OFFICIAL32 According to the lecture, why was heap roasting used in the producing of copper?

[00:00.00]NARRATOR: Listen to part of a lecture in an earth science class. [00:07.10]The professor is discussing an area of the United States called the Copper Basin.[00:12.94]MALE PROFESSOR: Now, you may not have heard of the Copper Basin.[00:17.21]It's in the Eastern United States, in the Tennessee River Valley. [00:21.69]It got its name because settlers discovered copper there in 1843. [00:26.54]And soon afterwards, it supported one of the largest metal mining operations in America. [00:32.68]At one time, four mining companies employed 2500 workers in the Copper Basin.[00:38.46]For that time period, it was a huge operation.
[00:42.50]Well, this mining operation turned the Copper Basin into a desert. [00:48.08]In the 1840s, when mining operations started, it was a dense green forest. [00:54.32]But in the 1940s, 100 years later, it was as barren as the Moon.[00:59.85]Efforts to reclaim the land and restore the basin to the fertile valley it once was...well, actually, those efforts are still ongoing. [01:09.52]It's been a long and tedious process. [01:12.52]In fact, it was many years before any results were seen. [01:16.62]Copper mining had gone on there for more than 90 years! [01:20.77]The damage couldn't be reversed overnight.
[01:23.33]Although I should mention that by 1996, the water in one of the rivers flowing through the basin was clean enough that it was the site of the Olympic white water kayaking competition. [01:35.79]And that river is still used now for recreation.
[01:39.82]But anyway... So ... let's analyze the problem.[01:44.35] It wasn't the mining itself that caused such massive destruction. [01:49.04]It was what happened after the copper ore was extracted from the mines. [01:53.44]It was a process called heap roasting.[01:56.77]Copper ore contains sulfur.[02:00.27] And heap roasting was a way to burn away the sulfur in the copper, so they'd be left with something closer to pure copper. [02:07.50]Well, in the process, large vats of raw copper ore are burned slowly, for two or three months actually, to lower the sulfur content. [02:18.06]And this burning—well, let's look at the results.
[02:22.71]First, the mines were fairly remote, so there was no way to bring coal or other fuel to keep the fires going. [02:29.54]So they cut down local trees for fuel.[02:32.88] And like I said, the fires burned for months—[02:37.36]uh, that's a lot of fires and a lot of trees. [02:40.80]Deforestation was occurring at a rapid rate. [02:44.70]And it was accelerated by the smoke from the burning ore. [02:47.73]Big clouds of sulfuric smoke, which was toxic to the trees, formed over the areas. [02:54.81]Trees that hadn't been cut for fuel were killed by the fumes.[02:59.36]The sulfur also mixed with the air and created sulfur dioxide. [03:04.84]And the sulfur dioxide settled in the clouds fell to the land in droplets of rain and sank into the soil.[03:12.37]This is what we now call acid rain. [03:16.34]You've probably heard of it.[03:18.54] But no one used the term back then.[03:21.47]Anyway, the acid rain created highly acidic soil.[03:26.62] Well, soon the soil became so acidic that nothing could grow, nothing at all. [03:33.94]Vegetation and wild life disappeared.[03:37.57]And it wasn't just the land and the air, it was the water too. [03:42.57]What do you think happen to the rivers? [03:45.25]Well, there are no trees to absorb the rain, and there was a lot of rain! [03:51.09]So the rain eroded the soil and swept it into the rivers. [03:56.12]This is called silting, when soil particles are washed into the rivers.[04:00.68] And the silting continued at an alarming rate.[04:04.31]But this was toxic soil and toxic runoff, the acid and metals in the soil made the once clear rivers flow bright orange.
[04:15.50]So it was really that one step in the process of producing copper...the problems just built up and up until there was a desert where a beautiful forest used to be. [04:27.40]OK. Now let's look at reforestation and land reclamation efforts.