发生压耙,影响正常生产[9]。同时,随着经济增长对矿产品的不断需求和选矿技术的不断创新发展,产生的尾砂越来越细甚至达到了超细的级别,超细尾砂比表面积大,导致屈服应力更大[10]。因此有必要对深锥浓密机内底部超细全尾砂料浆的屈服应力进行研究。
目前,对于全尾砂料浆的屈服应力的研究主要是为了分析其对管道输送阻力的影响,研究发现料浆中固相质量分数[11-12]、尾砂粒径分布[13-14]、外加剂[15-16]、料浆中离子强度[17]、时间与温度[18-19]等诸多因素均对全尾砂料浆的屈服应力有显著的影响,研究方法通常是根据料浆的固相质量分数和组成,应用干料与水搅拌制备成料浆,进行屈服应力测量,忽略了高浓度料浆形成过程中添加的高分子絮凝剂[20-21]以及絮凝过程对料浆流变特性的影响。
为此,本文首先开展不同条件下的超细尾砂絮凝沉降实验获得高浓度尾砂料浆,再对高浓度尾砂料浆进行原位屈服应力测试,并通过絮对凝前后料浆总有机碳(TOC)测试来分析超细尾砂对絮凝剂的吸附情况,进而分析不同絮凝剂吸附对尾砂料浆屈服应力的影响。
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因铁矿、铜矿、金矿等金属矿的尾砂的主要成分为SiO2[15, 22-23],本文采用人造尾砂(石英砂)作为实验材料,避免尾砂中其他矿物成分对絮凝与屈服应力的影响[24-25]。人造尾砂的SiO2质量分数为99.87%,密度为2604.04 kg·m−3,−10 μm颗粒体积分数为70.62%,属于超细尾砂[26],比表面积为0.799 m2·g−1。采用阴离子高分子絮凝剂Rheomax® DR 1050作为研究用絮凝剂。
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本文重点研究不同pH和不同絮凝剂单耗条件下的絮凝剂吸附情况与高浓度尾砂料浆的屈服应力,因此固定初始时尾砂料浆的固相质量分数为25%,固定絮凝剂溶液中絮凝剂的质量分数为0.025%,固定絮凝剂单耗为15 g·t−1时设置pH分别为8、9、10、11,固定pH为11时设置絮凝剂单耗(FD)为0~45 g·t−1。每组实验中尾砂料浆和絮凝剂溶液的总体积为750 mL。基本实验过程如图1所示:首先进行尾砂絮凝沉降实验,获得高浓度的絮凝尾砂料浆;然后进行TOC测定,确定上清液中TOC含量;最后进行高浓度絮凝尾砂料浆的屈服应力测试。
应用可拆卸的沉降筒进行静态絮凝沉降实验。首先应用干的人造尾砂和水配制尾砂料浆,并Ca(OH)2溶液调节料浆的pH,将尾砂料浆导入沉降筒后根据絮凝剂单耗加入絮凝剂溶液,上下晃动使尾砂与絮凝剂混合后进行沉降,应用高速摄像机实时记录固液分界面高度,1 h后取上清液进行浊度测试,14 h后记录固液分界面的高度以分别计算上清液和高浓度料浆的体积,然后取上清液进行TOC测定,最后排干上清液后对下部高浓度尾砂料浆进行屈服应力测试。应用可拆卸的沉降筒,既考虑了絮凝对屈服应力的影响,又避免了取样测试对料浆的扰动,从而提高了屈服应力测试的精度。
应用岛津TOC−L总有机碳分析仪进行TOC测定。分别对絮凝前的水、絮凝剂溶液和絮凝后的上清液进行TOC测定,根据碳平衡计算出高浓度尾砂料浆中的TOC含量,进而计算出絮凝剂吸附效率与絮凝剂吸附量,具体计算如式(1)、(2)所示。
$$\begin{split} & {{\rm{TOC}}_{\rm{floc}}} \times {V_{\rm{floc}}} + {{\rm{TOC}}_{\rm{slurry}}} \times {V_{\rm{slurry}}} = {{\rm{TOC}}_{\rm{super}}} \times \\ & {V_{\rm{super}}} + {{\rm{TOC}}_{\rm{sedi}}} \times {V_{\rm{sedi}}} \end{split}$$ (1) 其中,
${{\rm{TOC}}_{\rm{floc}}}$ 、${{\rm{TOC}}_{\rm{slurry}}}$ 分别为絮凝沉降前絮凝剂溶液和水的TOC质量浓度, mg·L−1;${{\rm{TOC}}_{\rm{super}}}$ 、${{\rm{TOC}}_{\rm{sedi}}}$ 分别为絮凝沉降后上清液和底部高浓度尾砂料浆的TOC质量浓度, mg·L−1;${V_{\rm{floc}}}$ 、${V_{\rm{slurry}}}$ 分别为絮凝剂溶液和水的体积,L;${V_{\rm{super}}}$ 、${V_{\rm{sedi}}}$ 分别为絮凝沉降后上清液和底部高浓度尾砂料浆的体积,L。$${{\rm{eff}}_{\rm{floc}}} = \frac{{{{\rm{TOC}}_{\rm{sedi}}} \times {V_{\rm{sedi}}}}}{{{{\rm{TOC}}_{\rm{floc}}} \times {V_{\rm{floc}}}}} \times 100$$ (2) 其中,
${{\rm{eff}}_{\rm{floc}}}$ 为絮凝剂吸附效率,%。$${m_{\rm{floc}}} = \frac{{{{\rm{eff}}_{\rm{floc}}}}}{{100}} \times \frac{\rm{FD}}{{1000 \times {\rm{SSA}}}}$$ (3) 其中,
