Performance comparison of La-loaded D201 resins in the simultaneous removal of nitrate and phosphate
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摘要: 采用La3+、La（OH）3以及La2（CO3）3对D201树脂进行改性，并从吸附动力学、等温线、共存离子影响以及再生等方面系统地对比3种La负载树脂的同步脱硝除磷性能。扫描电镜及成分分析结果表明，D201表面能够负载不同形态的La，以La2（CO3）3形态负载时La含量最高。吸附数据表明，La2（CO3）3型D201树脂除磷效果优越，吸附容量高且受干扰离子影响小。经Na2CO3溶液再生5次后，各树脂均能保持良好的再生吸附效果，同步脱硝除磷性能稳定。树脂中季铵官能团（-R4N+）对硝酸根选择性较高，使得硝氮吸附受共存离子的影响较弱；磷酸根吸附归因于与-R4N+静电作用及与负载的各形态La形成沉淀、发生配体交换等，受体系pH值变化较为敏感，弱碱性条件可促进吸附。Abstract: In this study, different forms of lanthanum (La), including La3+, La(OH)3 and La2(CO3)3, were employed to modify D201 resin. The obtained resins were critically examined and compared with each other on the simultaneous removal characteristics of nitrate and phosphate, from the perspective of kinetics, isotherms, influence of coexisting ions, and regeneration. SEM images and component analysis revealed that different species of La were evidently loaded on the D201 surface and the highest La content was obtained on the La2(CO3)3-loaded D201 resin. In addition, the La2(CO3)3-loaded D201 resin maintained favorable dephosphorization performance against coexisting ions. Notably, the original and La-loaded D201 resins could retain the adsorption efficiency after being regenerated by Na2CO3 5 times, an indication of stable denitration and dephosphorization performance. R4N+ groups preferentially captured nitrate so that there was a negligible influence of coexisting ions on nitrate uptake. The phosphate adsorption was attributed to electrostatic interactions with R4N+ groups and chemical precipitation or ligand exchange with the loaded La. Moreover, it was more sensitive to the solution pH change, as supported by the improvement under weak alkaline conditions.
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Keywords:
- lanthanum /
- D201 resin /
- denitration /
- dephosphorization
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金属半固态加工具有高效、简易、节能等特点，已成为材料加工界中应用前景广阔的新型成形技术.半固态合金浆料的制备是金属半固态成形技术的基础和关键，它要求浆料原始组织为均匀细小的球形非枝晶组织.为此，人们开发了机械搅拌[1]、电磁搅拌[2]、超声波搅拌[3]以及新近开发的熔体混熔技术[4].但是，产生的问题是增加了额外的设备和工艺环节，提高了生产成本.为了保持半固态合金的组织特性和成形优势，降低工艺成本，开发新的半固态制备技术显得极为重要.
稀土是我国特有的宝贵自然资源，广泛应用于黑色金属和有色金属.长期以来，认为稀土元素的加入能够优化微观组织.而且大多数的研究都专注于稀土元素(如Ce，La，Y，Nd和混合稀土)对铸造铝硅合金中共晶硅和初生硅晶粒的影响[5-8]，而有关稀土对铝硅合金初生相的影响的研究报道并不多.一些研究发现稀土对铸造铝硅合金初生相具有明显的细化作用[9-11]，基于这些研究提出了一种新的细化机制：即通过稀土在铝合金中产生的共晶反应来优化合金的凝固组织.为进一步探讨稀土对铝合金凝固组织的影响，力争发现一些尚未被认知的现象或规律，本项工作将选择具有亚共晶成分的A356合金为研究对象，研究稀土La在适当工艺条件下对A356合金初生相形貌的影响.
1 实验方法
以工业纯铝、纯镁、Al-50 % Si中间合金为原料，按照A356合金(即Al-7 % Si-0.3 % Mg)配制符合试验要求的亚共晶铝硅合金.经Magix (PW2424) X荧光光谱仪分析，所用合金的实际成分(质量分数)如表 1所示.
表 1 试验用A356铝合金成分
首先预热石墨坩埚，预热温度为200~300 ℃.然后把Al-Si中间合金锭和铝锭依次加入坩埚，在电阻炉中熔化炉料，熔化温度为700 ℃.炉料熔毕，除净熔渣，加入自行配制的熔剂(成分为50 % KCl和50 % NaCl)覆盖熔体液面，防止合金元素过渡烧损和氧化；升温到750 ℃时，将预热到200~300 ℃的金属镁块压入熔池中心，并缓慢回转和移动，时间为3~ 5 min；将炉温保持在730~750 ℃，六氯乙烷分2~3次压入合金液内精炼合金液，待精炼剂反应完毕，静置1~ 2 min，升温到760~780 ℃，分别加入0.5、1.5和2.0 wt%的La作为细化剂，并迅速移入预定温度的装置进行等温处理，根据文献[12]可知，Al-La共晶反应温度为640 ℃，因此将熔液分别在650 ℃和630 ℃温度保温1 min，然后将熔液浇入尺寸为准28 mm×70 mm的紫铜铜模中.为了获得不同稀土La加入量对A356显微组织的影响，选取了从0.5 wt%到3.5 wt% 5个不同的添加量在630 ℃条件下保温1 min后浇入铜模中.
