Metals are present everywhere around us and are one of the major materials upon which our economies are built. Economic development is deeply coupled with the use of metals. During the 20th century, the variety of metal applications in society grew rapidly. In addition to mass applications such as steel in buildings and aluminium in planes, more and more different metals are in use for innovative technologies such as the use of the speciality metal indium in LCD screens. A lot of metals will be needed in the future. It will not be easy to provide them. In particular in emerging economies, but also in industrialised countries, the demand for metals is increasing rapidly. Mining and production activities expand, and with that also the environmental consequences of metal production. In this course, we will explore those consequences and we will also explore options to move towards a more sustainable system of metals production and use. We will focus especially on the options to reach a circular economy for metals: keeping metals in use for a very long time, to avoid having to mine new ones. This course is based on the reports of the Global Metals Flows Group of the International Resource Panel that is part of UN Environment. An important aspect that will come back each week, are the UN Sustainable Development Goals, the SDGs. Those are ambitious goals to measure our progress towards a more sustainable world. We will use the SDGs as a touching stone for the assessment of the metals challenge, as well as the solutions we present in this course to solve that challenge.
Stock Dynamics and Modelling
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A Circular Economy of Metals: Towards a Sustainable Societal Metabolism
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从本节课中
Dynamics of Metal Systems
Week 3 and all subsequent weeks focus on solving the metals challenge. Obviously, we need to make changes in the metals system to reach a more sustainable situation and reconcile the different Sustainable Development Goals. When considering changes, it is important first to understand the system. We will be discussing stocks and flows of metals in society and see how they interact. In society, we do not just obey the laws of justice and economics, but also the laws of nature. It is important to realise that when contemplating solutions for the metals challenge. This week will be rather theoretical but will provide important information for the coming weeks.
与讲师见面
Ester van der VoetDr.
Institute of Environmental Sciences
Hello, welcome to the second lecture of week 3.
From the previous lecture we have now an image of the stocks in society or
urban minds, the materials that we have to close the loop.
In this lecture, we will explore the dynamics of those stocks.
To go towards a circular economy, we need to understand how these stocks
are built up, and when they will become available for recycling.
Mining companies have exploitation plans for mines, and
something similar should be done also for our urban mines.
Mining companies, well they explore,
they see where is metal in the ground, they think about how to get it out,
when to get it out, where to start, what to keep until later.
Also, those aspects, although obviously they are different for
urban mines but aspects like that should be in an urban mining plan.
For the investigation of stock dynamics,
we have an analytic tooled called Material Flow Analysis.
And then the dynamic very end of that dynamic material flow analysis.
The basic principle for dynamic material flow analysis or
for any type of material flow analysis, is the law of mass conservation.
Whatever float in 2D economy has to come out again,
there is no loss of materials.
There are three types of stocks with different dynamics, and
we need to understand that because we want to use the material to recycle for
our circular economy.
There's inflow-driven stocks, those are goods that have a very short lifespan.
So when they're consumed they become waste fairly quickly,
there's not really a stock formation, you could say.
Examples are food or packaging we use it and
then we immediately discard it, it becomes waste.
In the metals area for example, we have aluminium drinking cans as clamps
also small batteries they have a lifespan maybe of several weeks or months.
But then they become waste, they have a very short lifespan.
Second type of stock is really stock driven,
we want to have the stock at a certain level and we want to adjust the inflows,
to build up or maintain the stock at a certain level.
Examples can be found mainly in the area of buildings and
infrastructure, where metals play a very important role.
And then we have the outflow driven stocks,
those are the stocks that have become obsolete, nobody wants them anymore.
Basically they are waste but they have not entered the waste stage yet.
It can be cables that are no longer used but
are left in the ground because it's too much trouble to dig them up.
This can also be hibernating stocks, like old mobile phones that we did not
throw away yet but also we don't use it anymore, they hang around in some drawers.
And old computers, things that we do not use anymore but
also have not yet entered to waste stage,
each of these three types of stocks has a different dynamic.
So we have into the different metal applications in
more detail as their dynamics are different.
Let's first go to the inflow-driven applications,
the example I mentioned were aluminium drinking cans, take those drinking cans.
We as consumers are not interested in building up stocks of drinking cans,
we buy drinks and then we consume them.
And the packaging, being the aluminium cans, we throw it away immediately.
So in this case there is no stock building,
we have an inflow equaling the outflow almost at the same time.
How that might work you can see in the following animation by Rubin Huella.
>> Inflow driven system.
As model, there is no stock at all,
after use the flow goes directly to waste.
In the real world there will always be a small stock with
a very small residence time.
>> Next, we'll have a look at the stock driven applications,
examples of that can be found in the build environment.