${m_{\rm{floc}}}$ 为人造尾砂表面单位面积的絮凝剂吸附量,mg·m−2;FD为絮凝剂单耗,g·t−1;SSA为人造尾砂的比表面积,m2·g−1。应用Haake RT V550流变仪进行屈服应力测试。同时,应用ZetaCompact Z9000电位计对不同pH条件下的Zeta电位(ζ)进行测量,但是忽略絮凝剂单耗对Zeta电位的影响[27]。
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应用人造尾砂料浆固液界面的初始沉降速率(ISR)、絮凝沉降后上清液的浊度(T)和底部沉积尾砂的固相质量分数(SSF)来综合表征人造尾砂料浆的絮凝沉降效果。不同pH和FD条件下的絮凝沉降效果如图2所示。
图 2 絮凝条件对絮凝沉降的影响。(a)pH;(b)絮凝剂单耗
Figure 2. Effects of conditions on flocculation and settling: (a) pH; (b) flocculant dosage
由图2(a)可知,当FD=15 g·t−1时,在pH=8~11的范围内,ISR、T和SSF随着pH单调递减,其中ISR由0.7635 mm·s−1降到0.4565 mm·s−1;T由982 NTU降到143 NTU,变化最为显著;而SSF由52.49%降到51.56%,变化较小。有研究表明pH和金属阳离子对絮凝都有影响[28],而本文应用Ca(OH)2溶液调节料浆的pH,不同pH条件下的OH−1和Ca2+共同影响人造尾砂颗粒表面的Zeta电位,导致Zeta电位随着pH的增加而不断增加,从−70.65 mV增加到−20.97 mV,从而影响絮凝效果。根据《污水综合排放标准》(GB8978—96)要求,采矿、选矿工业悬浮物的二级标准为质量浓度不超过300 mg·L−1,因此实验范围内的最优pH为11。
由图2(b)可知,当pH值为11时,在FD=0~45 g·t−1的范围内,ISR随着FD的增加而先增大后减小,在FD=40 g·t−1时达到最大值0.5059 mm·s−1;而T随着FD的增加而先减小后增大,在FD=40 g·t−1时达到最小值112 NTU。同时,根据FD=0~15 g·t−1范围ISR和T的明显变化说明絮凝剂的絮凝作用较好,但是絮凝作用却不利于静态沉降时SSF的提高,SSF随着FD的增加而不断减小直至在FD=20 g·t−1达到稳定值51.26%。这是因为,在高分子絮凝剂作用下,人造尾砂颗粒形成絮团,导致絮团内部的包裹水不易排出而使得固相质量分数降低,也说明了仅依靠静态絮凝沉降较难获得较高浓度的尾砂料浆,可通过引入耙架的剪切作用与导水杆的导水作用来进一步提高絮凝尾砂料浆的浓度[7-8]。
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高分子絮凝剂与尾砂颗粒的絮凝作用属于桥接絮凝,絮凝剂分子在尾砂颗粒表面的有效吸附是絮凝的前提[29]。根据絮凝前后TOC的变化来分析絮凝剂吸附情况,不同pH和FD条件下的絮凝剂吸附情况如图3所示。由图3可知,不同条件下
${{\rm{TOC}}_{\rm{super}}}$ 远小于${{\rm{TOC}}_{\rm{floc}}}$ ,说明絮凝剂被大量吸附,但是${{\rm{TOC}}_{\rm{super}}}$ 略高于${{\rm{TOC}}_{\rm{slurry}}}$ ,说明仍有少量絮凝剂未被人造尾砂颗粒吸附。图 3 絮凝条件对絮凝剂吸附效率的影响。(a)pH;(b)絮凝剂单耗。
Figure 3. Effects of conditions on efficiency of flocc adsorption: (a) pH; (b) flocculant dosage
由图3(a)可知,在FD和初始尾砂料浆的固相质量分数不变的条件下,在pH值8~11的范围内,
${{\rm{eff}}_{\rm{floc}}}$ 随着pH的增大而不断增大,说明在增大pH有助于絮凝剂的吸附,这是因为本文中在pH增大时Zeta电位和Ca2+浓度均不断增大,从而促进高分子絮凝剂在人造尾砂颗粒表面的吸附。由图3(b)可知,当pH值为11时,在FD=0~45 g·t−1的范围内,
${{\rm{TOC}}_{\rm{super}}}$ 随着FD的增大而不断增大,${{\rm{eff}}_{\rm{floc}}}$ 随着FD的增大而不断减小,说明随着FD的增大未被吸附的絮凝剂也不断增多。因为在混合速率与混合时间一定的条件下,人造尾砂颗粒能够吸附的絮凝剂有限,从而导致在有限时间内不能被吸附的絮凝剂增多。 -
在不同pH和FD条件下,通过絮凝沉降实验得到浓缩(未添加水泥等胶结剂)超细尾砂料浆。通过流变仪测试浓缩超细尾砂料浆的屈服应力,并根据图3中的絮凝剂吸附效率和絮凝剂添加量计算出人造尾砂表面单位面积的絮凝剂吸附量,所得结果如图4所示。
图 4 絮凝条件对屈服应力的影响。(a)pH;(b)絮凝剂单耗
Figure 4. Effects of conditions on yield stress: (a) pH; (b) flocculant dosage
屈服应力随着pH和FD的变化趋势与