从所获铸锭的中部截取试块制作金相试样.经制备金相试样的相关程序后，在ZEISS Axioskop2光学显微镜上观察试样的凝固组织；为比较稀土La以及工艺条件对凝固组织的影响，用MIAPS(Microimage Analysis & Process)图相分析软件对每个试样测定平均等积圆直径和形状因子.
2 实验结果与分析
图 1是A356铝合金添加不同含量的稀土La在共晶温度之下(630 ℃)保温1 min后铜模浇注并水淬的铸态金相图.当加入的稀土量较少时，合金中存在一些粗大晶粒，如图 1（a）显示.从图 1中可见，随着La的加入量的增加，初生相的尺寸有明显的改变，晶粒变得越来越细小、圆整.图 2是A356铝合金添加不同含量的稀土La在共晶温度之上(650 ℃)保温1 min后铜模浇注并水淬的铸态金相图.由图 2可见，在650 ℃条件下保温的初生相大多成蔷薇花状，只有极少量的细小晶粒.因此，不同的等温温度对添加La的铝合金具有不同的细化效果.因此可知：在共晶温度之下某一温度(如630 ℃)保温，从该合金凝固的角度来说，此时已发生铝镧共晶反应.该反应中的产物：α-Al，与A356铝合金中初生α-Al相具有相同晶格类型和常数，满足结晶的热力学条件，可作为非自发结晶核心，从而细化合金初生α-Al晶粒和二次枝晶间距.
用MIAPS(Microimage Analysis & Process)图相分析软件测定晶粒平均等积圆直径.从图 3中可看出加入不同量得稀土La所得的初生相晶粒尺寸也不同，且在630 ℃条件下保温的初生相晶粒尺寸及形貌均优于在650 ℃条件下所得.
图 4是A356铝合金添加不同含量的稀土La处理后浇入铜模快速凝固和水淬的铸态显微组织.由图 4（a）可看到未添加稀土的合金组织中初生α-Al晶粒比较粗大，而且呈现明显的树枝状晶.而加入稀土La的合金, 初生相晶粒的形貌和尺寸逐步发生明显的变化，初生α-Al大多呈现为蔷薇花状或细小的球状，凝固组织中几乎没有粗大的树枝晶.由图 4(b)-(f)可看出，低过热度浇注的A356合金的凝固组织随着La含量增加，初生相先是逐渐变得细小，圆整.当La含量增加到1.5 wt%时，初生相细小颗粒越来越多，蔷薇花状的晶粒越来越少.当继续增大La含量时，A356合金的初生α-Al反而呈现出较多的蔷薇花状和树枝状，尺寸也有增加的趋势.因此，在本研究中，获得半固态球状组织的适宜La含量应为1.5 %.
图 5中亦可看出初生α-Al晶粒尺寸及形貌随稀土La添加量的变化.由图 5可见，在630 ℃条件下保温，随着La添加量的增加，初生相形貌和尺寸先变好后变坏，这个结果与图 4所得结果相符.
根据一些研究[12-14]报道：在铝熔体中加入稀土，在Al-RE二元相图的富铝角，铝和大部分的稀土元素（如La，Y，Sc等等）会在600 ℃之上发生共晶反应.在Al-La合金中，当一定量的La存在于系统中时，将于640 ℃时发生如下共晶反应[12]：L→ α -Al + La3Al11，共晶产物为α-Al和La3Al11相.而实验中所使用的A356铝合金液相线温度为615 ℃，可以推断，上述共晶反应在合金熔体凝固之前就已经发生.这意味着在铝合金熔体凝固之前，熔体中已经存在一部分α-Al晶粒.如果这个共晶反应能得到足够的α-Al，那么该共晶反应所结晶出的α-Al晶粒就可为A356铝合金初生相的结晶提供非自发形核的衬底，增加结晶核心数量，达到细化合金初生α-Al相和二次枝晶间距的目的.因此，稀土通过共晶反应来细化铝合金组织的方法不同于传统的细化机理和方法.
3 结论
La的加入对A356铝合金凝固组织具有细化的效果，且在本实验中，La的最佳加入量为1.5 wt%，此时能获得细小的初生相晶粒.La的细化机理与铝镧的相互作用有关，铝镧在640 ℃发生的共晶反应能够为A356铝合金初生相的形成提供形核核心.
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图 2 原D201及La负载D201树脂同步脱硝除磷性能与吸附时间关系
Fig 2. Simultaneous removal of nitrate and phosphate using the original and La-loaded resins as a function of time
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图 3 原D201及La负载D201树脂同步脱硝除磷等温线
Fig 3. Adsorption isotherms of nitrate and phosphate using the original and La-loaded resins
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图 4 原D201及La负载D201树脂在不同共存离子体系下的同步脱硝除磷性能
Fig 4. Performance of the original and La-loaded resins in terms of simultaneous removal of nitrate and phosphate in the systems with different coexisting ions
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