Liquid purification or separation – Processes – Using magnetic force
Reexamination Certificate
2001-07-11
2003-10-14
Reifsnyder, David A. (Department: 1723)
Liquid purification or separation
Processes
Using magnetic force
C210S748080, C210S175000, C210S198100, C210S222000, C210S243000, C209S012100, C209S127200, C095S028000, C096S001000, C096S003000
Reexamination Certificate
active
06632369
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains generally to methods and devices for collecting ions from a multi-species plasma. More specifically, the present invention pertains to methods and devices for collecting ions from a multi-species plasma after the ions of the plasma have been separated according to the respective masses of the constituent elements. The present invention is particularly, but not exclusively, useful for collecting the relatively low mass particles in a multi-species plasma.
BACKGROUND OF THE INVENTION
A plasma mass filter that is designed to separate low mass particles from high mass particles is disclosed in U.S. Pat. No. 6,096,220 which issued to Ohkawa for an invention entitled “Plasma Mass Filter” and which is assigned to the same assignee as the present invention. U.S. Pat. No. 6,096,220 is incorporated herein by reference. In overview, a plasma mass filter includes a substantially cylindrical shaped wall which surrounds a hollow chamber. A magnetic field is generated in the chamber which is generally axially oriented and an electric field is generated within the chamber which is oriented substantially perpendicular to the longitudinal axis of the chamber. Importantly, for operation of a plasma filter, the electric field has a positive potential on the axis relative to the wall which is usually at a zero potential. When a multi-species plasma is injected into the chamber, the plasma interacts with the crossed electric and magnetic fields, resulting in the rotation of the plasma about the chamber axis. In response to the crossed electric and magnetic fields, each ionized or charged particle in the multi-species plasma will travel on a predictable trajectory about the axis as it transits the chamber. The particular trajectory is dependent on the mass to charge ratio of the orbiting particle. With this in mind, the plasma mass filter is designed so that high mass particles will travel on unconfined orbits that allow the high mass particles to strike and be collected on the wall of the chamber. On the other hand, the low mass particles will have orbits that are smaller than the chamber radius, and hence are confined inside the chamber so as not to strike the chamber walls. Thus, the orbiting low mass particles eventually exit the chamber at one end of the cylinder where they need to be collected.
When salt vapors, such as sodium hydroxide vapors (NaOH), are introduced into the vacuum chamber, along with the multi-species plasma discussed above, a mechanism for collecting the low mass particles (light ions) in the plasma can be provided. To appreciate how this can be accomplished, certain characteristics of sodium hydroxide need to be considered. Specifically, these characteristics need to be considered in a context where ions derived from a salt, such as sodium hydroxide (NaOH) come into contact with a solid wall or plate. For this consideration, reference here is first made to FIG.
1
.
With reference to
FIG. 1
, it is known that when plasma ions derived from a salt come into contact with a solid wall
10
they will be neutralized and reform into a salt. If the wall
10
is cold enough (e.g. <100° C.), the salt will deposit faster than it evaporates and a layer of the salt will grow. Initially, the salt will deposit as a solid, forming a protective layer on the wall. However, as this layer grows, its surface temperature will increase, since salts are good insulators and there is a heat load on the wall from the plasma. Eventually, however, the surface will melt to create a molten layer
12
. Deposition of additional salt will increase the thickness of the molten layer
12
. In a steady state, molten salt will flow away at the same rate as the plasma deposits it. In addition, other ion species from the plasma will be neutralized and collected. Under these conditions, if the throughput of ions from other species is small enough, compared to the throughput of ions from the salt, then the other species will be incorporated into the molten layer. Thus, the other species will either be dissolved in the molten salt or will form micro-crystals that will be carried along by the molten salt. This provides a mechanism of draining both the molten salt and the other species off of the collector.
It happens that the draining of the molten salt off of the collector depends on the rate at which the molten salt is deposited. If the deposition rate is small, then gravity is the primary force driving the flow of the molten salt. In this case, the molten salt can be drained off the bottom of a flat collector plate. On the other hand, if the deposition rate is high, then the primary force driving the flow of the molten salt is the “plasma wind” force that arises because the molten layer absorbs momentum from the plasma. In that case, the molten salt can be drained off the side of a vertical collector plate. Or, the collector can be formed as a series of concentric cones and the molten layer can be drained off the back of the cones.
For the conditions mentioned above, the molten salt collector works best on salts that have a low vapor pressure in the molten state such as sodium hydroxide (NaOH). Accordingly, sodium hydroxide is used only as an example herein.
As mentioned above, if the deposition rate of the sodium hydroxide is relatively small, gravity is the primary force driving the molten layer. For example, if the plasma deposits sodium hydroxide onto a flat collector plate that is tilted at an angle &agr; with respect to vertical, as shown in
FIG. 1
in a steady state, the gravitational force is balanced by the viscous drag:
0
=
ρ
g
cos
α
+
η
∂
2
u
∂
z
2
,
where &rgr; is the mass density, &eegr; is the viscosity, and u is the velocity of the molten NaOH along the collector plate. Inertial terms in the fluid equations have been neglected. Solving this equation with the boundary conditions u(z=0)=0 and ∂u/∂z(z=d)=0 gives:
u
(
z
)
=
ρ
g
cos
α
η
d
2
(
z
d
-
z
2
2
d
2
)
,
where d is the local thickness of the molten layer
12
.
The average axial velocity is then:
u
_
=
1
d
∫
0
d
u
ⅆ
z
=
ρ
g
d
2
cos
α
3
η
.
The thickness of the molten layer, d, is determined by conservation of mass:
∂
∂
y
(
ρ
u
_
d
)
=
m
Γcosα
,
where y is the distance along the plane of the collector, &Ggr; is the ion flux, and m is the average ion mass. Solving this equation gives:
d
=
(
3
η
m
Γ
ρ
2
g
y
)
1
/
3
≈
0.053
(
y
R
w
)
1
/
3
mm
u
=
(
g
m
2
Γ
2
3
ρη
y
2
)
1
/
3
cos
α
≈
0.42
cos
α
(
y
2
R
w
2
)
1
/
3
cm
/
s
,
where it is assumed that d=0 and hence {overscore (u)}=0 at y=0. Also, the following values for the parameters have been used:
&rgr;
&eegr;
&Ggr;
m
R
w
1800 kg/m
3
0.004 Pa-s
0.05 mol/m
2
/s
13.33 amu
0.6 m
Note that in this example the thickness d is not explicitly dependent on the tilt angle &agr;. This is because the reduction in the driving force is cancelled by the spreading out of the ion flux. Assuming that y<8R
W
, we find that d<0.1 mm. The flow velocity is less than 1 cm/s, indicating that it takes around a minute for a deposited atom to leave the collector plate. In this case, a trough can be placed below the collector plate to catch the molten sodium hydroxide.
As the plasma imparts a heat load on the molten NaOH layer, the surface temperature of the NaOH is given by:
T
s
=T
m
+qd/&kgr;,
where T
m
=322° C. is the melting temperature of sodium, q≈0.5 M
Agnew Stephen F.
Cluggish Brian P.
Putvinski Sergei
Archimedes Technology Group, Inc.
Nydegger & Associates
Reifsnyder David A.
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