${m_{\rm{floc}}}$ 随着pH和FD的变化趋势相似。由图4(a)可知,当FD=15 g·t−1时,在pH值为8~11的范围内,屈服应力和${m_{\rm{floc}}}$ 随着pH的增大而不断增大。由图4(b)可知,当pH值为11时,在FD=0~45 g·t−1的范围内,屈服应力和${m_{\rm{floc}}}$ 随着FD的增大也不断增大,并且经过絮凝(FD>0)的浓缩超细尾砂料浆的屈服应力明显大于非絮凝(FD=0)的浓缩超细尾砂料浆的屈服应力,说明絮凝作用对屈服应力有较大的影响。料浆的屈服应力与料浆内固体颗粒间的相互吸引力有关,吸引力越大,屈服应力约大[30]。不同于经典的DLVO理论,高分子絮凝剂絮凝后的尾砂料浆里尾砂颗粒之间的相互作用力不仅包括范德华力和双电子层作用力,更重要的是因为尾砂颗粒表面吸附的絮凝剂而产生的桥接作用力,桥接作用力主要与絮凝剂性质、料浆中离子浓度、颗粒大小等因素有关[31-32]。由图4(a)可知,因为Zeta电位和Ca2+的影响导致人造尾砂颗粒表面吸附的絮凝剂量增加,从而增大了桥接作用力。同时从图4(b)可知,虽然图3(b)中絮凝剂吸附效率随着FD的增加而降低,但是因为絮凝剂单耗不断增大,所以人造尾砂颗粒表面吸附的絮凝剂量随着FD的增大也不断增加,进而增大了桥接作用力。桥接作用力的增大,导致絮凝沉降形成的浓缩超细尾砂料浆内的絮网结构强度更大,从而需要更大的剪切力来破坏絮网结构,也就导致屈服应力增大。
为了进一步分析屈服应力与
${m_{\rm{floc}}}$ 的关系,根据图5中屈服应力与${m_{\rm{floc}}}$ 的关系,可初步建立适用于本文人造尾砂的基于${m_{\rm{floc}}}$ 的屈服应力模型,如式(4)所示。$$ y = 12497x + 103.19,{R^2} = 0.9465 $$ (4) 其中,y为屈服应力,Pa;x为尾砂颗粒表面单位面积上絮凝剂的吸附量,mg·m−2;R2为可决系数。
由式(4)可知,屈服应力与
${m_{\rm{floc}}}$ 近似呈线性关系,因此在实际中需要通过控制${m_{\rm{floc}}}$ 来降低料浆屈服应力,保证料浆的流动性,从而预防深锥浓密机内的压耙。但是,从图4可知,${m_{\rm{floc}}}$ 主要受FD的影响,因此需要综合考虑絮凝效果(ISR、T和SSF)与屈服应力来综合确定FD的最优范围,最终确定本文FD的范围为15 g·t−1。在pH值为11、FD=15 g·t−1时,ISR =0.4565 mm·s−1,T=143 NTU,SSF=51.56%,屈服应力为243.18 Pa。此时屈服应力仍然较大,因为本文的絮凝沉降时间是14 h,时间相对较长,并且本文尾砂超细,因此深锥浓密机在长时间进料而不排料充填时,可采用底流循环活化[33]等方式来降低屈服应力。另外,从图2和图5对比分析可以发现,不同于动态絮凝沉降获得的高浓度尾砂料浆和搅拌制备形成的高浓度尾砂料浆[12, 14],本文通过静态沉降得到的浓缩超细尾砂料浆的屈服应力随着固相质量分数的增加而降低,这是因为强度高的絮网结构包裹大量水导致固相质量分数降低的同时也增加了屈服应力。
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通过对不同絮凝条件下获得的浓缩超细尾砂料浆的屈服应力进行原位测量,并通过对絮凝前后料浆总有机碳的测试来分析超细尾砂颗粒表面的絮凝剂吸附量,进而总结了絮凝沉降对浓缩超细尾砂料浆屈服应力的影响规律,主要结论如下:
(1)絮凝沉降对浓缩超细尾砂料浆的屈服应力有显著影响。不同絮凝条件下,pH和FD通过影响尾砂颗粒表面的絮凝剂吸附量影响浓缩超细尾砂料浆的屈服应力,在本文的实验范围内,屈服应力随着pH和FD的增大均不断增大。
(2)综合考虑尾砂料浆的絮凝沉降效果和所得浓缩超细尾砂料浆的屈服应力,本文最佳絮凝条件为pH值为11和FD=15 g·t−1,在此最优条件下ISR =0.4565 mm·s−1,T=143 NTU,SSF=51.56%,屈服应力为243.18 Pa。
(3)浓缩超细尾砂料浆的屈服应力随尾砂颗粒表面单位面积的絮凝剂吸附量的增大而增大,初步建立了适用于本文尾砂的基于絮凝剂吸附量的屈服应力模型。
Effect of flocculation sedimentation on the yield stress of thickened ultrafine tailings slurry
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摘要: 深锥浓密机内底部料浆的屈服应力过高容易导致压耙,为此通过对不同絮凝沉降条件下获得的浓缩超细尾砂料浆的屈服应力进行原位测量,并通过对絮凝前后料浆总有机碳的测试来分析超细尾砂颗粒表面的絮凝剂吸附量,进而分析了絮凝沉降对浓缩超细尾砂料浆屈服应力的影响规律。研究发现,絮凝沉降对浓缩超细尾砂料浆的屈服应力有显著影响,pH和絮凝剂单耗通过影响尾砂颗粒表面的絮凝剂吸附量进而影响浓缩超细尾砂料浆的屈服应力,屈服应力随着pH和絮凝剂单耗的增大均不断增大。综合考虑尾砂料浆的絮凝沉降效果和所得浓缩超细尾砂料浆的屈服应力,最佳絮凝条件是pH值为11和絮凝剂单耗为15 g·t−1,在此最优条件下料浆固液界面的初始沉降速率为0.4565 mm·s−1,沉降后上清液浊度为143 NTU,底部沉积尾砂料浆的固相质量分数为51.56%、屈服应力为243.18 Pa。