Let's take for example window frames, aluminium window frames.
We want those window frames for our windows but once they are there,
we won't buy new ones unless the old ones are broken.
That's what we call a saturated stock, a stock that is at a level that we want it
to be, and the flows are only maintained to keep the stock at that level
Window frames stay in place for decades say around 50 years,
then they have to be replaced because the building is being renovated or
maybe its being demolished, a new building has to be built.
So the equation is then a bit different,
the outflow does not equal the inflow of that same year but
it equals the inflow of 50 years ago.
This 50 years, we call the lifespan or the residence time.
Residence times for different applications can be quite
different they range from zero years up to centuries.
For buildings an average that is being taken in studies is 50 years,
sometimes it's shorter some building are already demolished after 20 or
30 years, sometimes it's much longer if it can be centuries even.
But 50 years is an average that we can take,
cars usually we use 15 years, also here there is a lot of
variation in the lifespan but 15 years is a good average.
Some household appliances, pots, pans, cutlery,
well they will last for say 30 years,
the shorter lifespans we see in office materials like pens and
paperclips, they only last for two yeas.
Mobile phones, computers they last three to five years,
packaging zero years, and you can make a generalized equation out of that.
The outflow out of stock equals the inflow of some years ago,
some being represented by the letter L in the equation meaning the life span.
While the stock is building up it means that the outflow is always
very much smaller than the inflow in the same year,
because it's the inflow of a number of years previously.
When in the end the stock its saturation level, the level that we wanted
to be the inflow will be reduced to the level that we need to maintain the stock.
That is, that we have to replace the material which goes to waste,
which we do not add to the stock itself.
You can see in this graph beside me, that the graph of the outflow
out of the stock has the identical shape to the graph of the inflow.
Only some years, and those number of years,
that is exactly the lifespan of the residents time behind.
So that means the inflow will grow until the saturation level,
the outflow will grow as well, to the same level.
So in the end, we have a stock that is in equilibrium,
we call that an equilibrium stock where the inflow equals
the outflow and the stock is at a saturated level.
>> Stock driven system.
If the inflow is bigger than the outflow, the stock increases.
This is a stock that is building up and has not yet reached an equilibrium.
If the inflow is as big as the outflow, the stock stays constant.
This happens when the stock has reached it's precise level and
remains in equilibrium.
The size of the stock is then equal to the flow multiplied by the residence time.
>> Let's go to the third type of stock, the outflow-driven stocks or
the hibernating stocks.
Those are stocks of applications that we no longer use but
also have not thrown away yet.
In the case of the cables they are just left in the ground,
we don't want to go through the trouble of digging them up, apparently.
And if we think about our old mobile phones well, they're just there in drawers
we don't use them anymore but also we don't want to throw them away,
maybe we think they are too valuable for that.
This is shown in the next animation.
>> Outflow driven system, the application is phased out.
The stock is not in use anymore,
there is no inflow, the stock hibernates,
sooner or later the stock goes to waste.
>> Is that good?
>> Yeah.
>> We've seen now the different types of stocks and their difference in dynamics.
So how is that relevant for a circular economy?
Well in the first place for all of these stocks,
the thing is that we have to catch the waste when it comes out.
For the inflow-driven applications, you see that well, the applications
are quite scattered, usually they end up in the household waste.
Separate collections systems will have to be established and probably
incentives will have to be created to get these things out of the household waste,
and prevent them from ending up in landfills or in incineration plants.
It's difficult to forecast either the inflows and
the outflows because they are quite susceptible to fashion.
It's different for the stock-driven applications,
here like an infrastructure and buildings we have well-regulated waste flows,
and we have a good idea of when they may become available,
a lot of it is already collected right now.
But a mining plant an urban mining plant,
must include estimates on the expected residence types,
also we need to estimate if and when the stocks are saturated,
that's essential information to build up our plans.
As for the outflow stocks or the hibernating stocks, they are,
as I mentioned before the ideal stocks for urban mining.
They are there, they are available now,
apparently there is a motivation not to throw them away.
So there are sort of the very good starting material for
our circular economy.
And after that we can gradually
go towards a circular economy
that encompasses all kinds
of stocks, that may work and
show in the next animation by Rubin Huella.
Hopefully, now you have an understanding of the system.
>> Do an application yet.
>> Hopefully now you have an understanding of the system of metals
based on the law of mass conservation.
This mass conservation, the realizations that what goes in has
to come out someday is basic information for a circular economy,
of course there's many other issues to be dealt with.
There is creating collection systems and
making incentives to actually operate those systems.
To make sure that everything is economically viable to develop recycling
technologies and so on and so on, most of them are not in this course.
But recycling will be the act of making new materials from old,
that's something for next week.
