Pandas tries to do something with indexes while only supposed to work with values.
#### Code Sample, a copy-pastable example if possible

```python
ra = pd.read_excel("Edata.xlsx", sheet_name="r-a").astype(float)

```
```python
d[i].min().min()

```
#### Problem description
Traceback (most recent call last):
  File "E:/загрузки/учеба/лабы/H2lab/Search.py", line 4, in <module>
    ra = pd.read_excel("Edata.xlsx", sheet_name="r-a").astype(float)
  File "E:\python\lib\site-packages\pandas\core\generic.py", line 5882, in astype
    dtype=dtype, copy=copy, errors=errors, **kwargs
  File "E:\python\lib\site-packages\pandas\core\internals\managers.py", line 581, in astype
    return self.apply("astype", dtype=dtype, **kwargs)
  File "E:\python\lib\site-packages\pandas\core\internals\managers.py", line 438, in apply
    applied = getattr(b, f)(**kwargs)
  File "E:\python\lib\site-packages\pandas\core\internals\blocks.py", line 559, in astype
    return self._astype(dtype, copy=copy, errors=errors, values=values, **kwargs)
  File "E:\python\lib\site-packages\pandas\core\internals\blocks.py", line 643, in _astype
    values = astype_nansafe(vals1d, dtype, copy=True, **kwargs)
  File "E:\python\lib\site-packages\pandas\core\dtypes\cast.py", line 729, in astype_nansafe
    return arr.astype(dtype, copy=True)
ValueError: could not convert string to float: '4→0'

---------------

Traceback (most recent call last):
  File "E:/загрузки/учеба/лабы/H2lab/Search.py", line 23, in <module>
    mins[i] = d[i].min().min()
  File "E:\python\lib\site-packages\pandas\core\generic.py", line 11620, in stat_func
    f, name, axis=axis, skipna=skipna, numeric_only=numeric_only
  File "E:\python\lib\site-packages\pandas\core\series.py", line 4083, in _reduce
    return op(delegate, skipna=skipna, **kwds)
  File "E:\python\lib\site-packages\pandas\core\nanops.py", line 123, in f
    result = alt(values, axis=axis, skipna=skipna, **kwds)
  File "E:\python\lib\site-packages\pandas\core\nanops.py", line 843, in reduction
    result = getattr(values, meth)(axis)
  File "E:\python\lib\site-packages\numpy\core\_methods.py", line 32, in _amin
    return umr_minimum(a, axis, None, out, keepdims, initial)
TypeError: '<=' not supported between instances of 'str' and 'int'
I encountered this problem i new version of pandas. It seems like pandas when coverting values to float or searching for min, tries to operate with indexes(which strings) as well.  I know it because "4→0" is a name of row or column. I can provide any fether info if you need it. It works ok with pandas 0.23.4

#### Expected Output
everything working without mistakes
#### Output of ``pd.show_versions()``

<details>
This convig works prety fine:
[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.23.4
pytest: None
pip: 19.1.1
setuptools: 39.0.1
Cython: 0.29.7
numpy: 1.15.1
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 7.0.1
sphinx: 1.8.1
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.0
openpyxl: 2.6.3
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: 4.3.3
bs4: 4.7.1
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

------------------------------------------

This doesn't:
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.0.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None
pandas           : 0.25.1
numpy            : 1.15.1
pytz             : 2018.5
dateutil         : 2.7.3
pip              : 19.1.1
setuptools       : 39.0.1
Cython           : 0.29.7
pytest           : None
hypothesis       : None
sphinx           : 1.8.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.3.3
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10
IPython          : 7.0.1
pandas_datareader: None
bs4              : 4.7.1
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.3
matplotlib       : 3.0.0
numexpr          : None
odfpy            : None
openpyxl         : 2.6.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.1.0
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : 1.1.0
xlwt             : None
xlsxwriter       : None

</details>