初步建立了适用于超细人造尾砂的基于絮凝剂吸附量的屈服应力模型,屈服应力随尾砂颗粒表面单位面积的絮凝剂吸附量的增大而增大,为实际生产中控制全尾砂絮凝沉降参数提供参考。Abstract: With the advantages of efficiency and economy, deep-cone thickener (DCT) has been increasingly applied in tailings management. The rake in the DCT is essential for obtaining high-concentration underflow slurry; thus, more emphasis was placed on the effects of rakes on the underflow concentration. However, high concentration means high yield stress, which may lead to rake blockage. Therefore, this study investigated the effects of flocculation and sedimentation on the yield stress of thickened ultrafine tailings slurry. First, flocculation and sedimentation experiments were conducted under a pH range of 8 to 11 and flocculant dosage of 0 to 45 g t−1 to obtain different thickened ultrafine tailings slurries. Then, the yield stress was measured through an in situ test. Finally, the amount of flocculant adsorbed on the tailings particle surface was analyzed by total organic carbon analysis. The amount of flocculant adsorbed on the tailings particles surface increased with the pH and flocculant dosage over the entire experiment range. Then, the yield stress increased with the increase in the amount of adsorbed flocculant, indicating that flocculation sedimentation has a significant influence on the yield stress. Based on the flocculation sedimentation behavior and yield stress, the optimal conditions were a pH of 8 and flocculant dosage of 15 g·t−1. Under these conditions, the initial settling rate of the solid–liquid interface was 0.4565 mm·s−1, supernate turbidity was 143 NTU, solid mass fraction of sediment was 51.56%, and yield stress was 243.18 Pa. The relationship between the amount of flocculant adsorbed and yield stress was investigated, and an empirical model for yield stress based on flocculant adsorption was established. It was found that the yield stress increased with the amount of flocculant adsorbed, providing a reference for the control of flocculation sedimentation parameters in actual production.
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Key words:
- tailings /
- flocculation sedimentation /
- total organic carbon /
- flocculant adsorption /
- yield stress
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