Goldbugs at work
外刊原文
There’s lots of gold in urban waste dumps
The pay dirt could be 15 times richer than natural deposits
GOLD MINING can be a dirty business. Even with relatively rich deposits, usually found in remote areas, you need giant excavators, huge crushing machines, lots of water and highly toxic chemicals, like acids and cyanide, to extract just ten grams of gold from a single tonne of ore (there are 31 grams in a troy ounce). At current rates, that is worth over $800.
A different sort of pay dirt, however, offers the prospect of a much greater return for urban miners: the printed circuit boards (PCBs) found in rapidly growing mountains of electronic waste. Estimates vary, but a tonne of PCBs could contain 150 grams or more of pure gold, which, because it does not tarnish, produces stable electrical connections. There are also other valuable materials used alongside gold, including silver, palladium and copper, which, if recovered, could push the total haul to well above $20,000 per tonne.
According to the UN, some 62m tonnes of electrical items, ranging from domestic goods to computers and mobile phones, were disposed of globally in 2022. Less than a quarter is reckoned to be recycled, at least in any formal way. Typically, the PCBs are removed and crushed before being either burned in a furnace to melt out metals or treated with chemical solvents, like strong acids. As these processes produce large carbon emissions and have poisonous by-products that are difficult to clear up, companies are developing a number of cleaner recycling methods. One of the more intriguing employs bacteria to do the actual extraction of metals.
Bioleaching, as this process is called, is an old idea: more than 2,000 years ago, the leaching of metal was seen to turn the water in copper mines blue. It was not until the 1950s, though, after bacteria were found to be responsible for the phenomenon, that the process was commercialised to recover leftover material in tailings, the liquid and solid waste from mining operations. Bioleaching relies on the metabolism of certain naturally occurring bacteria, such as Acidithiobacillus ferrooxidans, which produce oxidising agents that dissolve metals into solution. The metals can then be recovered with various separation and filtration methods. When used outdoors on tailings the process can take months and is not very efficient.
Contained within a custom-built modern processing plant, however, bioleaching can be sped up and improved with the use of a combination of similar bacteria. These, too, are naturally occurring and safe to work with as they are non-pathogenic. The trick, according to Bioscope Technologies, an urban-mining company building a bioleaching plant in Cambridge, England, is to keep the bacteria in their preferred conditions. This includes a carefully controlled acidic environment, a warm temperature below 50°C and a good supply of oxygen bubbled into their breeding tanks. Once these pampered bacteria are mixed with crushed PCBs in a reaction chamber they digest many of the metals within a day or two.
The resulting liquid is then treated in a number of ways to recover the metals. Gold quickly precipitates out with the addition of a little water. An electric current passed through the liquid in an enclosed system recovers copper in a manner similar to electroplating. Having done their stuff, the bacteria are returned to their breeding tanks before being used again.
The idea is to create an enclosed, circular recycling system that accelerates a natural process and is sustainable, says Jeff Bormann, Bioscope’s chief executive. Trial production runs are already under way with full production due to begin in January. The Cambridge plant has the capacity to process 1,000 tonnes of PCBs a year, although plans are already being discussed to build a much bigger one.
Bioscope was set up as a separate entity by N2S, a sister company that specialises in recycling IT equipment, and which supplies Bioscope with the crushed PCBS. Before being treated, Bioscope mechanically separates out plastics and fibreglass, the base material on which the circuits are built. Plastics can be sent for recycling and the company extracts silica from the fibreglass for use in specialised ceramics.
The process is capable of recovering almost all the gold, silver, copper and palladium from the crushed PCBs, although the exact amount depends on the type of circuits being recycled. Servers and telecoms equipment tend to contain the most precious metals, domestic appliances less so. The recovered metals are pure enough to be used again in electronics.
At the end of all the treatments, nothing is yet being thrown away for good. This is because new bioleaching methods are being developed to get at some of the non-precious, though still valuable, metals in PCBs. The recovery of tin was recently added, says Mr Bormann, with zinc, gallium and tantalum planned for next year. The goldbugs have plenty of work ahead.
参考译文
掘金者在行动
城市垃圾堆中蕴藏大量黄金
电子废料含金量竟是天然矿藏的15倍
传统金矿开采是一项劳心费力的产业。即便在富矿区，也需要投入大量机械设备和化学药剂，才能从一吨矿石中提取区区10克黄金，按当前市价不过800美元。
但城市“矿工”们发现了一条更为丰厚的“淘金路”：与日俱增的电子垃圾中的印刷电路板（PCBs）。据测算，每吨 PCBs 中可能蕴含150克以上的纯金。这些性质稳定的黄金在电路中发挥着关键作用。若将金、银、钯、铜等贵重金属一并回收，单吨价值可突破2万美元，堪称城市中的“富矿”。
联合国统计显示，2022年全球电子垃圾产量高达6200万吨，涵盖家电、电脑、手机等各类电子产品。其中仅不到四分之一经过规范回收。传统回收方式要么采用高温焚烧，要么使用强酸溶解，不仅耗能巨大，还会产生难以处理的有毒物质。为此，企业纷纷探索更环保的回收技术，其中最引人瞩目的莫过于利用细菌提取金属。
这种被称为“生物浸出”的技术可谓历史悠久。早在两千多年前，人们就发现铜矿中的积水会变成蓝色。但直到20世纪50年代，科学家才确认这一现象源于细菌作用，并开始将其应用于尾矿处理。这些天然细菌通过代谢作用产生氧化剂，能够将金属转化为溶液，再通过分离过滤加以回收。不过，在露天环境下处理尾矿往往耗时数月，效率欠佳。
英国创新企业 Bioscope Technologies 别出心裁，在剑桥建造了现代化生物浸出工厂。他们采用多种无害细菌组合，显著提升了处理效率。公司掌握的关键技术在于精准调控细菌生长环境，包括酸碱度、温度和氧气供应等要素。在这种“温室”环境中，细菌能在短短一两天内完成金属分解。
处理后的溶液经过多道工序提取金属。只需加入少许清水，黄金便会自然沉淀；通过电镀原理，还能高效回收铜。完成“工作”的细菌则被送回培养池继续服务。
Bioscope 首席执行官 Jeff Bormann 表示，公司致力于打造一个加速自然过程的封闭循环系统。目前试生产已经启动，预计明年1月全面投产。虽然剑桥工厂年处理能力为1000吨 PCBs，但扩建更大规模工厂的计划已在酝酿中。
作为 IT 回收巨头 N2S 的子公司，Bioscope 在处理 PCBs 前会先分离出塑料和玻璃纤维。这些材料也不会被浪费：塑料可以回收利用，玻璃纤维中的二氧化硅则可用于特种陶瓷生产。
这套工艺几乎能完全回收电路板中的金、银、铜、钯等金属，其中以服务器和通信设备的“含金量”最高。回收金属的纯度完全符合电子工业标准，可以直接重复使用。
目前，工厂对各类物质都实现了“物尽其用”。科研人员正在开发新的生物浸出技术，瞄准电路板中的锌、镓、钽等非贵金属。Bormann 透露，锡的回收技术已经投入使用，明年将开发更多金属的回收工艺。在这片城市“矿区”，科技淘金仍在继续。
感谢大家阅读。
来源：The Economist 《经济学人》最新一期
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